Answer: A layout manager is an object that can be associated with a container, such as a JPanel. The layout manager is responsible for setting the sizes and positions of components that are added to the container. Different types of layout manager, such as BorderLayout and GridLayout, implement different policies for laying out the components.

Answer: For example:

• JLabel -- just displays a line of text to the user.
• JCheckBox -- lets the user specify a yes/no option, represented by a box that has two states, checked or unchecked. The user can click the box to toggle the state.
• JTextField -- an input box where the user can type one line of text.

(Other components you should know include JPanel, JTextArea, JSlider, and JRadioButton.)

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
public class DisplayRect extends JPanel implements MouseListener {
     private int x,y;  // Upper-left corner of the rectangle.
     public DisplayRect() {
     
     }
     protected void paintComponent(Graphics g) {
         super.paintComponent(g);
         g.fillRect(x,y,100,50);
     }
     public void mousePressed(Event evt) {
     
     
     }
     public void mouseReleased(Event evt) { }
     public void mouseClicked(Event evt) { }
     public void mouseEntered(Event evt) { }
     public void mouseExited(Event evt) { }
}

Answer: Four lines have to be added. In the constructor, the panel must be registered to listen for mouse events from itself. This can be done with addMouseListener(this) [or with this.addMouseListener(this)]. Then the response to the mouse must be programmed in the mousePressed() method. When the user clicks at a point (a,b), the rectangle must be moved so that its upper left corner is at (a,b). Inspecting the paintComponent method, we see that the instance variables x and y are used as the upper left corner of the rectangle in the fillRect command. So, we must set x and y to the point where the user clicked the mouse. The mouse coordinates are given in mousePressed as evt.getX() and evt.getY(), so we just have to set x = evt.getX() and y = evt.getY(). Finally, in order to make the change visible on the screen, we need to call repaint() to make sure that the panel is redrawn using the new position of the rectangle.

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
public class DisplayRect extends JPanel implements MouseListener {
     private int x,y;  // Upper-left corner of the rectangle.
     public DisplayRect() {
        addMouseListener(this);
     }
     protected void paintComponent(Graphics g) {
         super.paintComponent(g);
         g.fillRect(x,y,100,50);
     }
     public void mousePressed(Event evt) {
         x = evt.getX();
         y = evt.getY();
         repaint();
     }
     public void mouseReleased(Event evt) { }
     public void mouseClicked(Event evt) { }
     public void mouseEntered(Event evt) { }
     public void mouseExited(Event evt) { }
}

Answer: They are specified by the MouseListener interface, so they have to be there in order for the class to properly implement MouseListener, even if they don't do anything.