Java swing error " 'void' type not allowed here " [duplicate]

Question

I'm getting an error 'void' type not allowed here

CODE:

public void create(JPanel jp){
    jp.add(new JButton().setPreferredSize(new Dimension(40, 40)));        
}

But when I use the same code without .setPreferredSize(new Dimension(40, 40)) it works fine.

WORKING CODE

public void create(JPanel jp){
    jp.add(new JButton());
}

Answer 1

setPreferredSize "returns" a void, so it cannot be passed as a parameter to add. You'd have to break up the code and separate to several statements:

public void create(JPanel jp){
    JButton myButton = new JButton();
    myButton.setPreferredSize(new Dimension(40, 40));
    jp.add(myButton);
}

Answer 2

Internal code of setPreferredSize from here http://grepcode.com/file/repository.grepcode.com/java/root/jdk/openjdk/6-b14/javax/swing/JComponent.java#JComponent.setPreferredSize%28java.awt.Dimension%29

public void setPreferredSize(Dimension preferredSize) {                    
  super.setPreferredSize(preferredSize);
}

When you are trying to execute this code

jp.add(new JButton().setPreferredSize(new Dimension(40, 40))); 

new JButton().setPreferredSize(new Dimension(40, 40)) return void, Thats why you are getting this 'void' type not allowed here

When you trying to add only new JButton() then its working fine because you are adding instance of JButton.

Solution :

JButton myButton = new JButton();
myButton.setPreferredSize(new Dimension(40, 40));
jp.add(myButton);