SCJP StudyGuide

Sun Certified Programmer for Java 6 Study Guide

SCJP Exam 310-065

Kathy Sierra Bert Bates

McGraw Hill

ISBN: 978-0-07-159106-5

Study Notes by Mahanthi Bukkapatnam

SCJP 6.0 Study Guide - CNTS

Mahanthi Bukkapatnam Page 2 of 69

Table of Contents

Introduction: Why these notes ............................................................................................ 5 Chapter 1: Declaration and Access Control........................................................................ 6

Identifiers and JavaBeans ............................................................................................... 6 Declare Classes ............................................................................................................... 7 Declare Interfaces ........................................................................................................... 8

Declare Interface Constants ........................................................................................ 9 Declare Class Members .................................................................................................. 9

Variable args ............................................................................................................. 10 Constructors .............................................................................................................. 10 Modifiers allowed only on methods ......................................................................... 11 Modifiers allowed only on instant variables............................................................. 11 Modifiers allowed only on local variables................................................................ 11 Transient variables .................................................................................................... 12 Volatile variables ...................................................................................................... 12

Enums ........................................................................................................................... 13 Chapter 2: Object Orientation........................................................................................... 14

Polymorphism............................................................................................................... 14 Overriding and Overloading ......................................................................................... 14

Overriding ................................................................................................................. 14 Overloading............................................................................................................... 15

Reference variable casting ............................................................................................ 16 Legal Return Type ........................................................................................................ 17 Constructors and Initialization...................................................................................... 19 Statics............................................................................................................................ 20 Coupling and cohesion.................................................................................................. 20

Chapter 3: Assignments .................................................................................................... 21 Literals, Assignments and Variables............................................................................. 21 Passing variables into methods ..................................................................................... 21 Array declaration, Construction and Initialization........................................................ 21

Initialization .............................................................................................................. 21 Wrapper classes and Boxing......................................................................................... 22

Boxing, == and equals()............................................................................................ 22 Overloading................................................................................................................... 23 Garbage Collection ....................................................................................................... 23

Chapter 4: Operators ......................................................................................................... 24 Chapter 5: Flow control, Exceptions and Assertions........................................................ 25

If statements .................................................................................................................. 25 Switch statements.......................................................................................................... 25 Loops and Iterators ....................................................................................................... 26

Basic for Loop........................................................................................................... 26 Enhanced for Loop (for arrays)................................................................................. 26 Labeled Statements ................................................................................................... 26

Exception handling ....................................................................................................... 27



Assertion ....................................................................................................................... 28 Chapter 6: String, I/O, Formatting and Parsing ................................................................ 29

String, StringBuffer and StringBuilder ......................................................................... 29 File and I/O ................................................................................................................... 29 Serialization .................................................................................................................. 29 Dates, Numbers and Currency ...................................................................................... 29 Parsing, Tokenizing and Formatting............................................................................. 29

Chapter 7: Collections....................................................................................................... 30 hashCode() and equals() ............................................................................................... 30 Collections .................................................................................................................... 31

Operations on a collection ........................................................................................ 31 Key Interfaces and Concrete classes......................................................................... 31 Ordered and Sorted ................................................................................................... 32 List interface ............................................................................................................. 33 Set Interface .............................................................................................................. 34 Map interface ............................................................................................................ 35 Queue Interface......................................................................................................... 36 Sorting Collections and Arrays................................................................................. 36 Comparable Interface................................................................................................ 36 Comparator Interface ................................................................................................ 37 Key Sort methods...................................................................................................... 37 Searching Arrays and Collections............................................................................. 37 Converting Arrays to Lists to Arrays........................................................................ 38 Using Lists ................................................................................................................ 39 Using Sets ................................................................................................................. 39 Using Maps ............................................................................................................... 39 Using Navigating (Searching) TreeSets and TreeMaps............................................ 39 Polling ....................................................................................................................... 40 Descending Order ..................................................................................................... 40 Backed Collection..................................................................................................... 40 Using PriorityQueue class......................................................................................... 40 Method overview for Arrays and Collections........................................................... 40 Method overview for List, Set, Map and Queue....................................................... 40

Chapter 7: Generics........................................................................................................... 41 Generics and Collections .............................................................................................. 41

Before Java 5............................................................................................................. 41 Generics and Legacy Code ....................................................................................... 42 Polymorphism and Generics ..................................................................................... 43 Generic Methods....................................................................................................... 43

Parameterized Types ..................................................................................................... 44 Class.......................................................................................................................... 44 Methods..................................................................................................................... 45

