GUI components in Java

It will describes about the GUI Components in java in detail.

1. GUI COMPONENTS M.SUJITHA, II-M.SC(CS&IT), Nadar Saraswathi College Of Arts and Science, Theni

2. Java includes libraries to provide multi-platform support for Graphic User Interface objects. Java's GUI components include labels, text fields, text areas, buttons. The Abstract Windowing Toolkit (AWT) also includes containers which includes these components . Containers include frames (windows), canvases which are used to draw and panels which are used to group components. Panels and canvases are contained in frames while buttons and other components can be placed either directly on frames or in panels inside the frames .

3. These GUI components are automatically drawn whenever the window is drawn. These GUI components are handled using Java's event model. When a user interacts with a component, an event is generated by the component that you interact with. For each component of your program, the programmer is required to designate one or more objects to "listen" for events from that component. If the program has a button labelled "start" you must assign one or more objects which will be notified when a user clicks on the button.

4. BUTTONS Button is a class in package java.awt which represents buttons on the screen. The constructor is: public Button(String label) which, when executed, creates a button with "label" printed on it. The button is large enough to display the label. There is also a parameter less constructor that creates an unlabeled button. Buttons respond to a variety of messages. "Action Listener" to a button with the method: public void addActionListener ( ActionListener listener);

5. ADDING BUTTONS TO A FRAME OR PANEL A command to add a button to a frame or panel: my Frame .add(start Button); This code is used in the constructor for the frame or panel. add(start Button) or simply add(start Button). The class extends Frame, which is part of java.awt. The constructor for Frame takes a String parameter and creates a window with the string in the title bar. The constructor for Button Demo calls the super class constructor, and then sets the size of the new Frame to be 400 x 200. The set Layout command tells the new Frame that new components should be added from left to right across the panel.

6. EXAMPLE CODING: import java.awt.*; public class Button Demo extends Frame { protected Button start Button, stop Button; // Constructor sets features of the frame, creates buttons, // public Button Demo() { super("Button demo"); // calls Frame constructor // which adds title to window setSize (400,200); // sets the size of the window //

7. Grid Layout. setLayout (new FlowLayout ()); // create two new buttons labels start and stop startButton = new Button("Start"); stopButton = new Button("Stop"); add( startButton ); // add buttons to frame add( stopButton ); // create an object to listen to both buttons: ButtonListener myButtonListener = new ButtonListener ();

8. startButton.addActionListener ( myButtonListener ); stopButton.addActionListener ( myButtonListener ); setVisible (true); // Show the window on the screen. // Trivial main program associated with ButtonDemo // public static void main(String args []) { // Create an instance of Buttons ButtonDemo app = new ButtonDemo () } }

9. ACTION LISTENERS FOR BUTTONS Objects which implement the Action Listener interface are required to implement a method action Performed which takes a parameter of type Action Event. When an action occurs to a button, all objects which have been added as Action Listener's for that button are notified by calling their action Performed method with a parameter with information on the exact event that occurred. T he system automatically creates the ActionEvent object and sends it along to the listener. It is need to manually create an ActionEvent object in this course.The most useful methods of ActionEvent are

10. INNER CLASSES A bit heavy to have to create a completely separate class in order to create a listener for the two buttons in our Button Demo class. Two alternatives are possible. 1.One is to let the frame itself be the Action Listener for the button. 2.public class Button Demo extends Frame implements Action Listener . This is the style suggested in Core Java for handling action events in simple cases. There is another style which is almost as simple, but more general. It involves the use of what are called "inner classes".

11. EXAMPLE SOURCE CODE: import java.awt.*; import java.awt.event .*; public class ButtonDemo extends Frame { protected Button startButton , stopButton ; public ButtonDemo () { startButton.addActionListener ( myButtonListener ); stopButton.addActionListener ( myButtonListener ); } public static void main(String args []) { { if (source == startButton ) System.out.println ("Start button "); else if (source == stopButton ) System.out.println ("Stop button "); } } }

12. ThetButton Listener is declared to be protected , it can only be used inside the containing class, Button Demo . The method Performed is still public . If it is declared as protected. These nested classes would now be contained within a single file named ButtonDemo.java. Another advantage of using nested classes is that all of the instance variables (and methods) of the outer class are visible inside the inner class. This can be handy for picking up information from other components of the outer class.

13. OTHER GUI COMPONENTS LABELS A  Label is a very simple component which contains a string . The constructors are public Label() // creates label with no text public Label// The methods available are public String getText () T he return label text public void setText (String s) It sets the label text the user can change the text in Labels.

14. TEXT FIELDS A   TextField  is a single line area that the user can type into . It is a good way of getting text input from the user. The constructors are public Text Field () . When the user types into a text field and then hits the return or enter key, it generates an event which can be handled by the same kind of Action Listener used with Buttons. If for some reason the user likes to be notified every time any change is made to the Text Field one can associate a Text Listener to the field.

15. TEXT AREAS Text Area  is a class that provides an area to hold multiple lines of text. It is fairly similar to  Text Field  except that no special event is generated by hitting the return key . The constructors are public Text Area( int rows, int columns ) // create text area with rows, columns, and displaying s Methods public void set Editable( booleans ) // if false the Text Area is not user editable public String get Text () // return text in Text Area public void set Text(String s) // sets the text