Why doesn't instanceof work with JPanel and JComponent?

Question

I feel like I'm missing something blindingly obvious here, so low hanging fruit for a Java guru:

I have code that looks like this:

private static void myFunc(JComponent c) {
        if (c instanceof JPanel) {
            //stuff
        }
        else if (c instanceof JMenu) {
            // other stuff
        }
}

Even though JPanel and JMenu are both subclasses of JComponent, the first instanceof gives an error:

Incompatible conditional operand types JComponent and JPanel

While the second one works fine. Why does it think that my JComponent could never be a JPanel?

Are you importing `javax.swing.JPanel`? – Jeremy May 05 '11 at 22:23 — Jeremy, May 05 '11 at 22:23

score 6 · Accepted Answer · answered May 05 '11 at 21:52

I suspect you're importing a different JPanel from somewhere. For the minute, try using the fully qualified types:

private static void myFunc(javax.swing.JComponent c) {
    if (c instanceof javax.swing.JPanel) {
        //stuff
    }
}

Beyond that, I can't think of any reason it wouldn't compile... if you can come up with a short but complete program which demonstrates the problem, that would help. This compiles fine:

import javax.swing.JComponent;
import javax.swing.JPanel;

public class Test {

    public static void myFunc(JComponent c) {
        if (c instanceof JPanel) {
            System.out.println("yes");
        }
    }
}

That does indeed appear to be the case. Curious what other JPanel I could possibly be importing... Thanks! — Morinar, May 05 '11 at 21:55

score 1 · Answer 2 · answered May 05 '11 at 21:54

1

I would double-check the imports.

Are you sure you've imported the very JPanel and the very JComponent class you wanted? Are they the following?

 import javax.swing.JPanel;
 import javax.swing.JComponent;

answered May 05 '11 at 21:54

Grzegorz Oledzki

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score 1 · Answer 3 · answered May 05 '11 at 21:54

1

Use this:

if(javax.swing.JPanel.isInstance(c)){

answered May 05 '11 at 21:54

Femi

64,273
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`.class`? But why would you do that? – Tom Hawtin - tackline May 05 '11 at 22:53
Right, it should be `if(javax.swing.JPanel.class.isInstance(c)){`, and that sidesteps the import questions as well as being a bit faster than `instanceof` (a micro-optimization, but why not). – Femi May 05 '11 at 23:01

score 1 · Answer 4 · answered May 05 '11 at 21:57

1

The following code compiles fine:

import javax.swing.*;

class Panel
{
  private static void myFunc(JComponent c) {
    if (c instanceof JPanel) {
      //stuff
    } else if (c instanceof JMenu) {
      // other stuff
    }
  }
}

answered May 05 '11 at 21:57

Oswald

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Why doesn't instanceof work with JPanel and JComponent?

4 Answers4

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