Chapter 8: Inner Classes ................................................................................................... 46 Overview and Key Points ............................................................................................. 46 Regular Inner class........................................................................................................ 47 Method Local inner class .............................................................................................. 48



Anonymous inner class ................................................................................................. 50 Static Nested class......................................................................................................... 51

Chapter 9: Threads............................................................................................................ 52 Overview....................................................................................................................... 52 Distinguish Thread and Thread of Execution ............................................................... 53 The Thread class ........................................................................................................... 53 Thread and the Runnable interface ............................................................................... 54 Defining Threads .......................................................................................................... 55 Instantiating Threads..................................................................................................... 56 Starting Threads ............................................................................................................ 57 Getting the name of the Thread .................................................................................... 57 Starting multiple threads ............................................................................................... 58 Thread Scheduler .......................................................................................................... 58 Thread States and Transitions....................................................................................... 59 Sleeping......................................................................................................................... 60 Thread Priorities............................................................................................................ 61 Yielding from a thread .................................................................................................. 61 Join() method ................................................................................................................ 62 Synchronization Code................................................................................................... 63 Key points about locking and synchronization............................................................. 64 Synchronizing static methods ....................................................................................... 65 More Synchronization points........................................................................................ 66 Thread interaction ......................................................................................................... 67 Exceptions related to threads ........................................................................................ 67 Summary - Points to keep in mind................................................................................ 68 Heading......................................................................................................................... 69



Introduction: Why these notes

To develop the habit of taking good notes. Help me in visualizing the concept and the relationship with other concepts. Help me pass the exam Help me to overcome my fear of taking exam.



Chapter 1: Declaration and Access Control

Identifiers and JavaBeans

Legal Identifiers

Legal Identifier

First char 1. $ 2. _ 3. Letter

Subsequent chars 1. $ 2. _ 3. Letter 4. Number

Size

Unlimited

Restrictions

Java Keywords cannot be used

Case

Case sensitive

Valid JavaBean method signatures

get set isMyStatus() public void addMyListener(MyListener m) public void removeMyListener(MyListener m)



Declare Classes

Access modifiers for a class

private, default, protected and public,

Non-access modifiers for a class

strictfp

final

abstract

Strictfp applicable to:

Only applicable to:

1. Class 2. Method



Declare Interfaces

Interface methods:

How to think about methods in an interface?

All the methods in an interface are abstract. No non-abstract method in an interface

Methods of an interface are public and abstract.

Interface constants:

Constants in an interface

Public, static and final (in any combination)

Static methods in an interface?

No not allowed

Restrictions for the methods in an interface

Cannot be

1. final 2. strictfp 3. native

Extends and interfaces

Interfaces can extend other interfaces. Only interfaces.

An Interface can extend multiple interfaces. Weird it is mentioned in page 123

Inplement and interface

No implementing of other interfaces or classes

Can a interface method be protected?

No. It can only be public.



Declare Interface Constants

One key rule to remember

Constants in an interface are: public static and final

Declare Class Members

final arguments

Final arguments cannot be modifying the argument in the method.

What modifiers cannot go together.

Abstract and static cannot go together.

Why? Abstract needs an object, static means no object is necessart.

The intention can be conflicting if you use both abstract and static together.

Other method modifiers

synchronized - Threads strictfp - floating point, scientific computing native - integrating legacy code.



Variable args

Key rules

Only 1 argument as var args

Last argument

Declared like the following: int x

not like the following: int x

How many arguments can be passed to a var arg variable?

0 or more.

Constructors

Return type of a constructor

Nothing.

What happens if a return type is specified

If a return type is specified, it becomes a method.



Modifiers allowed only on methods

Modifiers allowed only for methods

1. abstract 2. synchronized 3. strictfp 4. native

Modifiers allowed only on instant variables

Only for the instance variables

1. transient 2. volatile

Modifiers allowed only on local variables

Only for local variables

1. final



Transient variables

Transient means

Please do not serialize this variable.

Volatile variables

Volatile means

In a multi-threading situation, please do not perform any caching for there variables.



Enums

2 places enums can be declared

Outside a class Inside a class

Can be declared in a method?

No Never

Access modifier rule when declared outside a class

Not private or protected.

Public or default allowed

Semicolon rule

Semicolon at the end of an enum declaration is optional

Method to get all the enums as an array?

values() returns the enum values as an array

Does the values() method return the enums in the order by which they are declared?

Yes

Block and overriding for individual enum

You can start a block and override specific methods for that particular enum.



