Chapter 5© copyright Janson Industries 20141 Java Variables ▮ Types of variables ▮ Creating, modifying and displaying ▮ Comparing ▮ Converting between

Chapter 5 © copyright Janson Industries 2014 1

Java Variables

▮ Types of variables

▮ Creating, modifying and displaying

▮ Comparing

▮ Converting between types

▮ TextField

Non-graded Assg


Variables

▮ Used to access data (and objects) in classes

▮ Variables are declared. That means defining the variable’s:

▮ Identifier (name)

▮ Type

▮ Java’s basic variable types are called Primitive Data Types


▮ Two types: Primitive & Referenced

▮ Primitive variables:▮ Hold a value

▮ Type names begin with lowercase letters

▮ Referenced variables:▮ Hold the storage location of an object

▮ Type names begin with uppercase letters

Variables


Primitive Data Types

▮ boolean: true or false (default is false)▮ char: single character, use single quotes ▮ byte: 8 bit whole number (-128 to 127)▮ short: 16 bit whole number (32,767 max)▮ int: 32 bit whole number (2**31 max)▮ long: 64 bit whole number ▮ float: floating point (7 decimal place max)▮ double: double precision (15 dec places)


Variable Name Rules

▮ Must begin with a character

▮ Can be made of letters, numbers, _, or $

▮ No▮ Special characters (%, #, @, etc.)▮ Spaces▮ Reserved words (new, class, static,

etc.)

▮ Are case sensitive!!


Defining and Assigning

▮ Primitive data type then variable name

▮ int numberOfDependents;

▮ char gender, maritalStatus;

▮ A value can be assigned when the variable is declared:▮float taxRate = .28;

▮ Or a value can be assigned later:▮ numberOfDependents = 2;


Values▮ Calculated values can be assigned

using standard operators: +, -, *, /. Ex:

▮ When dividing two integers, decimal remainder truncated

▮ Dividing float and integer, integer “promoted” to float

▮ Standard order of operator precedence

int counter;counter = counter + 1; counter++; ++counter;


Values▮ Use PRINT

System.out.print(“The answer is: ”);System.out.print(answer);

▮ Assuming answer is boolean with value of true, results in:

The answer is: true

▮ Or PRINTLN. Does a carriage return so

▮ To get same result

System.out.print(“The answer is: ”);System.out.print(answer);

System.out.println(“The answer is: ” + answer);

+ concatenates

text or variables


Pre/Post Inc/Decrement▮ Standalone Pre/Post expressions (+

+counter; counter++;) do the same thing

▮ Within a larger expression they are different

▮ Result is: Counter = 1

Counter = 2

Counter = 2

Counter = 3

int counter = 1;

System.out.println("Counter = " + counter);

System.out.println("Counter = " + ++counter);

System.out.println("Counter = " + counter++);



Pre/Post Inc/Decrement

▮ To do the same thing without Pre/Post expressions requires more coding

int counter = 1;


counter = counter + 1;



counter = counter + 1;



Values▮ Some math functions are performed with

java supplied methods not operators

▮ These methods are stored in the Math class

▮

▮ Math is part of java.lang package▮No need to import or declare Math

because java.lang automatically imported

▮ To invoke a static method▮Classname.methodName(parms)

Chapter 5 © copyright Janson Industries 2014 12Look at documentation to see required parms


To raise 12 to the fourth power:

double answer; answer = Math.pow(12, 4);


Referenced Types

▮ String & StringBuffer are examples▮ StringBuffer provides more flexibility

when manipulating Strings

▮ Class name then variable name▮ String name, address, phoneNumber;▮ StringBuffer nameSB;

▮ Like primitives, you can assign a value when declaring the variable or assign a value later


Reference Types

▮ Have to create and assign an object to a reference variable

▮ Object created with the “new” keyword▮ String name = new String(“Joe”);▮ StringBuffer nameSB = new

StringBuffer(“Joe”);

▮ Alternative for String objects (not recommended)▮ String name = “Joe”;


Referenced Types

▮ You can declare a referenced variable for a class and then “instantiate” and assign the object to the variable

▮ Assuming Customer is a java class, the variable cust points to a Customer object that contains the values

Customer cust;

cust = new Customer(“Joe”, “1 Main St.”,

“Enid, OK 65654”);


For example: String

▮ If we set the value of a to Joe

▮ A String object with the text “Joe” is placed in memory and a = A1

▮ If we then set b to “Joe”, a 2nd String object with “Joe” is

placed in memory and b = B2

a = new String(“Joe”);

