A Quick Intro to Java for C and C++ Programmers

Scott Hudson

HCI 631

Fall 2000

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About this lecture

Will be assuming you are a fluent C or C++ programmer.

This is going to be a quick introduction to the language.

I'm not going to teach anything much about object-oriented programming.

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What is Java

Java is a programming language designed by James Gosling at Sun – Designed to be "squirted" across a network into

a device, the primary device right now being a web browser

– Interpreted from byte codes (virtual machine approach)

– Dynamically loaded

=> very portable (except...)

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What is Java

"A pretty good object oriented language with C++ camouflage on"

Once was "The latest thing" and massively hyped

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Why was there hype about Java?

At the right place at the right time (important need on the web) – (Supposed to be) Highly portable (finally

is)– Seen by lots of companies as a way to

break the Microsoft monopoly (didn’t work)

=> huge amounts of money involved=> major corporate politics/warfare

(Sun & Netscape vs. Microsoft)

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Beyond the Hype (and loss thereof)

Not much fundamentally new here – we have seen almost all the concepts in other

languages before

But, its fairly well designed– more than can be said for a lot of things– Its better than C++ for many/most uses – It was built by someone who knew the

"landscape" of programming languages and has some taste

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Beyond the Hype (and loss thereof)

In my opinion its the best current language with wide platform support (backing of many solid implementations)

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Goals

Downloadable into a wide range of devices/platforms

Good OO language to replace C++ – Relatively small/simple/clean language– Camouflaged as C++ for sneak attack

when C++ was the dominant language C++ programmers will feel at home

Lots of checking and safety

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Major differences from C++

Generally simplified No pointers

– Actually everything is a pointer (reference), but their are no dangling pointers!

– No pointer dereferencing (where you used to say "->" you now always use ".")

Garbage collection added – Big big win

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What's been added from C (all in C++)

O-O concepts & information hiding Stronger typing Real constants (not just macros) Overloading (two functions with the

same name, but different parameter types)

New comment style ("//" to eol)

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What's been added beyond C++

More checking Exceptions (they were in C++, but..) (Limited) signature based typing

("interfaces" -- a big win) Runtime type identification (plus

other reflection capabilities) Built in threads and synchronization

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What's been added beyond C++ (cont.)

Packages (collections of classes in a namespace)

Dynamic runtime loading Strings (as objects) A root class (Object) Unicode characters (16 bit chars) 64 bit longs, bytes, booleans

– (literals true and false)

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What's been added beyond C++ (cont.)

A (small) container library (hashtables, etc.)

Special "doc" comments /** ... */ + post-processing to produce hyperlinked API documentation – html + special @directives in the

comments

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Things that are missing (but we probably won't miss):

Virtual functions (everything is virtual!)

Virtual base classes Virtual destructors Implicit constructor invocation (and

other constructor/destructor weirdness)

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Things that are missing (but we probably won't miss):

Arcane type conversion rules (and generally loose typing)

16 different meanings for const extern (now have strong load-time

checking) ->* operator (member pointers) :: (scope operator)

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Things that are missing (but we probably won't miss):

void * Null terminated strings Declaration vs. definition (always

done together, need not declare before use)

(Brain dead) multiple inheritance Templates

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Things that are missing (but we probably won't miss):

Operator functions Macros, include files, and

conditional compilation (the preprocessor)

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Other things that are missing (that we might miss): Independent functions Pointers to functions Default parameters Unsigned types Bare metal performance(?)

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So is it too slow?

Can get roughly 4x of C code performance (on many things you care about) using JIT compilers

Plenty fast for interactive apps– Spend a lot of time in native drawing code

anyway

Faster development time => more optimization time => faster code!

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More details on differences with C++ Lexical level

Designed to look almost exactly like C++ (which is almost identical to C)

Minor differences, but not worth mentioning

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More details on differences with C++ Syntactic level

Designed to look a lot like C++ – which was designed as a superset of C– but cleaned up most egregious parts

Changes: – inheritance syntax– initialization in constructors– split definition vs. declaration (and scope

operator) – protection declaration

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More details on differences with C++Control Flow All C/C++ flow control constructs

except goto– added labeled break and continue to

handle the few remaining legit uses– if/else for while do/while switch/case/break – return break continue

throw try/catch/finally to support exceptions

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Exception syntax

try { …throwable except = new my_exception();throw(except); …

} catch (my_exception ex) {… do something to recover …} catch (Exception ex) { … }finally {

… code that is always executed …}

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More details on differences with C++Control Flow (cont.) Now have real booleans

– can't test integers and pointers directly – "if (ptr)" must be "if (ptr != null)"

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Classes

The major construct in Java (like most object oriented languages) is the class.

