instantiate object via class literal?

[Richard Hayward]

Just got to page 738 k&b's book, where they introduce class literals.

Is it possible to use a class literal to instantiate an object?
I thought this should work:

package p738;

import javax.swing.*;


class ClassLiteralTest {

  public static void main(String[] args){
    Class cl;
    JComponent jc;

    try {
      cl = Class.forName("JCheckBox");
    } catch (Exception e) {}

    jc = (JCheckBox) new cl();
  }

}

But it doesn't.

I thought I'd need the cast to assure the compiler that the object being created is indeed something descendant from JComponent and therefore assignable to jc.


Regards
Richard

[Gaurangkumar Khalasi]

Use newInstance() method. http://docs.oracle.com/javase/7/docs/api/java/lang/Class.html#newInstance()
Like jc = cl.newInstance();

[Paul Clapham]

Yes, there is.

But that isn't the way to do it.

Check out the API documentation for the Class class (that's java.lang.Class). You'll see methods with names like newInstance() which you can use, and there are more complicated ways if you need to create an object using a constructor which takes parameters.

But if you're a beginner, you probably don't need to do this. Beginners who create designs using reflection (which is what you're asking about) usually do so because they haven't yet learned about the ordinary way to do whatever it is they are designing.

[Campbell Ritchie]

It’s

Class.forName("foo.Foo").newInstance();

There are other ways to get the Class<?> object. It depends on the class in question having an accessible no‑arguments constructor, and declares a list of checked Exceptions as long as your arm. You may need a class cast, too.

[Matthew Brown]

Also note, what you're talking about there is a Class object. That's not the same as a class literal. A class literal is something like String.class, which is a way of getting the Class object for a class known to the compiler at compile time. But I don't think you'd ever use one to create a new object, because if you can use a class literal you can just use a new statement.

[Paul Clapham]

And by the way, this code:

cl = Class.forName("JCheckBox");

will probably throw an exception, unless you happen to have created a class named "JCheckBox" which isn't in a package. In particular it won't load the javax.swing.JCheckBox class because the name of that class is "javax.swing.JCheckBox" and not "JCheckBox".

And this line of code:

} catch (Exception e) {} 

guarantees that you won't know what that exception was. You should never do this in real code, unless you're sure you don't care what exception was thrown and you're sure you don't need to know what it is when it is thrown. (There could be cases where you really don't give a rat's.) Instead use this code:

} catch (Exception e) {
  e.printStackTrace();
}

[Richard Hayward]

Thanks to you all for your help. Much appreciated.

I've now made myself a small program to play with, to try and understand this aspect of java a little better.

It's only short, so I've posted it below for the enjoyment of any future readers of this thread. I hope that's appropriate.

The user can choose from 3 radio buttons. Depending on which they choose, a variable c of type Class is set to JCheckBox.class or JButton.class and so on.
When the user clicks on the green panel, c is used to create a component of the chosen type.

Regards

Richard

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

class ClassLiteralDemo implements ActionListener, MouseListener {

  JRadioButton bRadio,bCheck,bButton;
  JPanel drawPanel;
  Class c=JRadioButton.class;

  private void createAndShowGUI() {
    JFrame frame = new JFrame("Click somewhere on the green panel");
    JPanel buttonPanel = new JPanel();
    buttonPanel.setLayout(new BoxLayout(buttonPanel, BoxLayout.Y_AXIS));
    frame.getContentPane().add(buttonPanel, BorderLayout.LINE_START);

    drawPanel = new JPanel();   
    drawPanel.setLayout(null);
    drawPanel.setBackground(Color.green);
    drawPanel.addMouseListener(this);
    frame.getContentPane().add(drawPanel, BorderLayout.CENTER);

    ButtonGroup btnGroup = new ButtonGroup();
    bRadio = new JRadioButton("Radio");
    bRadio.addActionListener(this);
    buttonPanel.add(bRadio);
    bCheck = new JRadioButton("Check");
    bCheck.addActionListener(this);
    buttonPanel.add(bCheck);
    bButton = new JRadioButton("Button");
    bButton.addActionListener(this);
    buttonPanel.add(bButton);
    
    bRadio.setSelected(true);
    btnGroup.add(bRadio);
    btnGroup.add(bCheck);
    btnGroup.add(bButton);

    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setSize(350,200);
    frame.setVisible(true);
  }

  // Implement ActionListener interface
  public void actionPerformed(ActionEvent e) {
    if (e.getSource()==bRadio) {
      c=JRadioButton.class;
    } else if (e.getSource()==bCheck) {
      c=JCheckBox.class; 
    } else if (e.getSource()==bButton) {
      c=JButton.class;  
    } 
  }

  // Implement MouseListener interface 
  public void mousePressed(MouseEvent e){}  
  public void mouseReleased(MouseEvent e){}
  public void mouseEntered(MouseEvent e){}
  public void mouseExited(MouseEvent e){}
  public void mouseClicked(MouseEvent e){
    try {
      JComponent jc = (JComponent) c.newInstance();
      jc.setLocation(e.getX(),e.getY());
      jc.setSize(40,40);
      drawPanel.add(jc);
      drawPanel.repaint();
    } catch (Exception ex) {
      ex.printStackTrace();
    }
  }

  public static void main(String[] args){
    //Schedule a job for the event-dispatching thread:
    //creating and showing this application's GUI.
    SwingUtilities.invokeLater(
      new Runnable() {
        public void run() {
          new ClassLiteralDemo().createAndShowGUI();
        }
      }
    ); 
  }

}

[Campbell Ritchie]

Not bad, but I don’t like the listener. As far as I am concerned, addActionListener(this) is non‑object‑oriented programming.

radioButtonButton.addActionListener(new ClassChangeListener(JRadioButton.class));

class ClassChangeListener implements ActionListener
{
   @Override
   public void actionPerformed(ActionEvent e)
   {
      c = clazz;
   }
}

That will work best if ClassChangeListener is a private inner class (i.e. inside the GUI class, and not marked static), so it has access to the field c. You will doubtless have worked out how to fill in the bits of that class which I have omitted.