Question

I have the following simple code:

package test;

import javax.swing.*;

class KeyEventDemo {
    static void main(String[] args) {
    UIManager.setLookAndFeel("javax.swing.plaf.metal.MetalLookAndFeel");
    } 
}

It generates the following error message:

KeyEventDemo.java:7: unreported exception java.lang.ClassNotFoundException; must be caught or declared to be thrown
    UIManager.setLookAndFeel("javax.swing.plaf.metal.MetalLookAndFeel");
                            ^
1 error

Does anybody know what is wrong?

Answer 1

Actually, the message is self explaining: UIManager.setLookAndFeel throws a bunch of checked exceptions that thus need to be caught (with a try/catch block) or declared to be thrown (in the calling method).

So either surround the call with a try/catch:

public class KeyEventDemo {
    public static void main(String[] args) {
        try {
            UIManager.setLookAndFeel("javax.swing.plaf.metal.MetalLookAndFeel");
        } catch ( ClassNotFoundException e ) {
            // TODO handle me
        } catch ( InstantiationException e ) {
            // TODO handle me
        } catch ( IllegalAccessException e ) {
            // TODO handle me
        } catch ( UnsupportedLookAndFeelException e ) {
            // TODO handle me
        }
    }
}

Or add a throws declaration:

public class KeyEventDemo {
    public static void main(String[] args) throws ClassNotFoundException, 
        InstantiationException, IllegalAccessException, 
        UnsupportedLookAndFeelException {
        UIManager.setLookAndFeel("javax.swing.plaf.metal.MetalLookAndFeel");
    }
} 

If you don’t want to handle each of them in a specific way, this can be made less verbose by using the Exception supertype:

public class KeyEventDemo {
    static void main(String[] args) {
        try {
            UIManager.setLookAndFeel("javax.swing.plaf.metal.MetalLookAndFeel");
        } catch (Exception e) {
            // TODO handle me
        }
    }
} 

Or with a throws declaration (note that this convey less information to the caller of the method but the caller being the JVM here, it doesn’t really matter in this case):

class KeyEventDemo {
    static void main(String[] args) throws Exception {
        UIManager.setLookAndFeel("javax.swing.plaf.metal.MetalLookAndFeel");
    }
}