CIS 121 February 25, 2000

On Wednesday, we discussed Graphical User Interfaces and saw an example of adding a Button Component to a Container. Today we will look at other things that can be added to a Container.

We have discussed part of the hierarchy of the AWT package. Let us examine some more of the hierarchy.

			Component
Button	Checkbox	Choice	Container	Label		TextComponent
			Window		TextArea		TextField
			Frame

In the Component class we have discussed three important methods:

public void setSize(int width,int height);
public void setBackground(Color c);
public void setVisible(boolean b);

Another important method from the Component class is

public void setLocation(int x,int y);

In the Container class there are two important methods:

public Component add(Component comp);
public void setLayout(LayoutManager mgr);

Components such as Button allow the user to send signals to the computer. We can interpret those signals and perform appropriate actions.

An event is normally associated with action such as a mouse button click. When a button is clicked an object called ActionEvent is generated. In order to capture this object and use it, we use the interface ActionListener. This interface has one abstract method called actionPerformed. The parameter to actionPerformed is an ActionEvent. Since ActionListener is an interface, we must implement actionPerformed.

In order for the interpreter to create an ActionEvent object, it must listen for events. An event listener acts as a communicator between the component and the ActionListener. Each component inherits a method called addActionListener. The parameter list of addActionListener is an ActionListener object. Recall that any class that implements the interface ActionListener can be passed into a method whose parameter is an ActionListener object. Therefore, when we implement the ActionListener interface and call the method addActionListener, we may use the keyword this as the parameter.

ActionEvent and ActionListener are both located in the package java.awt.event so this package must be imported in any class definition where we use these classes.

There are several Listener Intefaces available to us.

ActionListener

public interface ActionListener extends EventListener{
   public void actionPerformed(ActionEvent e);
}

WindowListener

public interface WindowListener extends EventListener {
   public abstract void windowActivated(WindowEvent e);
   public abstract void windowClosed(WindowEvent e);
   public abstract void windowClosing(WindowEvent e);
   public abstract void windowDeactivated(WindowEvent e);
   public abstract void windowDeiconified(WindowEvent e);
   public abstract void windowIconified(WindowEvent e);
   public abstract void windowOpened(WindowEvent e);
}

ItemListener

public interface ItemListener extends EventListener {
   void itemStateChanged(Event e);
}

MouseListener

public interface MouseListener extends EventListener {
   public void mouseClicked(MouseEvent e);
   public void mousePressed(MouseEvent e);
   public void mouseReleased(MouseEvent e);
   public void mouseEntered(MouseEvent e);
   public void mouseExited(MouseEvent e);
}

All of these interfaces are located in the java.awt.event package and that package must be imported if we implement the interfaces.

Now let us examine the components availabe to us and how we can handle events generated by these components. There are two methods we can use to tell what triggered the event. One is the getActionCommand, and the other is getSource. We looked at an example of use getActionCommand to test the text of a button. We can use getSource to see what object generated the event.

Button

A Button a graphical representation of a push-button in the container

Constructors:

public Button();
public Button(String label);

Event Generated: ActionEvent
Listener Interface: ActionListener

Example:

Source Code:

Gui.java

import java.awt.*;

class Gui extends Frame {

   public Gui(String s) {
      super(s);
      setBackground(Color.yellow);
      setLayout(new FlowLayout());
      Button pushButton = new Button("press me");
      add(pushButton);
   }

}

Ex_1.java

class Ex_1 {

   public static void main(String[] args) {
      Gui screen = new Gui("Example 1");

      screen.setSize(500,100);
      screen.setVisible(true);
   }

}

Label

A label is a text string that may be used to label other components. Constructors:

public Label();
public Label(String label);
public Label(String label,int alignment);

Event Generated: None

Example:

Source Code:

Gui4.java

import java.awt.*;

class Gui4 extends Frame {

   public Gui4(String s) {
      super(s);

      setLayout(new FlowLayout());

      Label l1 = new Label("NAME:          ",Label.LEFT);
      Label l2 = new Label("AGE:    ",Label.CENTER);
      Label l3 = new Label("ADDRESS:",Label.RIGHT);

      add(l1);
      add(l2);
      add(l3);
   }

}

Ex_4.java

class Ex_4 {

   public static void main(String[] args) {
      Gui4 screen = new Gui4("Example 4");

      screen.setSize(500,100);
      screen.setVisible(true);
   }

}

Checkboxes

Checkboxes are components that have two states: on or off. A special case of checkboxes are radio buttons. With radio buttons only one choice can be on.