Chapter 2: Object Orientation

Polymorphism

Extends rule

A class can extend only one class.

Abstract methods requirement

Declaration should end with a semi colon.

Overriding and Overloading

Overriding

Some rules

Arguments must be the same

Return type must be the same or must be a subtype

access level can be less restrictive

overriding method can throw any number of unchecked exceptions.

Checked exceptions must be narrower or fewer.

Can you override methods that are final or static

No. Final means you cannot override it. Compiler Error If it is private in the super class, then you can redefine it.

Static If the subclass has a method by the same name, it is called redefining, not overriding. No compiler error.



Overloading

Some rules

Must Change Arguments must change

Optional 1. Return type 2. Access modifier 3. Throw new or broader exceptions

See Page 115



Reference variable casting

Compiler while casting

The compiler makes sure that the two types belong to the same inheritance tree

Animal animal = new Animal() Dog dog = (Dog) animal;

The above code, compiles but fails at runtime.

Downcast

You want to call a specific method.

How to think about downcast

Watch out for this point.

(I was not aware of this before)

The compiler checks to see if the class is in the same inheritance hierarchy and then allows it.

If the compiler knows for sure it is not the same type, it will give an error.

Downcast in not that safe

Animal animal = new Animal(); Tiger tiger = new Tiger(); tiger = new Animal();

This above code give an compile error. tiger reference cannot take an Animal object.

If we provide a cast like: tiger = (Tiger)new Animal();

The code compiles, but at runtime, gives a class cast exception.

Casting across inheritance tree

What happens when you cast say an Animal to a String.

Example: Animal a = new Animal();



String name = (String) a;

You will get a compiler error.

How to think about upcasting?

It is safe. You can do it explicitly or implicitly.

Legal Return Type

Overriding methods return type

The return type can be a subtype of the overridden method.

Covariant what is it

Need to do some research.

Within the bounds of covariant returns.

Overloading requirements

Just changing the return type is not enough.

The parameters should change for overloading to work.

Valid return values

On Page 128 and 129 there are 6 rules.

In a method with an object reference return type, you can return any object type that can be implicitly cast to the declared return type.



Constructors and Initialization

Default constructor

If you dont provide a constructor, the compiler creates a default constructor.

Syntax of a default constructor

A constructor with no arguments.

Scenario in which the default constructor will not be created

If a non default constructor is provided, default constructor will not be created by the compiler.

First statements in a constructor

this() or super().

this() or super can only occur as the first line in the constructor, if used other than the first statement, you will get a compiler error.

Prerequisite for calling an instance method or access an instance variable from the constructor

The call to super() must complete.

Can you call the constructor from other methods?

No, constructors can be called from other constructors only.

Key Rule for constructors

The first line in a constructor must be a call to super() or a call to this(). No exceptions.

If the constructor does not have this() or super() what will the compiler put in?

super();



If you dont put a this or super as the first line, what will the compiler insert?

super()

The super class does not have a no arg constructor, what the rule that the subclass constructor has to follow.

The first line of the constructor will have to call the super class constructor with the proper arguments.

When constructors are chained, and you dont call super() what will happen.

KaBoom the stack will explode.

Can you have this() and super() in the same method.

How can you? You cannot.

Statics

Coupling and cohesion



Chapter 3: Assignments

Literals, Assignments and Variables

Passing variables into methods

Array declaration, Construction and Initialization

Initialization

When is the instance initialization block run?

Immediately after the call to super().

Before the construction of the current object begins.

What exception is thrown from a static init block?

ExceptionInInitializationError



Wrapper classes and Boxing

Are Wrapper classes immutable?

Yes

Given a string representation of a value, how do you get its primitive?

parseXXXX()

Parse gets a primitive

Given a string representation, how do you get the wrapper object

valueOf()

valueOf() you get a wrapper Object

Given a wrapper how do you convert into other primitives?

xxxxValue();

From wrappers, how do you get the Hex and the Octal version of the value?

toHexString() and toOctalString();

Boxing, == and equals()

Boolean

== return ture if the values are same



Byte

== return ture if the values are same

Character

== return true if the values are within \u0000 to \u007f. Else false

Short and Integer

-128 to +127 return true, else false.

Overloading

Order for matching a method for overloading

It is done in the following order.

Primitive widening

Auto Boxing

Var args

Garbage Collection

How to invoke the garbage collector

System.gc();

finalize() method(?)



