What is meant Observer Design Pattern?

Observer Design Pattern:
          One object changes state, all of its dependents are updated automatically.

        

Where to use & benefits

• One change affects one or many objects.
• Many object behavior depends on one object state.
• Need broadcast communication.
• AKA “Publish-Subscribe”.
• Maintain consistency between objects
• keep classes from becoming too tightly coupled, which would hamper reusability.
• Related patterns include 

Example:
import javax.swing.*;
import java.awt.event.*;
import java.util.*;

class DisplayForm extends JFrame {
InputFormObserver input = new InputFormObserver();
InputForm inputForm ;
Observable obsInput;
JTextField display;
//...
public DisplayForm() {
addWindowListener(new WindowAdapter() {
public void windowClosing(WindowEvent e) {
System.exit(0);
}});
inputForm = new InputForm();
obsInput = inputForm.getInputInfo();
obsInput.addObserver(input);

display = new JTextField(10);
display.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {

}
});
getContentPane().add(display);
setTitle("Observer form");
setSize(200,100);
setLocation(200,100);
setVisible(true);

}

private class InputFormObserver implements Observer {
public void update(Observable ob, Object o) {
doSomeUpdate();
if (obsInput.countObservers()>0)
obsInput.deleteObservers();
obsInput = inputForm.getInputInfo();
obsInput.addObserver(input);
}

}
public void doSomeUpdate() {
display.setText(inputForm.getText());
JOptionPane.showMessageDialog(DisplayForm.this,
"This form has been updated");
}
public static void main(String args[]) {
DisplayForm df = new DisplayForm();
}
}
class InputForm extends JFrame {
public InformDisplay inform = new InformDisplay();
//...
JTextField input= new JTextField(10);

public InputForm() {
JPanel panel= new JPanel();
input.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
inform.notifyObservers();
}
});
panel.add(new JLabel("Enter: "));
panel.add(input);
addWindowListener(new WindowAdapter() {
public void windowClosing(WindowEvent e) {
System.exit(0);
}});
getContentPane().add(panel);
setTitle("Observable form");
setSize(200,100);
setVisible(true);
}
public Observable getInputInfo() {
return inform;
}

public String getText() {
return input.getText();
}

private class InformDisplay extends Observable {
public void notifyObservers() {
setChanged();
super.notifyObservers();
}
public String getChange() {
return input.getText();
}
}
//...
}

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