A B C

1 Joe

2 Joe

3

ab


For example: String▮ If we change the value of a

▮ A String object with the value “Art” is placed in memory

▮ a = C3 and the first Joe is not referenced

a = new String(“Art”);

A B C

1 Joe

2 Joe

3 Art

ab


String

▮ However if we define c as equal to a:

▮ c = C3

A B C

1 Joe

2 Joe

3 Art

abc

String c = a;


Strings▮ Are immutable (unchangeable)

▮ Assigning a new value to a String variable creates a new String object▮ It does not change any existing String object that the variable references

▮ There are now two String objects▮ One with the text “Art” the other “Joe”

String c = "Art";

c = “Joe”;


String▮ However, using the shortcut (i.e. not explicitly creating a new object) will not necessarily result in a

new object

▮ Only one object with text “Art”

▮ Two objects with text “Art”

String c = "Art";

String b = “Art”;

String c = "Art";

String b = new String(“Art”);


String▮ Even this…

▮ ..results in two objects with text “Art”

▮ Beware of the shortcut

▮ This is also why StringBuffers are better▮ Their value can be changed (more on this later)

String b = new String(“Art”);

String c = "Art";


Reference Variables

▮ If two Customer variables created as follows:

▮ How many objects are there?Customer cust1 = new Customer(“Walmart”);

Customer cust2 = new Customer(“Walmart”);


Reference Variables

▮ 2 objects exist

A B C

1 Walmart

2

3 Walmart

cust1 cust2


Reference Variables

▮ If the following statements are also executed:

▮ How many objects are there?

▮ How many objects are referenced?cust1 = new Customer(“Target”);

cust2.setCustName(“Sam’s”);


Reference Variables

▮ 3 objects exist, 2 are referenced

▮ For an alternative explanation of variables:

▮ http://www.javaranch.com/campfire/StoryCups.jsp

A B C

1 Walmart

2 Target

3 Sam’s

cust1 cust2


Null Pointer Exception

A B C

2 Target

3

cust1 cust2

Customer cust1, cust2;

cust1 = new Customer(“Target”);

cust2.setCustName(“Sam’s”);

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException

at packageName.className.methodName(fileName.java:lineNumber)

▮ Occurs when using a reference variable that is not assigned to an object (pointer is null)


Primitive Variables ▮ Defining these two integers:

▮ Results in a memory allocation of:int a = 1; int b = 2;

A B C

1 1

2 2

3


Primitives

▮ If we change the value of a

▮ The value 1 is replaced with 3

a = 3;

A B C

1 3

2 2

3


Referenced Types

▮ Proof: Printing Customer variable c would result in memory address (hash code)Customer c = new Customer();

c.setContactPerson("Joe Samaritan");

c.setContactPhone("555-3333");

c.setCustName("Kindness Foods");

c.setShipToStreet("1 Milk of St.");

c.setShipToCity("Human");

c.setShipToState("ME");

c.setShipToZip("03234");

System.out.println(c);



Why address?

▮ Because that is the value of the reference variable▮ If you wanted to see the Customers name,

use c.getName()

▮ Actually the Customer object's toString method is invoked when println executed▮ “Hey, we didn’t code a Customer.toString()

method!”▮ toString inherited from Object class▮ Object.toString returns address of an object


We’ll prove it!


In Customer, click Source, then Override/Implement Methods

Click toString and specify it as the first method

after the class variables, then click OK


Inserts new toString method that calls Object's (the superclass’) toString method


We’ll change to return customer name


When we run CustApp and print the Customer variable c

Try it: change Customer and CustApp


Another Example

ObjecttoString

PersontoString

NametoString

Person is an objectName is an objectPerson has a name

is a is a

has a


Inheritance Example

// Name.java

public class Name extends Object {

String first;

String last;

String middle;

public Name(String f, String l)

{ first = f; last = l; middle = “ “;}

public Name(String f, String l, String m)

{ first = f; last = l; middle = m; }

}

▮ We’ll create classes called Name and Person

Method Overloading


Example

// Person.java

public class Person extends Object { int age; Name name;

public Person() { age = 0; }

public Person(int a, String f, String l) { name = new Name(f, l);

age = a; }

public Person(int a, String f, String l, String m){name = new Name(f, l, m);age = a;

} }

Person “has a” name

Person “is an”object


Example

▮ PersonFrame will instantiate 3 Person objects// PersonFrame.java

import java.awt.*;

import java.awt.event.*;

public class PersonFrame extends Frame implements WindowListener, ActionListener {

Button showButton = new Button("Show");

Label outputLabel1 = new Label();



Person newPerson1 = new Person(32, "John", "Smith");

Person newPerson2 = new Person(27, "Mary", "Jones", “Jo");

