Java 如何调用存储在 HashMap 中的方法？(爪哇)

How to call a method stored in a HashMap? (Java)

提问by cwhiii

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:

我有用户将在命令行/终端 Java 程序中输入的命令列表（i、h、t 等）。我想存储命令/方法对的散列：

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

So that I can have code something like:

这样我就可以有类似的代码：

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().

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

这可能吗？如果没有，有什么简单的方法可以做到这一点？

采纳答案by aioobe

With Java 8+ and Lambda expressions

使用 Java 8+ 和 Lambda 表达式

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

使用 lambdas（在 Java 8+ 中可用），我们可以这样做：

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"
    }
}

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

在这种情况下，我很懒惰并重用了Runnable接口，但也可以使用Command我在 Java 7 版本的答案中发明的-interface。

Also, there are alternatives to the () -> { ... }syntax. You could just as well have member functions for helpand teleportand use YourClass::helpresp. YourClass::teleportinstead.

此外，还有() -> { ... }语法的替代方案。您也可以为helpandteleport和使用YourClass::helpresp使用成员函数。YourClass::teleport反而。

• A great Lambda cheat sheetover at Programming.Guide.

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

• Programming.Guide 上有一个很棒的Lambda 备忘单。

• 此处的 Oracle 教程：Java 教程™ – Lambda 表达式。

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Java 7 and below

Java 7 及以下

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:

您真正想要做的是创建一个接口，命名为 instance Command（或重用 instance Runnable），并让您的地图的类型为Map<Character, Command>。像这样：

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"

    }
}

Reflection "hack"

反射“黑客”

With that said, you canactually do what you're asking for (using reflection and the Methodclass.)

话虽如此，您实际上可以做您所要求的（使用反射和Method类。）

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");
    }
}

回答by Erich Kitzmueller

Though you could store methods through reflection, the usual way to do it is to use anonymous objects that wrap the function, i.e.

虽然您可以通过反射存储方法，但通常的方法是使用包装函数的匿名对象，即

  interface IFooBar {
    void callMe();
  }


 'h', new IFooBar(){ void callMe() { showHelp(); } }
 't', new IFooBar(){ void callMe() { teleport(); } }

 HashTable<IFooBar> myHashTable;
 ...
 myHashTable.get('h').callMe();

回答by DVD

If you are using JDK 7 you can now use methods by lambda expression just like .net.

如果您使用的是 JDK 7，您现在可以像 .net 一样通过 lambda 表达式使用方法。

If Not the best way is to make a Function Object:

如果不是，最好的方法是创建一个函数对象：

public interface Action {    void performAction(); }

Hashmap<string,Action> cmdList;

if(!cmdList.containsKey('h'))
    System.out.print("No such command.") else    cmdList.getValue('h').performAction();