我应该如何投放具有多个边界的Java通用？ [英] How should I cast for Java generic with multiple bounds?

问题描述

是否可以将Java中的对象转换为组合的通用类型？

我有一个方法：

  public static< T extends Foo& Bar> void doSomething（T object）{
 // do stuff 
} 
  

这个方法没有问题，如果我有一个类实现两个接口（Foo& Bar）。

问题是当我需要调用这个方法我需要的对象传递给它作为 java.lang.Object 接收，我需要强制转换以使编译器愉快。

问题出在了函数如下：

  public void problemFunction（Object o）{
 if（o instanceof Foo& o instanceof Bar）{
 doSomething（（Problematic cast）o）; 
} 
} 
  

}

解决方案

不幸的是，没有法律效力，你可以做出来满足这种情况。必须有一个已知的类型来实现您需要的所有界面作为界限，以便您可以投射到它。

 接口Baz扩展Foo，Bar {} 
 
 public void caller（Object w）{
 doSomething（（Baz）w）; 
} 
  

如果已知其他类型，例如 Baz ，为了满足边界，你可以测试这些类型，并在你的调用者中有一个分支，调用 doSomething 并转换这些类型。

您还可以使用委托，创建您自己的类 Baz ，它满足 doSomething 。然后将您传递的对象包装在 Baz 类的实例中，并将该包装传递给 doSomething p>

 私人静态类FooBarAdapter实现Foo，Bar {
 private final Object adaptee; 
 FooBarAdapter（Object o）{
 adaptee =（Foo）（Bar）o; 
} 
 public int flip（）{return（（Foo）adaptee）.flip（）; } 
 public void flop（int x）{（（Foo）adaptee）.flop（x）; } 
 public void blort（）{（（Bar）adaptee）.blort（）; } 
} 
 
 public void problemFunction（Object o）{
 doSomething（new FooBarAdapter（o））; 
} 
  

Is it possible to cast an object in Java to a combined generic type?

I have a method like:

public static <T extends Foo & Bar> void doSomething(T object) {
    //do stuff
}

Calling this method is no problem if I have a class that implements both interfaces (Foo & Bar).

The problem is when I need to call this method the object I need to pass to it is received as java.lang.Object and I need to cast it to make the compiler happy. But I can't figure out how to make this cast.

edit:

The problem lies in a function like this:

public void problemFunction (Object o) {
  if ( o instanceof Foo && o instanceof Bar) {
      doSomething((Problematic cast) o);
  }
}

}

解决方案

Unfortunately, there is no legal cast that you can make to satisfy this situation. There must be a single type known to implement all of the interfaces that you need as bounds, so that you can cast to it. The might be a type you create for the purpose, or some existing type.

interface Baz extends Foo, Bar { }

public void caller(Object w) {
  doSomething((Baz) w);
}

If other types are known, like Baz, to meet the bounds, you could test for those types, and have a branch in your caller that calls doSomething with a cast to those types. It's not pretty.

You could also use delegation, creating your own class Baz that meets the bounds required by doSomething. Then wrap the object you are passed in an instance of your Baz class, and pass that wrapper to doSomething.

private static class FooBarAdapter implements Foo, Bar {
  private final Object adaptee;
  FooBarAdapter(Object o) {
    adaptee = (Foo) (Bar) o;
  }
  public int flip() { return ((Foo) adaptee).flip(); }
  public void flop(int x) { ((Foo) adaptee).flop(x); }
  public void blort() { ((Bar) adaptee).blort(); }
}

public void problemFunction (Object o) {
  doSomething(new FooBarAdapter(o));
}

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