Well, we all have seen interactive applications that perform any function whenever the state of the key is changed. But how do internal things work? How to implement them in Java? To answer all these questions we have Java KeyListener. Java KeyListener is an interface in the java.awt.package and it is useful in interacting with the user whenever the state of the key is changed.
In this article, we will learn about Java KeyListener, its declaration, and its methods with some detailed examples. So let’s get started with our article.
JavaKeyListener is an interface in java.awt.event package. Java KeyListener operates the event whenever the state of the Key is changed. The class that processes the KeyEvent implement the Java KeyListener.Java KeyListener interface consists of 3 Methods.
The following syntax can declare the java KeyListener interface:
public interface KeyListener extends EventListener
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Whenever the state of the key is changed, an object must be present in the program, which helps in implementing the interface. An event will be generated by typing the key, pressing the key, or releasing the key; for this, we will use some methods. Java MouseMotionListener consists of three methods, which are as follows:
In this below code, we are generating output that handles Java KeyListener. In this code, we are counting the words we are typing in the output console. In this program, we have created a class called “KeyListenerExample1” that consists of different methods in it that are responsible for the working of Java KeyListener.
import java.awt.*;
import java.awt.event.*;
public class KeyListenerExample1 extends Frame implements KeyListener {
Label l;
TextArea area;
KeyListenerExample1() {
l = new Label();
l.setBounds (20, 50, 200, 20);
area = new TextArea();
area.setBounds (20, 80, 300, 300);
// adding KeyListener to the text area
area.addKeyListener(this);
// adding label and text area to frame
add(l);
add(area);
// setting size, layout and visibility of frame
setSize (400, 400);
setLayout (null);
setVisible (true);
}
public void keyPressed(KeyEvent e) {}
// overriding the keyReleased() method of KeyListener interface
public void keyReleased (KeyEvent e) {
String text = area.getText();
String words[] = text.split ("\\s");
l.setText ("Words: " + words.length + " Characters:" + text.length());
}
public void keyTyped(KeyEvent e) {}
public static void main(String[] args) {
new KeyListenerExample1();
}
}
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In the above program, as we can see we have used methods to obtain words and characters when we are typing in the output console. Here, as shown above we have typed “Coding Ninjas” and as a result, words and characters are shown in the output.
In the below code, we are going to see the state of the key in the output whether the key is typed, pressed, or released and for doing so we have used a class called “KeyListenerExample” in which we have used methods and we have overridden the methods to check the state of the key.
import java.awt.*;
import java.awt.event.*;
public class KeyListenerExample extends Frame implements KeyListener {
// creating object of Label class and TextArea class
Label l;
TextArea area;
// class constructor
KeyListenerExample() {
l = new Label();
l.setBounds (20, 50, 100, 20);
area = new TextArea();
area.setBounds (20, 80, 300, 300);
// adding the KeyListener to the text area
area.addKeyListener(this);
// adding the label and text area to the frame
add(l);
add(area);
// setting the size, layout and visibility of frame
setSize (400, 400);
setLayout (null);
setVisible (true);
}
// overriding the keyPressed() method of KeyListener interface where we set the text of the label when key is pressed
public void keyPressed (KeyEvent e) {
l.setText ("Key Pressed");
}
// Overriding the keyReleased() method of KeyListener interface where we set the text of the label when key is released
public void keyReleased (KeyEvent e) {
l.setText ("Key Released");
}
// Overriding the keyTyped() method of KeyListener interface where we set the text of the label when a key is typed
public void keyTyped (KeyEvent e) {
l.setText ("Key Typed");
}
// Main method
public static void main(String[] args) {
new KeyListenerExample();
}
}
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In the above output, as we can see we have typed “Coding Ninjas” and with the help of the above program, we can see the state of the key is shown that it is released in the output.
Example 3
In the below code, we are going to see the state of the key in the output whether the key is typed, pressed, or released and for doing so we have used a class called “JavaKeyListenerExample” in which we have used methods and we have written this program without overriding the methods unlike the above example, to check the state of the key.
import java.awt.FlowLayout;
import java.awt.Frame;
import java.awt.Label;
import java.awt.TextField;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;
public class JavaKeyListenerExample implements KeyListener
{
Label lb1, lbl2, lb;
TextField tf1;
Frame fr;
String s;
JavaKeyListenerExample()
{
//create a frame
fr = new Frame("KeyEventListener Example");
lb1= new Label(" Key Events will be displayed based on the actions", Label.CENTER);
lbl2= new Label();
lb= new Label();
tf1 = new TextField(20);
fr.setLayout(new FlowLayout());
fr.add(lb1);
fr.add(tf1);
fr.add(lbl2);
tf1.addKeyListener(this);
fr.setSize(460,250);
fr.setVisible(true);
}
public void keyPressed(KeyEvent ev)
{
lbl2.setText("Pressed Key");
}
public void keyReleased(KeyEvent ev)
{
lbl2.setText(" Released key");
}
//events to be done on typing key
public void keyTyped(KeyEvent ev)
{
lbl2.setText("Key is typed");
//set the visibility as true
fr.setVisible(true);
}
public static void main(String[] args)
{
new JavaKeyListenerExample();
}
}
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In the above output, as we can see we have typed “Coding Ninjas” and with the help of above program, we can see the state of key is shown that it is released in the output.
The Java KeyListener interface is identified in Java. awt.Java KeyListener is alerted whenever the state of the Key is changed.
What are the methods to implement Java KeyListener?
The methods to implement Java KeyListener are KeyPresses(KeyEventev): This method will be initiated when Key is pressed. KeyReleased(KeyEventev): This method will be initiated when a key is released. KeyTyped(KeyEventev): This method will be initiated when Key is typed.
Which command does not generate any events?
The command known as TextArea objects does not generate any events.
How do you use the KeyReleased method?
The keyReleased() function is implemented once when you release a key.
Why swings are used in Java?
Swings are used for creating window-based applications. Swing in Java is part of the Java foundation class, which is platform-independent and lightweight.
Conclusion
This article extensively discussed Java KeyListener and its different methods. We started with an introduction of Java KeyListener, then saw some of the ways of Java KeyListener, and after that, we have seen some examples of handling Java KeyListener.
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