Chapter 4: Operators

Assignment Operators

instanceof Comparison

RACL

Relational Operators

Arithmetic Operators

Conditional Operators

Logical Operators

TODO



Chapter 5: Flow control, Exceptions and Assertions

If statements

Switch statements

Valid data types allowed for a switch statement.

Char, byte, short, int or enum.

What data types are not allowed.

Long, float and double.

Requirement for the case constant

Case constant must evaluate to the same type as the switch expression.

The case constant must be a compile time constant

If the case constant is final is that enough.

No, if it is a final variable, it must have been initialized.

Case constant ranges

The case constant has to fall within the valid range of the switch expression data type.

Can you have more than one label using the same value.

No. The compiler will get confused.

Are boxing allowed for case constants?

Yes.



Loops and Iterators

Basic for Loop

Key requirement for condition.

You can have only 1 test expression

Last two things that happen in a for loop.

1. Iteration expression 2. evaluating the conditional expression

Enhanced for Loop (for arrays)

Key requirement for the declaration section of the enhanced for loop

Declaration should declare a new variable. Cannot use an old variable.

Key requirement for the expression section

Expression needs to be a array.

Labeled Statements

Can you label any statement?

Yes



Exception handling

A return statement is encountered, will the finally block execute

Yes. Definitely. (This is a point I learnt from this book)

Will finally always execute?

Yes, whether an exception happens or not.

In a try-catch-finally code block, which sections are optional.

Either one of the catch or finally blocks are required.

You cannot ignore both catch or finally blocks.

Unwinding the stack

Order of the exception handlers.

Specific Exception handlers come first

Generic Exception handlers come last.

Else it will result in a compile error.

What are unchecked exceptions?

Error and RuntimeExceptions

Common exception

See page 382



Assertion

What exception is thrown when an assertion fails

AssertionError

Example of assertions

assert( x < y );

assert( x < y ) : Message goes here..;

Requirement for the expression after :

It has to be capable of being converted to a String.

Void cannot be converted to a String. So a method returning void cannot come in the part.

JVM Command line switches

-ea -enableassertions

-da -disableassertions

Selective enabling

-ea:com.packagename.com



Chapter 6: String, I/O, Formatting and Parsing

String, StringBuffer and StringBuilder

File and I/O

Serialization

Dates, Numbers and Currency

Parsing, Tokenizing and Formatting

TODO



Chapter 7: Collections

hashCode() and equals()

Overriding equals

Strings and Wrappers override equals and make good hashing keys

Equals contract.

Reflexive

Symmetric

Transitive

Consistent

Null

equals() and hashCode()

If equals() of 2 objects is true, hashCode() must be true

If hashCodes() are not equal, then equals() has to be false.

instanceof operator

In the equals() method, use instanceof to see if the class is proper class.

Transient variables and equals()

When overriding equals(), do not use transient variables for comparision.

Why? The explanation is a TODO item



Collections

Operations on a collection

What are the operations?

1. Add

2. Remove

3. Retrieve

4. Find/Search an object or a group of objects in the collections

5. Iterate through a collection.

Key Interfaces and Concrete classes

Key Interfaces

Collection List Set

NavigableSet SortedSet

Queue Map

NavigableMap SortedMap

Key Concrete Classes

Maps HashMap Hashtable TreeMap LinkedHashMap

Sets HashSet LinkedHashSet TreeSet



Lists ArrayList Vector LinkedList

Queues PriorityQueue

Utilities Collections Arrays

Ordered and Sorted

Difference

Ordered is different from sorting

But a sorted collection is in a natural ordered.

Sorted collection is both sorted and ordered.



List interface

Main concept of a list

Cares about the index.

Has methods related to the index

Get(int index) indexOf(Object o) add(int index, Object obj)

You want a fast iteration? Which list class do you use?

ArrayList

Main difference between ArrayList and Vector

Vector methods are synchronized, will have a performance hit.

You want a fast insertion and deletion, which list class you would want to use?

LinkedList



Set Interface

Main point about a set

A set does not allow duplicates

Main way to think about set classes

HashSet and TreeSet

And HashSet has a cousin called LinkedHashSet

HashSet and hashCode() relation

The more efficient is the hashCode() method, the efficient is the access performance with HashSet

LinkedHashSet and HashSet difference

HashSet objects are not ordered.

LinkedHashSet the insertion order is maintained.

Tree Set

Ascending Order by natural order.

TreeSet implements NavigableSet.

NavigableSet implements SortedSet



Map interface

Main point about a set

Cares about unique identifiers

Relies on the equals() method

HashMap

Unsorted and unordered map

More efficient your hashCode(), the more efficient is the access performance.

