This document discusses inheritance and GUI components in Java. It begins by reviewing how to create basic GUI frames and components using classes like JFrame and JOptionPane. It then explains the concepts of inheritance, where a subclass inherits attributes and methods from its superclass. Regarding GUIs, the document shows how custom GUI frames can be created by extending the JFrame class and overriding its methods. This allows creating specialized frame classes with customized functionality and properties.
3. Plan
▪ Review:
– Created a frame – JFrame class
JFrame jFrame = new JFrame();
jFrame.setTitle("GUIs are awesome!");
jFrame.setSize(400,300);
jFrame.setVisible(true);
jFrame.setDefaultCloseOperation(jFrame.EXIT_ON_CLOSE);
4. Plan
▪ Review:
– Created a frame – JFrame class
– Accessed the content pane of the frame – Container class
Container CP = jFrame.getContentPane();
5. Plan
▪ Review:
– Created a frame – JFrame class
– Accessed the content pane of the frame – Container class
– Created a pop-up window to give message to the user –
JOptionPane class
JOptionPane.showMessageDialog(jFrame, "Java is so much fun!");
JOptionPane.showMessageDialog(null, "Java is so much fun!");
6. Plan
▪ Review:
– Created a frame – JFrame class
– Accessed the content pane of the frame – Container class
– Created a pop-up window to give message to the user –
JOptionPane class
– Created a pop-up window to get data from the user -
JOptionPane
String s = JOptionPane.showInputDialog(jFrame, "What year were
you born in?");
int i = Integer.parseInt(s);
7. Plan
▪ Review:
– Created a frame – JFrame class
– Accessed the content pane of the frame – Container class
– Created a pop-up window to give message to the user –
JOptionPane class
– Created a pop-up window to get data from the user –
JOptionPane
▪ Look at components that we can add to the content
pane: JButton, JLabel, JTextField…
▪ Inheritance and Interfaces
9. GUIS
▪ Many different types of GUI objects
▪ https://web.mit.edu/6.005/www/sp14/psets/ps4/java-
6-tutorial/components.html - good reference
10. GUI components
Pushbutton.
▪ JButton – from the javax.swing package
▪ The text passed to the constructor is the text inside the
button.
▪ Also generates an action event – will get to later.
JButton okButton = new JButton("OK");
JButton cancelButton = new JButton("Cancel");
JButton acceptButton = new JButton("Accept", acceptIcon);
Name of the button Text on the button
11. GUI components
Text, images
▪ JLabel – from the javax.swing package
▪ The text passed to the constructor is the text inside the
Label.
▪ Can also specify an ImageIcon object which can contain
an image.
▪ No action event.
JLabel myText = new JLabel("Welcome to my program!");
12. GUI components
Images
▪ ImageIcon – from the java.swing package
▪ Used to hold an image.
▪ Can then be passed to constructor of JLabel or JButton
to actually show the image.
▪ No action event.
ImageIcon myIm = new ImageIcon("SpongeBob.jpg");
JLabel myLabel = new JLabel(myIm);
13. GUI components
Text input
▪ JTextField – from the java.swing package
▪ Allows user to enter a line of text.
▪ Constructor: initial text, the width in characters or both
▪ Has action event when user hits Enter key while it is active
(has blinking vertical line)
JTextField myTF = new JTextField();
JTextField myTF = new JTextField("Please enter your text here");
JTextField myTF = new JTextField("Please enter your text here", 40);
14. GUI components
Text input
▪ JTextArea – from the java.swing package
▪ Allows user to enter multiple lines of text.
▪ Constructor: Default text, rows and width
▪ Set the size of the object:
JTextArea myTA = new JTextArea();
JTextArea myTA = new JTextArea("Please enter your text here");
JTextArea myTA = new JTextArea("Please enter your text here", 5 , 30);
myTA.setColumns(30);
myTA.setRows(10);
15. GUI components
Drop down box
▪ JComboBox – from the java.swing package
▪ Gives the user options that they can choose from a
drop down box.
▪ Constructor: String array of choices
▪ Can also set the top box to be editable or not.
