Download Chapter 6 Objects and Classes

Chapter 7 Objects and Classes
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1
Motivations
After learning the preceding chapters, you are capable of
solving many programming problems using tools such
as selections, loops, methods, and arrays. However,
these Java features are not sufficient for developing
graphical user interfaces (GUI) and large scale
software systems. Suppose you want to develop a
graphical user interface as shown below. How do you
program it?
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
2
Objectives















To describe objects and classes, and use classes to model objects (§8.2).
To use UML graphical notations to describe classes and objects (§8.2).
To demonstrate defining classes and creating objects (§8.3).
To create objects using constructors (§8.4).
To access objects via object reference variables (§8.5).
To define a reference variable using a reference type (§8.5.1).
To access an object’s data and methods using the object member access operator
(.) (§8.5.2).
To define data fields of reference types and assign default values for an object’s
data fields (§8.5.3).
To distinguish between object reference variables and primitive data type
variables (§8.5.4).
To use classes Date, Random, and JFrame in the Java library (§8.6).
To distinguish between instance and static variables and methods (§8.7).
To define private data fields with appropriate get and set methods (§8.8).
To encapsulate data fields to make classes easy to maintain (§8.9).
To develop methods with object arguments and differentiate between primitivetype arguments and object-type arguments (§8.10).
To store and process objects in arrays (§8.11).
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3
OO Programming Concepts
Object-oriented programming (OOP) involves
programming using objects.
• An object represents an entity in the real world that
can be distinctly identified.
• For example, a student, a desk, a circle, a button,
and even a loan can all be viewed as objects.
• An object has a unique identity, state, and behaviors.
• The state of an object consists of a set of data fields
(also known as properties) with their current values. The
behavior of an object is defined by a set of methods.
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4
Classes
• Classes are constructs that define objects of the
same type.
• A Java class uses variables to define data fields
and methods to define behaviors.
• The behavior of an object also known as its
actions which is define by methods.
• Additionally, a class provides a special type of
methods, known as constructors, which are
invoked to construct objects from the class.
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
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5
More Classes
The access level modifiers will determine whether other
classes can use a particular field or invoke a particular
method.
There are two level of access control:
• public, or package-private //top level
• public, private, protected, or package-private // member level
A class may be declared with the modifier public, which is visible to
all classes anywhere. However, if a class doe not have modifier
(public or private, or else), then, it is only visible within its own
package.
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6
Objects
An object has both a state and behavior. The state defines
the object, and the behavior defines what the object does.
Class Name: Circle
A class template
Data Fields:
radius is _______
Methods:
getArea
Circle Object 1
Circle Object 2
Circle Object 3
Data Fields:
radius is 10
Data Fields:
radius is 25
Data Fields:
radius is 125
Three objects of
the Circle class
The State of an object, know as properties or attributes is represent
as data fields.
A circle object has data fields of radius, which is the property define
circle.
A rectangle object has data fields width and height, which are
properties and characterize a rectangle.
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
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7
More Objects
• A Java class uses variable to define data
fields and methods.
• A class provides methods of special type,
know as constructors which is invoked to
create a new object.
• A constructor can performs any actions,
but they do initial action. Example:
Circle3 myCircle = new Circle3 (5.0);
class
object
constructor (keyword new that creates new object
myCirlce from Circle 3 class with the argument of 5.0)
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
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8
Constructors
The declaration for a method or a constructor
type of the arguments for that method or
constructor.
Constructors are a special kind of methods that
are invoked to construct objects.
Constructors with no parameters is referred to as
new ClassName();
a no-arg constructor.
•Constructors must have the same name as the class itself.
•Constructors do not have a return type—not even void.
•Constructors are invoked using the new
Example:
new Circle();
new Circle(5.0);
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9
Create Objects
A class provides the blueprint for objects; that you create an object
from a class.
Each statement below taken from CreateObjectDemo program
creates an object and assigns it to a variable:
Point originOne = new Point (2, 9);
//create object Point
Rectangle recOne = new rectangle
(originOne, 10, 20);
//create object Rectangle
Rectangle rectwo = new Rectangle (50, 10);
//Create object Rectangle
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
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10
Classes
class Circle {
/** The radius of this circle */
double radius = 1.0;
/** Construct a circle object */
Circle() {
}
Data field
Constructors
/** Construct a circle object */
Circle(double newRadius) {
radius = newRadius;
}
/** Return the area of this circle */
double getArea() {
return radius * radius * 3.14159;
}
Method
}
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
11
UML Class Diagram
Unified Modeling Language (UML) is an object-oriented analysis
and design language from the Object Management Group (OMG).
