Example of using callbacks in Java This code executes this.doSomething and calls a user-specified callback when its finished. This is a synchronous method, but you could imagine that doSomething forked into a new thread and executed onTaskComplete later.

Callback.java

public interface Callback {
    void run(Object result);
}

main.java

public class main {
    public main() {
        this.doSomething(new Callback() {

            @Override
            public void run(Object result) {
                System.out.println(result.hashCode());
            }
            
        });
    }

    public void doSomething(Callback onTaskComplete) {
        // some action is performed...
        // ...
        Object result = new Object();
        onTaskComplete.run(result);
        return;
    }
}