Classes are a type definition: They define the data and operations of an object. – You can create several instances of

objects; several things with that data and those operations

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Classes

For C programmers you can think of this for now as a struct with the functions operating on the struct "inside" the struct.

For C++ programmers you can think of this as a class.

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class point_list { int x; int y; point_list next; double distance_to(point_list other) { double dx, dy; dx = (double)(x - other.x); dy = (double)(y - other.y); return Math.sqrt(dx*dx + dy*dy); } double distance_to_next() { if (next == null) return 0; else return distance_to(next); }}

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What have we got here.

Three instance variables– C++ == fields

Two methods– C++ == member functions– Note: There are no functions in Java, just

methods (notice that we had to say Math.sqrt() to get a square root).

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What have we got here.

If we had two points: pt1 and pt2, then we could compute the distance between them aspt1.distance_to(pt2)

Except... we have some problems here what are they? (p, v, c, after)

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Problem #1: self containment

Looks like a point_list contains a point_listclass point_list {...point_list next;

But, recall everything is a pointer, so this is a pointer (reference) to a point_list, so we are ok

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Like C++ Two forms of data: primitive and objectsPrimitive types:byte 8 bit signed integershort 16 bit signed integerint 32 bit signed integerlong 64 bit signed integerboolean true of falsechar 16 bit unicode characterfloat single precisiondouble double precision

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If its not a primitive its an object

Note: this includes strings & arrays Declaring instance variables of

object types actually declares references to objects of that class– null is a possible value– need to be initialized to refer to object

Referenced with “.” instead of “->”(back to problems)

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Problem #2: Visibility

Classes also provide information hiding and the default is to not allow full access to instance variables or methods.

So a better version would be...

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public class point_list { protected int x; protected int y; protected point_list next; public double distance_to(point_list other) { double dx, dy; dx = (double)(x - other.x); dy = (double)(y - other.y); return Math.sqrt(dx*dx + dy*dy); } public double distance_to_next() { if (next == null) return 0; else return distance_to(next); }}

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Specifying protection Members can be: public protected

private

<unlabeled>

Accessible to all

Accessible to classes within the same package and subclasses

Accessible only within the class itself

Inside package: protected

Outside package: private(no subclass access outside package).

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Specifying protection Classes can be: public, unlabeled, or private Almost all classes are public

(anybody can use them) Can have private or local classes

– only accessible in package– rare– can protect constructors instead

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Point of style

Its a good idea for all but the simplest classes to not have any public instance variables.– Instead have a protected variable with

read and write access methods. – This lets you change your mind later

without breaking all the code that uses yours.

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Point of style

protected int _x; public int x() {return _x;} public void set_x(int val)

{_x = val;} Seems tedious, but…

This has saved my sorry butt many times! (back to problems)

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Problem #3: no constructor

Haven't provided a way to create any useful objects of this type

Java initializes instance variables: int et al. 0

boolean false

float, double 0.0

char ‘\0’

Object null

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Initialization

You can also explicitly initialize instance vars

protected int x = 0; protected int y = 0; protected point_list next = null;

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Declaring constructors

Typically, you want provide a constructor. – If you don't provide a constructor, one

(that does nothing and takes no parameters) will be provided for you.

Constructor looks like a method with the same name as the class but no return type:

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Declaring constructors

public point_list( int xv, int yv, point_list nxt) { x = xv; y = yv; next = nxt; }

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Declaring multiple constructors

Often want to provide several constructors– Java does not do default parameters

public point_list(int xv, int yv) { this(xv,yv, null); } public point_list() { this(0,0); }

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Declaring multiple constructors

Notice that we can "chain" constructors together using "this()". – We can also invoke the super class

constructor using "super()". – Must be the first thing in the constructor.

(back to problems)

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Some misc. basics

All parameters are pass-by-value. – However, for all object types we are

passing references by values, so...

Can return at most one value. Arithmetic, etc. operations

(precedence, etc.) are the same as C/C++, except...

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Some misc. basics

Arithmetic, etc. operations are the same as C/C++, except... – Use of comma operator is limited to for

statements. – Can apply bitwise operations to booleans

(but not mixed int/boolean) to get non-shortcutting operations.