The constructor is denoted as
ClassName (parameterName: parameterType)
The Method is denoted as
methodName(parameterName: parameterType): retrunType
Circle
UML Class Diagram
Class name
radius: double
Data fields
Circle()
Constructors and
methods
Circle(newRadius: double)
getArea(): double
circle1: Circle
radius = 1.0
circle2: Circle
radius = 25
circle3: Circle
UML notation
for objects
radius = 125
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public class Circle3 {
/** The radius of the circle */
private double radius = 1;
/** The number of the objects created */
private static int numberOfObjects = 0;
/** Construct a circle with radius 1 */
public Circle3() {
numberOfObjects++;
}
public Circle3(double newRadius) {/** Construct a circle with a specified radius */
radius = newRadius;
numberOfObjects++;
}
public double getRadius() {/** Return radius */
return radius;
}
public void setRadius(double newRadius) { /** Set a new radius */
radius = (newRadius >= 0) ? newRadius : 0;
}
/** Return numberOfObjects */
public static int getNumberOfObjects() {
return numberOfObjects;
}
/** Return the area of this circle */
public double getArea() {
return radius * radius * Math.PI;
}
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
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}
13
1 public class TestCircle3 {
3 /** Main method */
4 public static void main(String[] args) {
5 // Create a Circle with radius 5.0
6
7 Circle3 myCircle = new Circle3(5.0);
8 System.out.println("The area of the circle of radius "
9
+ myCircle.getRadius() + " is " + myCircle.getArea());
10
12
Circle3 yourCircle = new Circle3(6.0);
13 System.out.println("The area of the yourcircle of radius "
14
+ yourCircle.getRadius() + " is " + yourCircle.getArea());
15
16
17
Circle3 hisCircle = new Circle3();
18 System.out.println("The area of the hiscircle of radius "
19
+ hisCircle.getRadius() + " is " + hisCircle.getArea());
20
21 }
22 }
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
14
1 public class TestCircle1 {
2 /** Main method */
3 public static void main(String[] args) {
4 // Create a circle with radius 5.0
5 Circle1 myCircle = new Circle1(5.0); // create new object
6 System.out.println("The area of the circle of radius "
7
+ myCircle.radius + " is " + myCircle.getArea());
9 // Create a circle with radius 1
10 Circle1 yourCircle = new Circle1(); // create new object
11 System.out.println("The area of the circle of radius "
12
+ yourCircle.radius + " is " + yourCircle.getArea());
13
14 // Modify circle radius
15 yourCircle.radius = 100; //Changes new object's with new value of 100
16 System.out.println("The area of the circle of radius "
17
+ yourCircle.radius + " is " + yourCircle.getArea());
18 }
19 }
21 // Define the circle class with two constructors
22 class Circle1 {
23 double radius;
25 /** Construct a circle with radius 1 */
26 Circle1() {
27 radius = 1.0;
28 }
29
30 /** Construct a circle with a specified radius */
31 Circle1(double newRadius) {
32 radius = newRadius;
33 }
34 /** Return the area of this circle */
36 double getArea() {
37 return radius * radius * Math.PI;
38 }
39 }
Example:
Defining Classes and
Creating Objects

Objective: Demonstrate
creating objects, accessing
data, and using methods.
TestCircle1
Run
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
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15

Example:
public class TV {
int channel = 1; // Default channel is 1
Defining Classes
int volumeLevel = 1; // Default volume level is 1
boolean on = false; // By default TV is off
and Creating
public TV() {
Objects, Accessing
}
public void turnOn() { //Turn on TV
Data and Using
on = true;
}
Methods
public void turnOff() { //Turn off TV
on = false;
}
public void setChannel(int newChannel) { // set a new channel
if (on && newChannel >= 1 && newChannel <= 120)
channel = newChannel;
}
public void setVolume(int newVolumeLevel) { // set a new volume
if (on && newVolumeLevel >= 1 && newVolumeLevel <= 7)
volumeLevel = newVolumeLevel;
}
public void channelUp() { //increase channel
if (on && channel < 120)
channel++;
}
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public class TestTV { // Main method
public static void main(String[] args) {
TV tv1 = new TV(); // create a TV
tv1.turnOn();
// turn on
tv1.setChannel(30); //set a new channel
tv1.setVolume(3); //set a new volume
TV tv2 = new TV(); //create a TV - object tv2 - TV class
tv2.turnOn();
//turn on
tv2.channelUp(); // trun on
tv2.channelUp(); //increase channel
tv2.volumeUp(); // increase volume
System.out.println("tv1's channel is " + tv1.channel //display state
+ " and volume level is " + tv1.volumeLevel);
System.out.println("tv2's channel is " + tv2.channel
+ " and volume level is " + tv2.volumeLevel);
}
}
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
17
Encapsulation
Encapsulation is one of the four fundamental OOP concepts. The
other three are inheritance (is the capability of a class to use the properties
and methods of another class), polymorphism (more than one form), and
abstraction (simplifying complex reality by modeling classes ).
