public class Main {

    private interface Delegate { void execute(String str); }

    private static void delegateCaller(Delegate delegate) {
        delegate.execute("Hello world");
    }

    public static void main(String[] args) {
        delegateCaller(new Delegate() {
            @Override
            public void execute(String str) {
                System.out.println(str);
            }
        });
    }
}