– == and != mean "refer to the same object".

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Some misc. basics

Like C++ can declare variables anywhere a statement is legal– including inside parens of for statements

Same scoping rules– scope limited to enclosing block: {... }

Don't have to declare before use.

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Hierarchical Types and Inheritance

The big wins in object-oriented programming come from hierarchical typing and inheritance.

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Hierarchical Types and Inheritance

Two things of importance:– Inheritance: the ability to derive the

implementation of something from some existing code (AKA getting someone else to do the work for you).

– Substitutability: the ability to write code that doesn't care about exactly what type it operates over (so you can substitute related but different types).

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Inheritance

In Java (like most OO languages) you can base the implementation of a class on another class. – Basically, you take what is in the other

class (the base or superclass) and then extend it to do new and/or different things.

– The new class (the derived class or subclass) preserves the API of the superclass

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Inheritance

The new class preserves the API of the superclass– so it does everything the superclass did

Consequently, an instance of a subclass can be substituted for an instance of a superclass– a variable that references the superclass

can safely be used to reference the subclass

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Declaring inheritance in class declarations Suppose we wanted to create a

colored_point_list which keeps a color value with each point...

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Declaring inheritance

public class colored_point_list extends point_list {

protected Color _pt_color; public Color pt_color() {return _pt_color;} public void set_pt_color(Color val) {_pt_color = val;}

public colored_point_list( int xv, int yv, point_list nxt, Color clr) { super(xv,yv,nxt); set_color(clr); }}

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Declaring inheritance

Note the extends clause– Gives the base class we are derived from– If there is no extends clause we

automatically inherit from Object (the Java root class).

– Java only supports single inheritance, so you only get to list one thing.

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Declaring inheritance

Note also that we call the superclass constructor (might as well let them do the work here) using the special form:"super(...);"

Like "this()" this must be the first thing in the constructor.

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Get substitutability

If we have a piece of code which declares a variable of type point_list, we can safely let it operate on an object of type colored_point_list.

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Another (different) use of “Super”

Can use “super” to refer to a method in the superclass that has been overridden by the subclass

public class grid8_point_list extends point_list { public void set_x(int val) { int rounded = (val / 8)*8 + ((val%8 >= 4)?1:0); super.set_x(rounded); } ...}

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Notes on this example

public class grid8_point_list extends point_list { public void set_x(int val) { int rounded = (val / 8)*8 + ((val%8 >= 4)?1:0); super.set_x(rounded); } ...}

Boy it’s a good thing we put that set_x() in there

We call set_x() don’t set x directly

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This Sometimes you need to refer to the

object itself in one of its methods (pass it somewhere). – Reserved word this is used for this purpose.

As in:

manager.put_in_table(this);

Note that instance variable references such as "_x" above are really shorthand for "this._x".

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Abstract classes

Its not necessary to completely define a class – This is useful to define the API to

something without requiring a particular implementation (see also interfaces).

– This is useful also if you have a lot of common functionality to put in a base class but details (i.e., implementation of particular methods) have to be provided by various subclasses.

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Abstract classes

(Must) declare the class "abstract" Declare each missing method

“abstract” and put a semicolon where the body would be.

abstract public class stack { abstract public void push(Object obj); abstract public Object pop(); abstract public boolean empty();}

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Abstract classes

Can't instantiate an abstract class (but can declare variables).

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Interfaces

Syntactically an interface declaration looks a lot like a class declaration (replace "class" with "interface"). – But, no variables and methods don't have

bodies.

Basically interfaces define an API, but no implementation

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Interfaces

Interfaces are used as a "promise". – Class can say:

"implements”interface_name" after extends clause

– This means that the class promises to implement the API of the interface.

– It doesn't say anything about how its going to do it, just that it will

– The compiler checks that it does.

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Interfaces

Can inherit from (“implement”) multiple interfaces.

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Substitutability for interfaces Can declare variables and parameters

with interface types – Variable can then refer to object of any type

that implements the interface Has promised to implement the API (&

compiler has checked) so this is completely type safe!

– Gives you a lot of flexibility to reimplement, etc.

– Advice: use this whenever you can

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Packages

Classes are organized into collections of related things– what constitutes one package is entirely

up to you.