Encapsulation can be described as a protective barrier that prevents
the code and data being randomly accessed by other code defined
outside the class. Access to the data and code is tightly controlled by an
interface.
Encapsulation provides a technique of making the fields in a class
private and providing access to the fields via public methods.
If a field is declared private, it cannot be accessed by anyone outside
the class, thereby hiding the fields within the class. For this reason,
encapsulation is also referred to as data hiding.
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved.
0132130807
The main benefit of encapsulation is the ability
to modify
our implemented code without breaking18the code
Benefits of Encapsulation:
• The fields of a class can be made read-only or write-only.
• A class can have total control over what is stored in its
fields.
• The users of a class do not know (like a blackbox) how
the class stores its data. A class can change the data type of
a field, and users of the class do not need to change any of
their code.
Link – example of Encapsulation program
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
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Example of
Data Field Encapsulation
Circle
The - sign indicates
private modifier
Circle3
-radius: double
The radius of this circle (default: 1.0).
-numberOfObjects: int
The number of circle objects created.
+Circle()
Constructs a default circle object.
+Circle(radius: double)
Constructs a circle object with the specified radius.
+getRadius(): double
Returns the radius of this circle.
+setRadius(radius: double): void
Sets a new radius for this circle.
+getNumberOfObject(): int
Returns the number of circle objects created.
+getArea(): double
Returns the area of this circle.
TestCircle3
Run
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
20
Declaring/Creating Objects
in a Single Step
ClassName objectRefVar = new ClassName();
Assign object reference
Create an object
Example:
Circle myCircle = new Circle();
//the variable myCirlce holds a reference
to a Circle object.
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
21
animation
Trace Code
Circle myCircle = new Circle(5.0);
Declare myCircle
SCircle yourCircle = new Circle();
yourCircle.radius = 100;
myCircle
no value
// Circle & SCircle - class
// myCircle & yourCircle– object
// new – keyword that creates object from the class Circle
// new Circle(5.0) - constructor
// (5.0)– argument(s) or instance variable value of 5.0.
//() - is null parameter, goes to default main method
parameter value.
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
22
animation
Trace Code, cont.
Circle myCircle = new Circle(5.0);
myCircle
no value
Circle yourCircle = new Circle();
: Circle
yourCircle.radius = 100;
radius: 5.0
Create a circle
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
23
animation
Trace Code, cont.
Circle myCircle = new Circle(5.0);
myCircle reference value
Circle yourCircle = new Circle();
yourCircle.radius = 100;
Assign object reference
to myCircle
: Circle
radius: 5.0
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
24
animation
Trace Code, cont.
Circle myCircle = new Circle(5.0);
myCircle reference value
Circle yourCircle = new Circle();
yourCircle.radius = 100;
: Circle
radius: 5.0
yourCircle
no value
Declare yourCircle
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25
animation
Trace Code, cont.
Circle myCircle = new Circle(5.0);
myCircle reference value
Circle yourCircle = new Circle();
: Circle
yourCircle.radius = 100;
radius: 5.0
no value
yourCircle
: Circle
Create a new
Circle object
radius: 0.0
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26
animation
Trace Code, cont.
Circle myCircle = new Circle(5.0);
myCircle reference value
Circle yourCircle = new Circle();
: Circle
yourCircle.radius = 100;
radius: 5.0
yourCircle reference value
Assign object reference
to yourCircle
: Circle
radius: 1.0
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rights reserved. 0132130807
27
animation
Trace Code, cont.
Circle myCircle = new Circle(5.0);
myCircle reference value
Circle yourCircle = new Circle();
yourCircle.radius = 100;
: Circle
radius: 5.0
yourCircle reference value
: Circle
Change radius in
yourCircle
radius: 100.0
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
28
Reference Data Fields
The data fields can be of reference types. For example,
the following Student class contains a data field name of
the String type.
public class Student {
String name; // name has default value null
int age; // age has default value 0
boolean isScienceMajor; // isScienceMajor has default value false
char gender; // c has default value '\u0000'
}
If a data field of a reference type does not reference any object, the
data field holds a special Java value, null.