Null and HashMap

HashMap allows one null key

Allows multiple null values.

Hashtable

Synchronized version of HashMap.

Does not allow anything that is null.

LinkedHashMap

Maintains insertion order

Slower to add and remove

Faster iteration

TreeMap

Sorted Order.



Queue Interface

Main point

FIFO first in, first out

PriorityQueue

?

Sorting Collections and Arrays

How do you sort a collection

Collections.sort( list );

Comparable Interface

Requirement for the sort method

Sort() method takes a list

The objects in the list must implement an interface called Comparable.

Comparable interface

You need to implement a single method called compareTo();

Example:

Int x = thisObject.compareTo( anotherObject );

-ve thisObject < anotherObject 0 same +ve thisObject > anotherObject

Implementation point

You need to have access to the source code in order to implement the compareTo() method



Key Method

compareTo()

Comparator Interface

What can you do with this interface

If you dont have access to the source code of the class that you want to sort, you can use the Comparator to do the compare operation.

Key method

compare()

Key Sort methods

Collections

Collections.sort(list) Collections.sort(list, comparator)

Arrays

Arrays.sort( array ) Arrays.sort( array, comparator)

Searching Arrays and Collections

Search method

binarySearch()

Successful search

Return the index of the element being searched

0 and +ve indicate successful search

-ve indicates unsuccessful search



Unsuccessful search

0 -> -1 1 -> -2 2 -> -3

As 0 is a valid searh index.

Collections state for searching

Sorted.

If not sorted, results unpredictable.

Comparator requirements.

If the sort is not done by the comparator, then the binarySearch() should not use the comparator.

If the comparator is used to sort the collection, then pass the comparator as an argument to the binarySearch() method

Converting Arrays to Lists to Arrays

List and sets

toArray()

Arrays

asList() Backed collections



Using Lists

Iterator

Iterator i3 = d.iterator();

While( i3.hasNext() ) { Dog d2 = I3.next(); //No need for the cast here as we are using generics }

Using Sets

What does the add method return

True/False. If there is a duplicate object already present in the collection, then add() would return false.

Using Maps

Using Navigating (Searching) TreeSets and TreeMaps

Key methods

Lower() less than Floor() less than or equal Higher() greater than Ceiling() greater than or equal

LowerKey() FloorKey() HigherKey() CeilingKey()



Polling

Key Idea

Peek returns a copy (?) to verify this

Poll return and removes the entry from the collection.

Key methods

pollFirst() pollFirstEntry() retrieves and removes the key value pair.

pollLast() pollLastEntry()

Descending Order

Key methods

TreeSet.descendingSet()

TreeMap.descendingMap()

Backed Collection

TODO

Using PriorityQueue class TODO

Method overview for Arrays and Collections TODO

Method overview for List, Set, Map and Queue TODO



Chapter 7: Generics

Generics and Collections

Before Java 5

Short coming of Collections before Java 5

Not type safe.

No way to check at compile time as to what type of object is being added to the collection.

A non-generic collection can hold any kind of object!

Advantage of Generics with collections.

1. Ensures that the exact type is added to the collection. 2. You dont have to do an explicit cast when retrieving the

object from the collection.



Generics and Legacy Code

Example

List myList = new ArrayList();


Public List changeStrings(ArrayList s) { }

Public List changeStrings(ArrayList s) { }

Will the generic code work with the legacy code?

Yes. No problem.

Will get a compiler warning.

Explain Type Erasure

After the code compiles, all the Generic type information is lost. The byte codes generated before and after generics were introduced, is exactly the same.

Generics when do they come into play.

Only at compile time.



Polymorphism and Generics

Base type


This works just fine.

Generic Type


This does not work.

Compare Generic and Array example

Object[] myArray = new JButton[3]; //Works fine

List list = new ArrayList // Does NOT work

Generic Methods

Base type


This works just fine.

What is the mechanism by which you tell the compiler that you will not be adding anything into the collection, but still let the method take in generic subtypes?

?

Public void addAnimal(List animals)

Public void addAnimal(List



Do you have an equivalent for extends?

Yes, with the super keyword.

we can have the following

public void addAnimal(? super Dog) { }

Which means, accept any class which is a supertype of Dog.

Key rule for using the ?

While declaring - It cannot be used in the RHS while creating the object

Reference declaration

When creating a typed collection you cannot use the ?

What is wrong with this statement?

The following does not compile:

List foo = new ArrayList



carRental object will have to deal with the Car type only

Can we use more than 1 parameter type

Yes.

