如何调用存储在 HashMap 中的方法?(爪哇) [英] How to call a method stored in a HashMap? (Java)

问题描述

我有一个命令列表(i、h、t 等)，用户将在命令行/终端 Java 程序中输入这些命令.我想存储命令/方法对的散列:

I have a list of commands (i, h, t, etc) that the user will be entering on a command line/terminal Java program. I would like to store a hash of command/method pairs:

'h', showHelp()
't', teleport()

这样我就可以有类似的代码:

So that I can have code something like:

HashMap cmdList = new HashMap();

cmdList.put('h', showHelp());
if(!cmdList.containsKey('h'))
    System.out.print("No such command.")
else
   cmdList.getValue('h')   // This should run showHelp().

这可能吗?如果没有，有什么简单的方法可以做到这一点?

Is this possible? If not, what is an easy way to this?

推荐答案

使用 Java 8+ 和 Lambda 表达式

使用 lambdas(在 Java 8+ 中可用)，我们可以这样做:

With Java 8+ and Lambda expressions

With lambdas (available in Java 8+) we can do it as follows:

class Test {

    public static void main(String[] args) throws Exception {
        Map<Character, Runnable> commands = new HashMap<>();

        // Populate commands map
        commands.put('h', () -> System.out.println("Help"));
        commands.put('t', () -> System.out.println("Teleport"));

        // Invoke some command
        char cmd = 't';
        commands.get(cmd).run();   // Prints "Teleport"
    }
}

在这种情况下，我很懒惰并重用了 Runnable 接口，但也可以使用我在 Java 7 版本中发明的 Command 接口回答.

In this case I was lazy and reused the Runnable interface, but one could just as well use the Command-interface that I invented in the Java 7 version of the answer.

此外，还有 () 的替代方案 ->{ ... } 语法.你也可以拥有 help 和 teleport 的成员函数，并使用 YourClass::help .YourClass::teleport 代替.

Also, there are alternatives to the () -> { ... } syntax. You could just as well have member functions for help and teleport and use YourClass::help resp. YourClass::teleport instead.

• Programming.Guide 上的一个很棒的 Lambda 备忘单.

此处的 Oracle 教程:Java 教程™–Lambda 表达式.

Oracle tutorial here: The Java Tutorials™ – Lambda Expressions.

你真正想要做的是创建一个接口，命名为Command(或重用Runnable)，并让你的地图类型<代码>映射<字符，命令>.像这样:

What you really want to do is to create an interface, named for instance Command (or reuse for instance Runnable), and let your map be of the type Map<Character, Command>. Like this:

import java.util.*;

interface Command {
    void runCommand();
}

public class Test {

    public static void main(String[] args) throws Exception {
        Map<Character, Command> methodMap = new HashMap<Character, Command>();

        methodMap.put('h', new Command() {
            public void runCommand() { System.out.println("help"); };
        });

        methodMap.put('t', new Command() {
            public void runCommand() { System.out.println("teleport"); };
        });

        char cmd = 'h';
        methodMap.get(cmd).runCommand();  // prints "Help"

        cmd = 't';
        methodMap.get(cmd).runCommand();  // prints "teleport"

    }
}

反射黑客"

话虽如此，您可以实际执行您的要求(使用反射和 Method 类.)

Reflection "hack"

With that said, you can actually do what you're asking for (using reflection and the Method class.)

import java.lang.reflect.*;
import java.util.*;

public class Test {

    public static void main(String[] args) throws Exception {
        Map<Character, Method> methodMap = new HashMap<Character, Method>();

        methodMap.put('h', Test.class.getMethod("showHelp"));
        methodMap.put('t', Test.class.getMethod("teleport"));

        char cmd = 'h';
        methodMap.get(cmd).invoke(null);  // prints "Help"

        cmd = 't';
        methodMap.get(cmd).invoke(null);  // prints "teleport"

    }

    public static void showHelp() {
        System.out.println("Help");
    }

    public static void teleport() {
        System.out.println("teleport");
    }
}

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