Null is a literal just like true and false, Boolean type, null is a
literal for a reference type.
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Default Value for a Data Field
The default value of a data field is null for a
reference type, 0 for a numeric type, false for a
boolean type, and '\u0000' for a char type.
However, Java assigns no default value to a local
variable inside a method.
public class Test {
public static void main(String[] args) {
Student student = new Student();
/* default values of data fields name, aga, isScienceMajor, and
gender for a Student object.*/
System.out.println("name? " + student.name);
System.out.println("age? " + student.age);
System.out.println("isScienceMajor? " + student.isScienceMajor);
System.out.println("gender? " + student.gender);
}
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}
Differences between Variables of
Primitive Data Types and Object Types
Created using new Circle()
Primitive type
int i = 1
i
1
Object type
Circle c
c
reference
c: Circle
radius = 1
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31
Copying Variables of Primitive
Data Types and Object Types
Created using new Circle()
Primitive type
int i = 1
i
1
Object type
Circle c
c
reference
c: Circle
radius = 1
Object type assignment c1 = c2
Primitive type assignment i = j
Before:
Before:
After:
i
1
i
2
j
2
j
2
After:
c1
c1
c2
c2
c1: Circle
C2: Circle
c1: Circle
C2: Circle
radius = 5
radius = 9
radius = 5
radius = 9
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
32
Garbage Collection
As shown in the previous figure, after the
assignment statement c1 = c2, c1 points to
the same object referenced by c2.
The object previously referenced by c1 is
no longer referenced. This object is
known as garbage. Garbage is
automatically collected by JVM.
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33
Garbage Collection, cont
TIP: If you know that an object is no
longer needed, you can explicitly assign
null to a reference variable for the
object. The JVM will automatically
collect the space if the object is not
referenced by any variable.
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
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34
The Date Class
Java provides a system-independent encapsulation of date
and time in the java.util.Date class. You can use the Date
class to create an instance for the current date and time and
use its toString method to return the date and time as a string.
The + sign indicates
public modifer
java.util.Date
+Date()
Constructs a Date object for the current time.
+Date(elapseTime: long)
Constructs a Date object for a given time in
milliseconds elapsed since January 1, 1970, GMT.
+toString(): String
Returns a string representing the date and time.
+getTime(): long
Returns the number of milliseconds since January 1,
1970, GMT.
+setTime(elapseTime: long): void
Sets a new elapse time in the object.
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The Date Class Example
For example, the following code
java.util.Date date = new java.util.Date();
System.out.println(date.toString());
// date is an object for the class library of java.util.Date
displays a string like Sun Mar 09 13:50:19
EST 2003.
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36
The Random Class
You have used Math.random() to obtain a random double
value between 0.0 and 1.0 (excluding 1.0). A more useful
random number generator is provided in the java.util.Random
class.
java.util.Random
+Random()
Constructs a Random object with the current time as its seed.
+Random(seed: long)
Constructs a Random object with a specified seed.
+nextInt(): int
Returns a random int value.
+nextInt(n: int): int
Returns a random int value between 0 and n (exclusive).
+nextLong(): long
Returns a random long value.
+nextDouble(): double
Returns a random double value between 0.0 and 1.0 (exclusive).
+nextFloat(): float
Returns a random float value between 0.0F and 1.0F (exclusive).
+nextBoolean(): boolean
Returns a random boolean value.
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The Random Class Example
If two Random objects have the same seed, they will generate
identical sequences of numbers. For example, the following
code creates two Random objects with the same seed 3.
Random random1 = new Random(3);
System.out.print("From random1: ");
for (int i = 0; i < 10; i++)
System.out.print(random1.nextInt(1000) + " ");
Random random2 = new Random(3);
System.out.print("\nFrom random2: ");
for (int i = 0; i < 10; i++)
System.out.print(random2.nextInt(1000) + " ");
From random1: 734 660 210 581 128 202 549 564 459 961
From random2: 734 660 210 581 128 202 549 564 459 961
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Displaying GUI Components
When you develop programs to create graphical user
interfaces, you will use Java classes such as JFrame,
JButton, JRadioButton, JComboBox, and JList to
create frames, buttons, radio buttons, combo boxes,
lists, and so on. Here is an example that creates two
windows using the JFrame class.