Each source file belongs to a particular package – listed at the top with the package

declaration: package sub_arctic.lib;

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Packages

Note: its possible to leave the package declaration out which indicates the special "unnamed package”– don't do this! it just gets confusing to

everyone concerned

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Packages Package names have multiple parts

(‘subpackages”) separated by "." – java.awt and java.awt.image

But, these don't have a special relationship between them

Packages are primarily to segregate the name space– class names must be unique within the

package, but not across all packages

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Packages

Also have an effect on protection rules – recall: protected members can be

accessed by classes in same package

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Imports

You can use package names to disambiguate types.

Recall we had an example that used the class Color. – Comes from the library package java.awt

and its full name is java.awt.Color. – If you get tired of writing out the full name,

you can "import" the class: import java.awt.Color;

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Imports If you get two classes with the same

name from different packages you use full names to disambiguate.

Since compiler knows how to find packages, this eliminates the need for #includes– you can also import from compiled code

without the source!)

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Packages

java.lang.* is imported automatically.

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Static members

Instance variables belong to (have a copy in) each instance.

Can also declare variables that belong to the whole class. – Do this with static:

class a_class { protected static int count = 0; public a_class() {count++;} }

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Static members

There is only one copy of the static variable– belongs to the class (shared by all

instances).

You can also have static methods. – These can be invoked without an instance

(using the class name): Math.sqrt(3.1415d)

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Initialization of statics You can use conventional initializers

(after =) – Executed when the class is loaded

Also have special static initialization blocks (outside of methods): static {... some code ...}– Also executed when class is loaded– Java loader takes care of loading classes

you depend on first

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Final members

You can declare classes, methods, and variables final. – For variables this means they can't be

assigned to after initialization (AKA constants):

public static final double PI2 = Math.PI*Math.PI;

– For methods, this means you cannot override the method in subclasses.

– For classes this means all the methods are final.

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Final members

You can declare final variables in interfaces

Java idiom: to collect a bunch of reusable constants together put them in an interface and "implement" them in the classes where they are used.

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Strings

In Java, strings are objects of class String– not array's of char, although there are

operations to get back and forth– Special literal e.g: "abc" represents a

String object.

Character escapes such as \n similar to C.

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Strings

Strings are immutable– operations create new strings, don't

modify existing ones.

"+" is used for string concatenation, and is treated specially – Any + with at least one String operand is

treated as string concatenation – The other operand is converted to a string

(this also works for String parameters)

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Conversion to Strings

For primitive types there is a standard (and obvious) conversion

For Objects, the toString() method is called – Object provides one (prints class name

and unique id) – Or you can override

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Arrays

Arrays are also objects in Java

Can declare as "int foo[]" or "int[] foo" – both declare a variable which refers to an

array of ints (initialized to null)

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Arrays

Array size is not part of the type– that gets established by the instance

int[] foo = new int[52];...foo = new int[42]

Each array object has a read-only field: length foo.length == 42

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Arrays Note that arrays are collections of

Object references. – All set to null by default.

Can initialize arrays to refer to particular object with special form similar to C/C++ int[] foo = {3, 5, 9};Object[] bar = {"one", new Stack(), new Integer()};

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Array types

Arrays are hierarchically typed If sub is a subclass of base then sub[] is a subclass of base[]

So you could have: base[ ] base_var = new sub[42];

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Checking types at runtime

Can determine if an object qualifies as a particular type at runtime using expression: "obj instanceof aclass"

– "null instanceof aclass" is always false.

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Checking and converting types at runtime

Can convert to a sub- or super-class with a C style cast expression: "(aclass)expr" – Does runtime type check and throws an

exception if this is not a legal conversion (i.e., object being cast is not of that type or a subclass thereof).

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Output to stdout

java.lang.System has lots of useful stuff for accessing the local environment including: – System.out, System.err, and System.in

(for stdout, stderr, and stdin). – Two operations of particular interest:

System.out.print() and System.out.println() print anything convertible to a string with or without a newline.

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Threads

Threads are built into the language Concurrent threads can greatly

simplify program structure and design for a number of things

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Threads

They also make testing and debugging nearly impossible

So unless your programs all work without testing (and/or there is no reasonable way around it), I advise: Just say no!

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Threads

Note: if you feel compelled to use threads with subArctic read that section in the manual! – non-obvious things you have to do or you

will get (nasty & hidden!) concurrency bugs

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Good luck...

... and, as always, if you or any of your programming team are caught or killed, I will disavow that I ever taught you anything about Java.

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