String[] times = {"10:00", "10:30", "11:00", "11:30", "12:00"};
JComboBox myCB = new JComboBox(times);
myCB.setEditable(true);
17. Inheritance
▪ In order to understand some GUI concepts, we have to
look at some inheritance concepts.
▪ Inheritance is not part of the syllabus of this course and
it will not be directly examined.
▪ But you need to understand it in order to understand
some GUI concepts.
18. Inheritance
▪ Let’s imagine we’ve created a class called Student
public class Student {
String name;
long studentNo;
int YearOfStudy;
Student(){
System.out.println("Student constructor.");
}
void MethodA() {
System.out.println("MethodA from Student.");
}
}
19. Inheritance
▪ Let’s imagine we’ve created a class called Student
▪ Now lets imagine that we want to make some other
classes based on this class:
– eg. PGStudent or UGStudent or PTstudent
▪ These classes will have a lot in common with Student
but they will have things that are unique to those
classes as well.
20. Inheritance
▪ We can create a class from
another class.
▪ We use the keyword:
extends
public class UGStudent extends Student{
String[] ListOfCourses;
float[] ListOfMarks;
float ReturnAveMark() {...}
}
public class PGStudent extends Student {
String Supervisor;
String DegreeType;
void SubmitThesis() {}
}
Inheritance
One class acquires the
members of another
class.
21. Inheritance
▪ The class we are inheriting from is called the superclass.
▪ The class that inherits is called the subclass.
Student
UGStudent PGStudent
Superclass
Subclass
22. Inheritance
▪ We can build a hierarchy of classes:
UniversityMember
Staff
Administrative
Academic
Technical
Student
UGStudent PGStudent
23. Inheritance
▪ Subclasses inherit the attributes and methods of the
superclasses
▪ Objects created from subclasses will have the attributes
and methods of the superclasses.
▪ They can then add their own attributes and methods.
24. Inheritance
public class Student {
String name;
long studentNo;
int YearOfStudy;
Student(){...}
void MethodA() {...}
}
public class UGStudent extends Student{
String[] ListOfCourses;
float[] ListOfMarks;
float ReturnAveMark() {...}
}
25. Inheritance
public class Student {
String name;
long studentNo;
int YearOfStudy;
Student(){...}
void MethodA() {...}
}
public class UGStudent extends Student{
String[] ListOfCourses;
float[] ListOfMarks;
float ReturnAveMark() {...}
}
This code isn’t
really here, but
these members
of Student are
available to
UGStudent.
26. Inheritance
▪ So if we have: public class UGStudent extends Student{
String[] ListOfCourses;
float[] ListOfMarks;
float ReturnAveMark() {...}
}
public class Student {
String name;
long studentNo;
int YearOfStudy;
Student(){
System.out.println("Student constructor.");
}
void MethodA() {
System.out.println("MethodA from Student.");
}
}
27. Inheritance
▪ Then in the main class, we can say:
PGStudent a = new PGStudent();
a.MethodA();
▪ Calling a method defined in the superclass from an object of the
subclass.
▪ The methods and attributes of the superclass are automatically
part of the subclass.
Object of the subclass
Method from the superclass
28. Inheritance
▪ We can also create objects of a subclass and assign it to
the superclass.
– An object declared to be of superclass A can refer to an instance
of subclass B or C.
– An object declared to be of class Student can reference to an
instance of UGStudent or PGStudent
Student a = new PGStudent();
PGStudent a = new PGStudent();
Object from the subclass
Assigned to superclass
29. Interfaces
▪ We have seen how one class extends another class.
▪ A related concept is implementing an interface.
▪ interface : like a class
– It has methods, attributes etc…
▪ …but…
– We don’t give any details i.e. no bodies to any method.
▪ So an interface just gives what a class would look
like to the outside, without giving any details.
30. Interfaces
▪ If we have an interface we can implement it.
▪ We say that a class implements an interface if it
gives the method bodies to all the methods in the
interface.
▪ We must write all the methods in an interface.
▪ We can write more methods that are not in the
interface.