Person newPerson3 = new Person(42, "John", "Public", "Q");


public PersonFrame()

{ setLayout(null); this.setSize(500, 400);

showButton.setBounds(275,300,50,25);

outputLabel1.setBounds(112,200,350,15);



add(showButton); this.setVisible(true);

showButton.addActionListener(this);

addWindowListener(this);

add(outputLabel1);add(outputLabel2);add(outputLabel3); }

public void actionPerformed(ActionEvent e) {

outputLabel1.setText("The new person is: " + newPerson1);



} Uses the default toString method to display

Example


public void windowActivated(WindowEvent e) {}

public void windowClosed(WindowEvent e) {}

public void windowClosing(WindowEvent e) {

this.dispose();

}

public void windowDeactivated(WindowEvent e) {}

public void windowDeiconified(WindowEvent e) {}

public void windowIconified(WindowEvent e) {}

public void windowOpened(WindowEvent e) {}

public static void main(String args[]) {

PersonFrame pf = new PersonFrame();

}

}

Example


Example

PersonFrame Person Name

PersonFrame has a Person object (actually 3)Person has a Name object

When we run PersonFrame and click the button...

has a

has a


Yuck!

Referenced variables hold an address not info!


Example

▮ We’ll override the inherited toString functions for both Name and Person

//Name

: : :

public String toString() {

if (middle.equals(" ")) return ( first + " " + last);

else if (middle.length() == 1) return (first + " " + middle + ". " + last);

else return (first + " " + middle + " " + last);}

//Person

: : :


return (name + ". Who is " + age + " years old." );}


Example

public void actionPerformed(ActionEvent e) {




} }

▮ Now when we try to print the newPerson variables:

▮ The Object class’ toString method (which would print out the address) is overridden by the Person toString method


Example

▮ The Person toString method tries to concatenate the Name object

▮ Fortunately, we overrode the Object class’ toString method in the Name class also

//Person

: : :


return (name + ". Who is " + age + " years old." ) ; }


Better?


Example

▮ We overrode (the inherited) Object class' toString method by defining toString methods for both Person and Name

▮ We accessed the Person and Name classes' toString methods in PersonFrame through composition (i.e. PersonFrame “has a” Person)


Example

▮ In PersonFrame (specialized type of Frame, aka a subclass of Frame) we created 3 instances/objects of type Person:

▮ In Person, we created an instance/object of type Name

Person newPerson1 = new Person(32, "John", "Smith");

Person newPerson2 = new Person(27, "Mary", "Jones", “Jo");

Person newPerson3 = new Person(42, "John", "Public", "Q");

name = new Name(f, l);


Inheritance Example

ObjecttoString

PersontoString

NametoString

Person is an Object (subclass of Object)Name is an Object (subclass of Object)

Both inherited toString

is a is a


Inheritance Example

Frame is a Component (subclass of Component)PersonFrame is a Frame (subclass of Frame)

Frame inherits setLayout, what does PersonFrame inherit?

is a

is a

ComponentsetLayout

PersonFramesetLayout getTitle

Frame setLayout getTitle


Composition Example

PersonFrame Person Name

PersonFrame has a PersonPerson has a Name

PersonFrame invokes Person's toString method

Person invokes Name's toString method

has a

has a


Comparison Operators▮ < less than▮ > greater than▮ >= greater than or equal to▮ <= less than or equal to▮ == equal▮ != not equal

▮ Order of precedence: relational then

equality

Relational

Operators

EqualityOperators


Comparison Operators

▮ Comparisons result in a boolean value (true or false)

▮ You can assign the result of a comparison to a boolean variable

boolean isOvertime = (hours > 40);


Converting Types

▮ Primitives can be converted to a larger type by simply equating/assigning it

▮ To convert from larger to smaller, use a “cast” operationlong a; int b = 1; char c = ‘2’;

a = b;

b = c;


Casting

▮ Converts a larger primitive type to a smaller primitive type

▮ General syntax:

type1Variable = (type1Name) type2Variable

long a; int b = 1; char c = ‘2’;

b = (int)a;

c = (char)b;


Primitives to Strings

long a = 1;

int b = 2;

char c = ‘c’;

String d, e, f;

d = String.valueOf(a);

e = String.valueOf(b);

f = String.valueOf(c);

▮ Casting and = do not work

▮ String class has static methods (.valueOf) that convert many primitives to strings


▮ Use the String valueOf methods to convert primitive types to Strings

▮ Notice no byte valueOf


Converting Types

▮ There are also a series of “Wrapper” classes for primitives

▮ Wrapper classes contain useful methods for manipulating primitives

byte Byteint Integerdouble Double : : : :