Public class RentalGeneric { //T is the parameter type }

Can the ? wildcard be used for classes?

NO

The following is not legal:

Public class NumberHolder



Chapter 8: Inner Classes

Overview and Key Points

What is the relationship the inner class shares with the outer class?

Inner class instance has access to all members of the outer class instance even Private member of the outer class

4 Types of inner classes

Regular Inner class

Method local Inner class

Anonymous inner class

Static Inner class



Regular Inner class

Example

Class MyOuter {

Private int x = 7; Class MyInner { Public void method() { System.out.println( X = + x ); } } }

Requitement for Instantiating an Inner class

You need to have an instance of the outer class.

2 places an inner class instance is instantiated

Within the outer class There is an implicit this reference to the outer instance.

Outside the outer class

Example for instantiating an inner class from the outside the outer class

MyOuter outer = new MyOuter(); MyOuter.MyInner inner = outer.new MyInner();

You need to have the outer instance, otherwise you cannot instantiate the inner class.

Weird point

Instantiating an inner class is the only scenario in which youll invoke new on an instance.



In other places, you invoke new to create a new instance.

How do you reference the outer class instance?

MyOuter.this

MyOuter is the class name.

Access modifiers for inner class

Since inner class is like a member of the outer class all the modifiers applicable for member can be applied for the inner class as well:

final

abstract

public

private

protected

static

strictfp

Method Local inner class

How do you define method local class and

Where can you instantiate it?

In a method.

Only after you declare the class, can you instantiate an object of the method local inner class



What method local inner object can and cant do?

The instance can be created only in the method in which the inner class is declared.

The inner class can access the outer class instance variables and methods.

It cannot access the local variables of the method in which it is defined.

It can access the local variables which are marked final and initialized(?)

It the method is static, then the inner class can only access the static members of the outer class. as there is no this reference associated with a static method.



Anonymous inner class

Plain old version 1

Class Popcorn { Public void pop() { } }

Class Food { Popcorn p = new Popcorn() { Public void pop() { } }; }

Plain Old Version 2

interface Popcorn { Public void pop() { } }

Class Food { Popcorn p = new Popcorn() { Public void pop() { } }; }

Key point to remember

Through anonymous inner classes, we are creating an instance of a new subclass (we dont know its name).



Static Nested class

Example

Class MyOuter { Static class MyInner { } }

Instantiation:

MyOuter.MyInner inner = new MyOuter.MyInner();

Key point

It can only access the static members of the outer class.



Chapter 9: Threads

Overview

What sections are in this chapter?

1. Start new Threads 2. Recognize thread states and transitions.

a. By what methods the transitions take place. 3. Object Locking 4. Write code to use wait, notify and notifyAll

Can you explain why this order exists?

1. First you need to know how to create a thread object and know how to start a thread.

2. A thread of execution will be going through a few stages like the following:

1) newly created thread. 2) Runnable eligible for running not waiting. 3) Running the thread is executing at this stage. 4) Blocked/waiting the thread is put on hold. 5) Dead the thread is finished executing.

3. Need for Object locking the underlying mechanism for blocking threads to allow for atomic operations (Rewrite this line). The keywords the language provides for enabling this functionality.

4. Inter-thread communication the methods provided on the Object itself.



Distinguish Thread and Thread of Execution

What is a java.lang.Thread

Thread is a object. It is created on the Heap.

Thread of execution

Thread of execution is a lightweight process, that has its own call stack.

The Thread class

What does a thread mean to you?

A thread means: a thread of execution that has its own call stack

What is one of the most important rule to keep in mind with regards to threads?

Very little is guaranteed.

What is the name of the main thread in Java

It is called main.

What are the important methods on the Thread object not on the object itself.

start() yield() sleep() run()

In which method does the action happen?

run() method.

The thread of execution will always begin by invoking the run() method.



Thread and the Runnable interface

What are the 2 ways by which you can define and instantiate a thread.

1. Extend the java.lang.Thread class. 2. Implement the Runnable interface.

Can you explain as to why extending the Thread class is not a good design.

SubClassing means that you are making a special class of the Thread class. This is very unlikely.

What are the 2 benefits of implementing the Runnable interface.

By implementing the Runnable interface, you can make the class be a part of a thread and at the same time you are free to extend other class.



Defining Threads

Of the 2 ways to create threads, which is preferable.

Implementing the Runnable interface

What happens if you overload the run() method like run(String s)? Will the overloaded method be called?

No.

Only the run() method will be invoked from the new thread of execution.

