如何在java中创建一个通用的单例类？ [英] how to create a generic singleton class in java?

问题描述

我想在java巫婆中创建一个通用的单例类，给出我通过方法参数传入的类的单例对象。如下面的代码：请帮助

  public final class Listener< T extends Object> {
 private Listener（）{
} 
 
 public static< T>听者LT;？> getInstance（Class< T> clazz）{
返回SingletonHolder.INSTANCE; 
 
 
 private static class SingletonHolder {
 public static final Listener INSTANCE = new Listener（）; 
 //私有静态最终地图< Class<扩展可运行>，监听器<？扩展
 // Runnable> INSTANCE = new ...; 
 
 
 $ / code> 




解决方案

Make一个枚举只有一个值， INSTANCE 等。即时单身。

如果我理解正确，我想你希望你的单例实现一些通用接口？例如：

  interface ListenerInterface< T> {
 void handleEvent（T event）; 
 
 
 
 
 
 $ b在这种情况下，你可以使用未经检查的转换来返回你的enum-as - 以一种通用的方式。这是安全的，只要你的单例没有实际使用实例（或者，只假设它是一个Object）。
 
 
  public enum MySingleton实现了ListenerInterface< Object> {
 INSTANCE; 
 
 @SuppressWarnings（unchecked）
 public static< T> ListenerInterface< T> getListener（）{
 return（ListenerInterface< T>）INSTANCE; 
} 
 
 public void handleEvent（Object event）{
 System.out.println（我是一个单身人士，一个事件发生了：+ event）; 
} 
 
} 
  
如果界面是生产者的消费者 - 也就是说，它返回一个 T   - 那么您只能安全地返回 null 。否则，当其他人尝试使用他们的 T 时，他们会认为是Foo，但实际上只是一个Object。
 
i want to create a generic singleton class in java witch gives singleton object of class which i pass in method parameter. some thing like below code : please help
public final class Listener<T extends Object>  {
    private Listener() {
    }

    public static <T> Listener<?> getInstance(Class<T> clazz) {
        return SingletonHolder.INSTANCE;
    }

    private static class SingletonHolder {
        public static final Listener INSTANCE = new Listener();
        // private static final Map<Class<? extends Runnable>,Listener<? extends
        // Runnable> INSTANCE = new ...;
    }
}

解决方案
Make an enum with just one value, INSTANCE or such. Instant singleton.

If I understand you correctly, I think you want your singleton to implement some generic interface? Something like:
interface ListenerInterface<T> {
    void handleEvent(T event);
}
In that case, you can use an unchecked cast to return your enum-as-singleton in a generified way. This is safe as long as your singleton doesn't actually use the instance (or else, only assume it's an Object).
public enum MySingleton implements ListenerInterface<Object> {
    INSTANCE;

    @SuppressWarnings("unchecked")
    public static <T> ListenerInterface<T> getListener() {
        return (ListenerInterface<T>) INSTANCE;
    }

    public void handleEvent(Object event) {
        System.out.println("I am a singleton. An event happened: " + event);
    }

}
If the interface is a producer instead of consumer -- that is, it returns a T -- then you can only return null safely. Otherwise, someone else will get a ClassCastException when they try to use their T, which they think is a Foo but is actually just an Object.

                        
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