▮ For example, each “Wrapper” class has a toString method that does the same thing as String’s valueOf


long a = 1;

int b = 2;

char c = ‘c’;

String d, e, f;

d = Long.toString(a);

e = Integer.toString(b);

f = Character.toString(c);

Primitives to Strings


Strings to Primitives

▮ The parseXXX methods (of each Wrapper class) convert Strings to primitive types

int a;

long b;

String c = “1”;

a = Integer.parseInt(c);

b = Long.parseLong(c);


Strings to Chars

▮ Use a String method called .charAt(#)

▮ A is set to 1

char a;

String c = “123”;

a = c.charAt(0);


Converting between types

▮ Assuming int a, double b, String c

LargerPrimitiv

e

SmallerPrimitiv

eb = a;

SmallerPrimitiv

e

LargerPrimitiv

ea = (int) b;

StringPrimitive

c = String.valueOf(a);c = Double.toString(b);

Primitive

Stringa = Integer.parseInt(c);


GUI Interface Components

▮ TextField: entry field to hold and display text defined by the programmer or user

▮ Usually used in conjunction with a label that defines the text to be entered


Defining Components

▮ As always, must import classes (not shown) then define the components:

▮ Then add to the frame:

Label custNameLabel = new Label("Enter the customer name:");TextField custNameTF = new TextField(25);Label custAddrLabel = new Label("Enter the customer address:");TextField custAddrTF = new TextField(25);

add(custNameLabel); add(custNameTF); add(custAddrLabel);add(custAddrTF);


Defining Properties

▮ Of course you will want to define their properties (e.g. text) either when created

▮ Or after creating using the “setters”

Label custNameLabel = new Label("Enter the customer name:");

Label custNameLabel = new Label();: : : : : :custNameLabel.setText(“Enter the customer name:”);


Positioning▮ To set position, turn off the default layout

scheme:

▮ Specify the components size and location with either the setSize and setLocation methods or the setBounds method

setLayout(null);

custNameLabel.setSize(160,10); custNameLabel.setLocation(5,35); custNameTF.setSize(200,20); custNameTF.setLocation(162,30); custAddrLabel.setBounds(5,100,75,75);custAddrTF.setBounds(162,100,75,75);


Positioning

custNameLbl.setSize(155,10);custNameTF.setSize(200,20); custNameTF.setLocation(162,30);

custAddrLb.setBounds(5,100,75,75);

custAddrTF.setBounds(162,100,75,75);

custNameLbl.setLocation(5,35);


Defining Components▮ Lots of other commonly set properties:

▮ Alignment▮ Font style▮ Font size

setLayout(null);Label l1 = new Label("stuff"); l1.setBounds(15,50,155,20); Label l2 = new Label("stuff", Label.RIGHT); l2.setBounds(15,80,155,20); Label l3 = new Label("stuff");l3.setAlignment(Label.CENTER); l3.setBounds(15,110,155,20); add(l1); add(l2); add(l3);

How does label know “stuff” is text

not a variable?


Putting it all together in a new visual class called Sale


Change size to 300, 229 and layout to null


Defined 5 labels, 4 text fields, and 1 buttonRename, align

Enable Close button - How? (Hint: 2 steps)


When data is entered and the calc button is clicked, the total should be calculated and displayed (as above) and the text

fields blanked out


Non-graded Exercise

▮ Need three primitives to hold qty, price and cost

▮ When button clicked:

▮ Retrieve qty and price, calculate cost with a 6.5% sales tax

▮ Build result message and place in result label

▮ Blank out other text fields

▮ What method holds this logic?


Non-graded Exercise

qty = Integer.parseInt(qtyTF.getText());

price = Double.parseDouble(priceTF.getText());

cost = price * qty * 106.5;

msgLbl.setText("The cost of this " +

"transaction is: $" + cost);

custNameTF.setText("");

itemNameTF.setText("");

qtyTF.setText("");

priceTF.setText("");

Sale saleTest = new Sale();

private int qty;

private double price, cost;


Non-graded Exercise

▮ If tested now nothing appears: why?

▮ If run and press button nothing happens: why?

▮ If price and qty entered, get wrong result: why?




Non-graded Assg

▮ Export Sale.java

▮ Send as an email attachment to [email protected]


Points to Remember

▮ Primitive variables contain a value

▮ Referenced variables contain an address

▮ Use toString(), casting, and Wrapper class methods to convert between types

▮ TextFields can be used for input and output

Documents

Chapter 5© copyright Janson Industries 20141 Java Variables ▮ Types of variables ▮ Creating, modifying and displaying ▮ Comparing ▮ Converting between