如何一般地实现存储在HashMap中的调用方法？ [英] How to generically implement calling methods stored in a HashMap?

问题描述

我想将某些字符路由到方法，所以当在命令行中键入char时，方法被执行。



根据答案如何调用存储在HashMap中的方法，我通过使用Command设计模式将这些字符映射到方法。



但是我想通常实现这一点，所以似乎需要实现反射才能使用方法类作为参数。我的尝试是在我的匿名类中的字段 private Method方法中获取一个 NullPointerException

---

这是我的代码：

I want to route certain chars to methods, so that when the char is typed in the command-line the method is then executed.

Based on the answer How to call a method stored in a HashMap, I'm mapping these chars to methods by using the "Command" design-pattern.

However I want to generically implement this, so it seems that I need to implement reflection in order to use the Method class as a parameter. My attempt is getting a NullPointerException on the field private Method method in my anonymous class...

---

Here is my code:

import java.lang.reflect.Method;

public interface InvokesMethod {

    public void invokeMethod() throws Exception;
    public void setMethod(Method method);
} // end of interface

import java.util.HashMap;
import java.lang.reflect.Method;

public class Terminal {

    public HashMap<Character, InvokesMethod> commands;

    public Terminal() {
        this.commands = new HashMap<Character, InvokesMethod>();

        try {
            this.setCommand('p',
                 this.getClass().getDeclaredMethod("printHelloWorld"));

        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    private void printHelloWorld() {
        System.out.println("Hello World!");
    }

    private void setCommand(char letter, Method method) {
        this.commands.put(letter, new InvokesMethod() {

            // NullPointerException starts here in the stack-trace:
            private Method method;

            @Override
            public void invokeMethod() throws Exception {
                method.invoke(null);
            }

            @Override
            public void setMethod(Method method) {
                this.method = method;
            }
        }).setMethod(method);
    }

    public void executeCommand(char letter) throws Exception {
        this.commands.get(letter).invokeMethod();
    }
} // end of class

public class Main() {

    public static void main(String[] args) {
        Terminal commandLine = new Terminal();

        try {
            commandLine.executeCommand('p');

        } catch (Exception e) {
            e.printStackTrace();
        }
    }
} // end of class

推荐答案

对于您没有启动的代码方法。请记住，执行与 null 您必须调用public static方法：

Regards to your code you didn't initiate method. Bear in mind that execute with null you must call public static method:

您的其他问题，您没有正确启动界面。这里是工作示例：

Your other issue , you didn't initiated interface properly. Here is working example:

InvokesMethodItf

public interface InvokesMethodItf {

public void invokeMethod() throws Exception;
public void setMethod(Method method);
} 

InvokesMethod

public class InvokesMethod implements InvokesMethodItf{

private Method method;

@Override
public void invokeMethod() throws Exception {
     method.invoke(null);
}

@Override
public void setMethod(Method method) {
    this.method = method;
}

}

终端

public class Terminal {

public HashMap<Character, InvokesMethodItf> commands;

public Terminal() {
    this.commands = new HashMap<Character, InvokesMethodItf>();

    try {
        this.setCommand('p',
             this.getClass().getDeclaredMethod("printHelloWorld"));

    } catch (Exception e) {
        e.printStackTrace();
    }
}

public static void printHelloWorld() {// method.invoke(null) looking for "static" method
    System.out.println("Hello World!");
}


private void setCommand(char letter, Method method) {

    InvokesMethodItf inv = new InvokesMethod();

    inv.setMethod(method);

    this.commands.put(letter, inv);
}

public void executeCommand(char letter) throws Exception {
    this.commands.get(letter).invokeMethod();
 }
}

主要

public class Main {
public static void main(String[] args) {
    Terminal commandLine = new Terminal();

    try {
        commandLine.executeCommand('p');

    } catch (Exception e) {
        e.printStackTrace();
    }
  }
}

输出：

Hello World!

这篇关于如何一般地实现存储在HashMap中的调用方法？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！