先給出一個小demo

package package2;
import java.util.function.Function;
public class Main {
    public static void main(String[] args){
        System.out.println("main");
        Main main = new Main();
        main.F(new method1());
        System.out.println("==========");
        main.F(new method2());
    }
    public void F(method method){
        Function f = Function.identity();
        method = (package2.method) f.apply(method);
        method.m();
    }
}

package package2;
public interface method {
    public void m();
}

package package2;
public class method1 implements method{
    public method1() {
        System.out.println("method1");
    }
    @Override
    public void m(){
        System.out.println("m1");
    }
}

package package2;
public class method2 implements method{
    public method2() {
        System.out.println("method2");
    }
    @Override
    public void m(){
        System.out.println("m2");
    }
}

運行結果：
main
method1
m1
==========
method2
m2
Process finished with exit code 0

個人理解：
有沒有發現只要繼承同一個接口，在調用的時候指定一下就可以換成其他的方法啦。很方便啊。