What are the 2 steps to define code to run in a separate thread by extending the java.lang.Thread class

1. Extend the java.lang.Thread class 2. Override the run() method

Example: public class Thread1 extends java.lang.Thread {

public void run() { System.out.println(Here you go.); }

}

How do you define behavior to be run in a separate thread using the Runnable interface.

Make the class implement the runnable interface and define the public void run() method.

Example: public class Runnable2 implements Runnable {

public void run() { System.out.println(Here you go.); }

}



Instantiating Threads

How do you instantiate a subclass of the java.lang.Thread class

Thread1 t1 = new Thread1()

How do you instantiate a thread when a class has implemented the Runnable interface?

Pass the instance of the class which implements Runnable as the argument to the constructor when creating an object of the Thread class.

Runnable2 r = new Runnable2(); Thread t2 = new Thread(r);

Overloaded constructors to keep in mind

Thread() Thread(String name)

Thread(Runnable r) Thread(Runnable r,String s)

When Thread object is created, is a thread started?

No. The Thread object has been created.

When is a new thread created? When is a new call stack created?

Call the start() method on the Thread class.



Starting Threads

What happens when a start() method is called on a Thread object?

A new Thread of execution starts a new call stack is created. The newly created thread in now in the Runnable state. When given a chance by the JVM scheduler, the run() method is called.

Once the run() method completes, what is the state of the thread?

Dead state.

Getting the name of the Thread

What is the method to get the name of the executing Thread?

getName()

On which class is this method declared?

Thread class.

How do you set the name of the Thread.

1. Using the constructor Thread(String name) or Thread(Runnable r, String name).

2. Calling the setName() on the Thread object.

How does an instance of a Runnable know the Thread name? How do you get the reference to the currently executing thread?

static Thread.currentThread() will return the reference to the currently executing thread.

Do threads get a default name?

Yes. If you dont provide a name, it may be given a default name.



Starting multiple threads

Can you start multiple threads?

Yes of course

Is the order of the threads that are being executed guaranteed?

No

Once a thread had been started, can you start it again

No. Never.

What will happen if you start a thread the second time?

An IllegalThreadStateException will be thrown at runtime.

Thread Scheduler

Who decides which thread should be run at any given moment

The Thread Scheduler

What does it do? It decides which thread is eligible to run.

It make sure that only 1 thread is run.

There will be only 1 currently running thread. What are some of the keywords related to scheduling?

Eligible Runnable state Currently running thread Queue behavior Runnable pool Influencing the scheduler



Can you control the scheduler?

No.

Can you influence the scheduler and if so how?

Yes.

By the following methods:

Method of the Thread class public static void sleep(Long millis) throws InterruptedException public static void yield() public final void join() throws InterruptedException public final void setPriority(int newPriority)

Methods of the Object class public final void wait() throws InterruptedException public final void notify() public final void notifyAll()

Thread States and Transitions

2 states already seen

New Runnable Dead

What other states can a thread be?

Running Waiting/Blocking/Sleeping

What is the common thing about waiting/blocking/sleeping

The thread is not eligible to run.

Not runnable.



What does Blocked mean?

Thread may be waiting for a resource or an objects lock.

Events that send a blocked thread to runnable state?

The event that sends the thread to runnable is

The availability of the resource Of the object lock is suddenly available.

What does Sleeping mean?

The code puts the Thread to sleep by calling the sleep() method.

What event makes the thread move from the sleep state to the runnable state

Once the sleep time expires, the thread wakes up and is now put in the runnable state.

What does wait() mean?

The thread waits for a notification from another thread.

Can one thread block another thread?

No. The way threading is defined is that 1 thread does not have the knowledge about other threads. It can block the current thread, never an another thread.

Suspend() stop() and resume() does this but these methods have been deprecated.

Sleeping

Key method

Thread.sleep();

What does interruption mean

When a thread is sleeping, it can be interrupted and it will then throws the InterruptedException.



What do I have to remember about this method?

Sleep() is a static method. You cannot put another thread to sleep. Only the currently executing thread.

Thread Priorities

What is the method used to set the priority?

setPriority()

Pre defined priorities

MIN_PRIORITY NORM_ PRIORITY MAX_PRIORITY

MIN 1 NORM 5 MAX 10

Yielding from a thread

Key method

Yield()

What does calling yield() do?

It allows the thread scheduler to make other thread of the same priority run

Is this behavior guaranteed?

No



Willing calling yield make the thread go to the blocked/waiting/sleeping State?