31. Interfaces
▪ So if we have:
interface MyInterface {
void MethodA();
void MethodB();
}
public class MyClass implements MyInterface{
public void MethodA() {
System.out.println(“My Method A");
}
public void MethodB() {
System.out.println(“My Method B");
}
}
• We can implement this as follows:
32. Interfaces
▪ Interfaces are helpful because I know that if MyClass
implements MyInterface, then I know that MyClass
must have:
– a method void MethodA()
– a method void MethodB()
▪ So if I create an object MyObject from a class that
implements MyInterface, I know I will be able to say
MyObject.MethodA()
33. Inheritance and Interfaces
▪ Inheritance gives us a way of creating classes from
other classes
▪ Interfaces give us a way of defining what methods a
class should have without defining the method.
35. Back to GUIs
▪ Now lets bring this back to GUIs…
▪ We have seen how the things that make a window are
objects of various GUI classes (like JFrame, JOptionPane,
JButton …)
▪ So far, we have created an object from these classes
and then changed their attributes.
▪ Instead we can inherit from these classes are create
out own, more specialized classes
- in particular JFrame
36. GUIs
import javax.swing.*;
public class MyFrame extends JFrame{
MyFrame(){
setTitle("My Frame.");
setSize(200,200);
setLocation(300,300);
setDefaultCloseOperation(EXIT_ON_CLOSE);
}
}
▪ Notice how this class calls the methods
setTitle() and setSize() – how can it do this?
JFrame
MyFrame
37. GUIs
▪ Now we can create objects of the
MyFrame class that we have just written:
public class L7 {
public static void main(String[] args) {
MyFrame mf = new MyFrame();
mf.setVisible(true);
}
}
We didn’t write this method – it was
inherited from the JFrame class, so it
is past of our MyFrame, because
MyFrame is a subclass of JFrame.
38. GUIs
public class MyBlueFrame extends JFrame{
public static void main(String[] args) {
MyBlueFrame mbf = new MyBlueFrame();
}
}
• Go one step further…
• We can even write this as the main class and have a main
method as one of its methods.
– Will have a main method
– This class should extend JFrame class
40. GUIs
• Let’s overload this constructor with a version that takes in
the size of the frame.
MyBlueFrame(String title, int x, int y){
setTitle(title);
setSize(x,y);
setLocation(300,300);
setDefaultCloseOperation(EXIT_ON_CLOSE);
}
• Now let’s say we want to set limits on the size of
the frame – these should be accessible by the user
• Let’s use class constants to do this!
41. GUIs
public static int MAX_X = 250, MAX_Y = 250;
public static void main(String[] args) {...}
MyBlueFrame(String title, int x, int y){
setTitle(title);
if (x > MAX_X)
x = MAX_X;
if (y > MAX_Y)
y = MAX_Y;
setSize(x, y);
setLocation(300,300);
setDefaultCloseOperation(EXIT_ON_CLOSE);
}
42. GUIs
public static int MAX_X = 250, MAX_Y = 250;
public static void main(String[] args) {...}
MyBlueFrame(String title, int x, int y){
setTitle(title);
setSize(x > MAX_X ? MAX_X : x, y > MAX_Y ? MAX_Y : y);
setLocation(300,300);
setDefaultCloseOperation(EXIT_ON_CLOSE);
}
• These are class constants
• We can use MAX_X instead of MyBlueFrame.MAX_X
because we are inside the MyBlueFrame class.
43. GUIs
void SetBackgroundToBlue() {
Container cp = getContentPane();
cp.setBackground(Color.BLUE);
}
• Let’s add a function that changes the background to blue…
• Why getContentPane() and not frame.getContentPane()?
• Because getContentPane() is an object method that is a
member of JFrame objects – objects created from our class
are JFrame objects!
44. GUIs
public class MyBlueFrame extends JFrame{
public static void main(String[] args) {
MyBlueFrame mbf = new MyBlueFrame("My blue frame");
mbf.SetBackgroundToBlue();
mbf.setVisible(true);
}
}
• In the main method, should I use mbf.SetBackgroundToBlue()
or SetBackgroundToBlue()?
• Is this an object method or a class method?
• Does this method belong to the class or does it only
correspond to a particular object?
• Object method!
