“Closures are poor man's objects and vice versa” - What does this mean?

Question

Closures are poor man\'s objects and vice versa.

I have seen this statement at many places on the web (including SO) but I don\'t quite

Answer 1

"Objects are a poor man's closures" isn't just a statement of some theoretical equivalence — it's a common Java idiom. It's very common to use anonymous classes to wrap up a function that captures the current state. Here's how it's used:

public void foo() {
    final String message = "Hey ma, I'm closed over!";
    SwingUtilities.invokeLater(new Runnable() {
        public void run() {
            System.out.println(message);
        }
    });
}

This even looks a lot like the equivalent code using a closure in another language. For example, using Objective-C blocks (since Objective-C is reasonably similar to Java):

void foo() {
    NSString *message = @"Hey ma, I'm closed over!";
    [[NSOperationQueue currentQueue] addOperationWithBlock:^{
        printf("%s\n", [message UTF8String]);
    }];
}

The only real difference is that the functionality is wrapped in the new Runnable() anonymous class instance in the Java version.