No. It may takes it to the runnable state at the most and this also is not guaranteed

Join() method

Key Method

Join()

Is this method static?

No. Why? This method is called on a thread object.

What does t.join() mean?

The currently executing thread is joined to the end of the thread on this the join() method is called.

The current thread is blocked.

t.join() means:

Join me (the current thread) to the end of t, so that t must finish before I (the current thread) can run again.

t.join(5000) means

wait until thread t is done, but if it takes longer that 5,000 milliseconds, then stop waiting and become runnable anyway



Synchronization Code

Explain Race Condition.

Multiple threads can access the same resource and corrupt data.

Before a method/operation completes, another thread races in and reads/updates the data.

What are 2 things to do to protect the data?

1. Mark the variable private 2. Synchronize the code that modifies the variable.

How does synchronization work?

With locks.

Acquiring locks are known as

1. Getting the lock 2. Locking the object 3. Locking on the object 4. Synchronizing on the object

What can you call the object whose lock we are acquiring?

Monitor

Lock and the monitor are they different.

Technically yes.

But the two can be used interchangeably



Key points about locking and synchronization

Only methods of or code blocks can be synchronized.

How many locks does an object have?

One

Do you need to synchronize all the methods of a class?

No, only the methods that access the mutable state needs to be synchronized.

How many threads execute a synchronized method at a given time?

Only 1 method can be executing an synchronized method at a given time.

Only after a thread completes executing a method, will any other waiting thread be given the opportunity to run this method.

If a thread goes to sleep, does it release its locks?

No. It does not release them.

Can a thread acquire multiple locks?

Yes.

Can a thread acquire the same lock twice?

Yes. If it already has it, no problem.



Synchronizing static methods

Can you synchronize static methods?

Which lock is acquired for static methods?

The lock on the methods java.lang.Class instance.

Public synchronize static method() { Return 3;

}

Public synchronize static method() { Synchronize(MyClass.class) {

Return 3; }

}

Equivalent to:

Public synchronize static method() { Class c1 = Class.forName(MyClass) Synchronize(c1) {

Return 3; }

}



More Synchronization points

How do you describe as to what a thread that is waiting on a synchronized method doing?

Blocked on the objects lock.

Goes into a waiting pool for that particular object.

Once the lock on the object is released (a thread which holds the lock relinquishes it after finishing the synchronized method), this thread may get the lock and only then it will go into the runnable state and then into the running state.

What is the necessary condition for a synchronized method or block to block synchronized method or block.

Only if the two are lock on the same object.

Key goal of synchronization

Threads working with the same data need to go one at a time.

Comment on static synchronized method and a non-static synchronized method.

Static synchronized method and a non-static synchronized method will not block each other.

If you access a static field using a non-static method, two threads might invoke that method using two different this instances.

A simple rule to make a class thread safe -

Methods that access changeable fields need to be synchronized.



Thread interaction

Important point about wait/notify

Wait(), notify() and notifyAll() must be called from a synchronized context.

A thread cant invoke a wait or notify method on an object unless it owns that objects lock.

If the wait() method is called on an object whose lock is not held by the currently executing thread, IllegalMonitorStateException is thrown.

IllegalMonitorStateException is not a checked exception.

Exceptions related to threads

Important exceptions

IllegalThreadStateException not a checked exception IllegalMonitorStateException not a checked exception

InterruptedException checked exception sleep() - static join() - non static wait() non static



Summary - Points to keep in mind

Final methods

wait notify notifyAll join() setPriority()

This means that a programmer cannot override the default functionality

Static methods

sleep() yield()

Since these are not final can a programmer override these methods.

When you see Obj.syncMethod() What do you think

The syncMethod() is synchronized on the obj.

InterruptedException

Anytime you see sleep(), wait() and join() you make sure that the InterruptedException is caught. Else, the code will not compile.

Calling start() on a dead thread

Will throw the IllegalThreadStateException at runtime

Calling wait on an object whose lock is not held

Will throw the IllegalMonitorStateException at runtime

Calling wait() from outside a

TODO: to find out as to what will happen.



synchronize block?

Key classes, methods and exceptions

java.lang.Thread start() (IllegalThreadStateException)

java.lang.Runnable Public void run()

public static void Sleep(long millis) throws InterruptedException public static void yield() public final void join() throws InterruptedException public final void setPriority(int newPriority)

public final void wait() throws InterruptedException (IllegalMonitorStateException) public final void notify() public final void notifyAll()

Runtime Exception, not checked exception

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Documents

SCJP StudyGuide