Constructors:

public Checkbox();
public Checkbox(String label);
public Checkbox(String label,Checkboxgroup group,boolean state);

Event Generated: ItemEvent
Listener Interface: ItemListener

Example:

Source Code:

Gui5.java

import java.awt.*;

class Gui5 extends Frame {

   public Gui5(String s) {
      super(s);

      setLayout(new FlowLayout());

      Checkbox c1 = new Checkbox("Eggs");
      Checkbox c2 = new Checkbox("Toast");
      Checkbox c3 = new Checkbox("Fries");
      Checkbox c4 = new Checkbox("Yogurt");
      Checkbox c5 = new Checkbox("Bagels");

      add(c1);
      add(c2);
      add(c3);
      add(c4);
      add(c5);

   }

}

Ex_5.java

class Ex_5 {

   public static void main(String[] args) {
      Gui5 screen = new Gui5("Example 5");

      screen.setSize(500,100);
      screen.setVisible(true);
   }

}

In order to create Radio Buttons we use the class CheckboxGroup. We create a new instance of a CheckboxGroup and pass this as a parameter to the third constructor for Checkbox.

Example:

Source Code:

Gui6.java

import java.awt.*;

class Gui6 extends Frame {

   public Gui6(String s) {
      super(s);

      setLayout(new FlowLayout());

      CheckboxGroup whatNext = new CheckboxGroup();

      Checkbox Continue = new Checkbox("Continue",whatNext,true);
      Checkbox Exit = new Checkbox("Exit",whatNext,false);

      add(Continue);
      add(Exit);

   }

}

Ex_6.java

class Ex_6 {

   public static void main(String[] args) {
      Gui6 screen = new Gui6("Example 6");

      screen.setSize(500,100);
      screen.setVisible(true);
   }

}

TextFields

A TextField allows you to either input or output textual information.

Constructors:

public TextField();
public TextField(int columns);
public TextField(String text);
public TextField(String text,int columns);

Event Generated: ActionEvent
Listener Interface: ActionListiner

TextAreas

A TextArea is similar to a TextField but it gives more than one line.

Constructors:

public TextArea();
public TextArea(String text);
public TextArea(int rows,int columns);
public TextArea(String text,int rows,int columns);
public TextArea(String text,int rows,int columns,int scrollbars);

Scrolling Lists

Constructors:

public List();
public List(int rows,boolean multipleSelections);

Event Generated: ActionEvent
Listener Interface: ActionListener

Example:

Source Code:

Gui7.java

import java.awt.*;

class Gui7 extends Frame {

   public Gui7(String s) {
      super(s);

      setLayout(new FlowLayout());

      List menu = new List(4,true);

      menu.add("Eggs");
      menu.add("Toast");
      menu.add("Fries");
      menu.add("Yogurt");
      menu.add("Bagels");
      menu.add("Pancakes");
      menu.add("Coffee");

      add(menu);

   }

}

Ex_7.java

class Ex_7 {

   public static void main(String[] args) {
      Gui7 screen = new Gui7("Example 7");

      screen.setSize(500,100);
      screen.setVisible(true);
   }

}

Drop Down Menus

A Drop Down Menu has two parts - The MenuBar and the Menu. MenuBar

A MenuBar is a bar containing the name of the menu.

Constructor:

public MenuBar();

Menu

A Menu is a list of items.

Constructors:

public Menu();
public Menu(String label);

Layout Managers

There are five main predefined Layout managers we can use. We have seen how FlowLayout works. Components are added to the container in rows and when there is not enough space left on the row to add a new component, it is placed on the next row.

There are three constructors for the FlowLayout class.

public FlowLayout();
public FlowLayout(int align);
public FlowLayout(int align,int hGap,int vGap);

The other predefined Layout manager we will consider is the GridLayout.

There are two constructors for the GridLayout class.

public GridLayout(int rows,int columns);
public GridLayout(int rows,int columns,int hGap,int vGap) throws IllegalArgumentException;

We may also choose to define our own Layout. We can do this by using the setLocation method from the Component class. We specify in pixels where the coordinates of the upper left-hand corner of the component should be, where (0,0) are the coordinates of the upper left-hand corner of the container.

Another important class in the awt is the Graphics class. This is an abstract class and cannot be instantiated by a constructor. In order to create a new Graphics object we use the getGraphics() method of a Component.

Your exercise for the rest of the class period is to take the source code for a couple of the components and implement Event handling.

Recall the methods in each of the interfaces, and depending on which interface you decide to implement make sure you implement the appropriate method. Recall the two methods available to you.

getSource returns an object
getActionCommand returns the string data from any text field