CODEWITHSUNDEEP
java
gobernador
gobernador

Reputation: 5739

Most efficient way to rule out classes from an interface's method

Up to this point, I have used the following code to restrict the application of some methods to instances of certain classes. For instance, using ItemListener, but this could be applied to many things,

public class mListener implements ItemListener {
    public void itemStateChanged(ItemEvent e) {

        if (!(e.getItemSelectable instanceof JCheckBox)) { //again, JCheckBox was chosen arbirtarily
            System.err.println("mListener can only be applied to a JCheckBox");
            return;
        }

    }
}

However, in a few places on the Oracle Java tutorials, I have seen the following code

public class mListener implements ItemListener {
    public void itemStateChanged(ItemEvent e) {
        JCheckBox box = null;

        try {
            box = (JCheckBox) e.getItemSelectable();
        } catch (ClassCastException ex) {
            System.err.println("mListener can only be applied to a JCheckBox");
            return;
        }

    }
}

Which is the best way to lock out classes that you don't want a method applied to? This is especially the case with implementing interfaces, where the parameter can't be changed.

Upvotes: 1

Views: 91

Answers (3)

gobernador
gobernador

Reputation: 5739

The answer may be this: catching a ClassCastException allows e to be any subclass of JCheckBox, whereas using instanceof restricts it solely to a JCheckBox.

Upvotes: 0

Jon Skeet
Jon Skeet

Reputation: 1503020

In both cases, this is a programming error. The listener is being added inappropriately. The correct response for this is almost certainly not just printing out a message where it will probably never be seen - it's an exception.

Simply casting will give you that exception without any work on your part, so just cast unconditionally, and don't try to cover up programming errors.

Upvotes: 6

Sean Patrick Floyd
Sean Patrick Floyd

Reputation: 299138

The second version is evil, as it uses exceptions for flow control. Read Effective Java by Joshua Bloch to learn why that is bad (Item 57: "Use exceptions only for exceptional conditions").

Upvotes: 3

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