TestFrame
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Run
39
animation
Trace Code
JFrame frame1 = new JFrame();
frame1.setTitle("Window 1");
frame1.setSize(200, 150);
frame1.setVisible(true); JFrame
frame2 = new JFrame();
frame2.setTitle("Window 2");
frame2.setSize(200, 150);
frame2.setVisible(true);
Declare, create,
and assign in one
statement
frame1 reference
: JFrame
title:
width:
height:
visible:
frame1 & frame2 – object created from Jframe class
setTtile, setSize, setVisible, setTitle – methods that set the properties
of the objects.
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
40
animation
Trace Code
JFrame frame1 = new JFrame();
frame1.setTitle("Window 1");
frame1.setSize(200, 150);
frame1.setVisible(true); JFrame
frame2 = new JFrame();
frame2.setTitle("Window 2");
frame2.setSize(200, 150);
frame2.setVisible(true);
frame1 reference
Set title property
: JFrame
title: "Window 1"
width:
height:
visible:
frame1.setTtile(“Window1”);
// setTitle method sets a title for Window 1
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41
animation
Trace Code
JFrame frame1 = new JFrame();
frame1.setTitle("Window 1");
frame1.setSize(200, 150);
frame1.setVisible(true);
JFrame frame2 = new JFrame();
frame2.setTitle("Window 2");
frame2.setSize(200, 150);
frame2.setVisible(true);
frame1 reference
: JFrame
title: "Window 1"
width: 200
height: 150
visible:
Set size property
//setSize method defined the size of 200 & 150 (width, height).
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42
animation
Trace Code
JFrame frame1 = new JFrame();
frame1.setTitle("Window 1");
frame1.setSize(200, 150);
frame1.setVisible(true);
JFrame frame2 = new JFrame();
frame2.setTitle("Window 2");
frame2.setSize(200, 150);
frame2.setVisible(true);
frame1 reference
: JFrame
title: "Window 1"
width: 200
height: 150
visible: true
Set visible
property
//setVisible method displays the window.
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43
animation
Trace Code
JFrame frame1 = new JFrame();
frame1.setTitle("Window 1");
frame1.setSize(200, 150);
frame1.setVisible(true);
JFrame frame2 = new JFrame();
frame2.setTitle("Window 2");
frame2.setSize(200, 150);
frame2.setVisible(true);
frame1 reference
: JFrame
title: "Window 1"
width: 200
height: 150
visible: true
frame2 reference
Declare, create,
and assign in one
statement
: JFrame
title:
width:
height:
visible:
frame2 – object created from Jframe class with another sets of
methods; setTitle, setSize, setVisible
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rights reserved. 0132130807
44
animation
Trace Code
JFrame frame1 = new JFrame();
frame1.setTitle("Window 1");
frame1.setSize(200, 150);
frame1.setVisible(true);
JFrame frame2 = new JFrame();
frame2.setTitle("Window 2");
frame2.setSize(200, 150);
frame2.setVisible(true);
frame1 reference
: JFrame
title: "Window 1"
width: 200
height: 150
visible: true
frame2 reference
: JFrame
title: "Window 2"
width:
height:
visible:
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
Set title property
45
animation
Trace Code
JFrame frame1 = new JFrame();
frame1.setTitle("Window 1");
frame1.setSize(200, 150);
frame1.setVisible(true);
JFrame frame2 = new JFrame();
frame2.setTitle("Window 2");
frame2.setSize(200, 150);
frame2.setVisible(true);
frame1 reference
: JFrame
title: "Window 1"
width: 200
height: 150
visible: true
frame2 reference
: JFrame
title: "Window 2"
width: 200
height: 150
visible:
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
Set size property
46
animation
Trace Code
JFrame frame1 = new JFrame();
frame1.setTitle("Window 1");
frame1.setSize(200, 150);
frame1.setVisible(true);
JFrame frame2 = new JFrame();
frame2.setTitle("Window 2");
frame2.setSize(200, 150);
frame2.setVisible(true);
frame1 reference
: JFrame
title: "Window 1"
width: 200
height: 150
visible: true
frame2 reference
: JFrame
title: "Window 2"
width: 200
height: 150
visible: true
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
Set visible
property
47
Adding GUI Components to Window
You can add graphical user interface components,
such as buttons, labels, text fields, combo boxes,
lists, and menus, to the window. The components are
defined using classes. Here is an example to create
buttons, labels, text fields, check boxes, radio
buttons, and combo boxes.
GUIComponents
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
Run
48