通用类中的java通用转换 [英] java generic casting in generic classes

问题描述

我在写一个通用的最大堆的简单实现。如果我写了

  public class FastMaxHeap< T> {
 
 T [] data; 
 int size; 
 
 static final int HEAP_SIZE = 10000; 
 
 @SuppressWarnings（unchecked）
 public FastMaxHeap（）{
 data =（T []）new Object [HEAP_SIZE]; 
 size = 0; 
} 
} 
  

编译。现在要实际实现堆，即写入maxHeapify（），我需要能够比较两个T。先验似乎是可能的一种选择是告诉编译器T实现了Comparable。但是，如果我键入替换< T>与< T实现Comparable>编译器抱怨 - 我该怎么做？

或者，我可以定义一个类

  public class HasValue {
 
 int value; 
 
公共HasValue（int值）{
 this.value = value; 
} 
  

}

理论上我应该能够比较两个HasValue对象，如x.value> y.value。但是如果我输入

  public class FastMaxHeap< T extends HasValue> {
 
 T [] data; 
 int size; 
 
 static final int HEAP_SIZE = 10000; 
 
 @SuppressWarnings（unchecked）
 public FastMaxHeap（）{
 data =（T []）new Object [HEAP_SIZE]; 
 size = 0; 
} 
} 
  

现在我得到一个ClassCastException。这里发生了什么？ Java泛型伤害了我的大脑。

解决方案

在第一种情况下 T extends Object 在运行时擦除为 Object 。在第二种情况下 T扩展了HasValue 被清除为 HasValue ，因此您需要拥有。

  data =（T []）new HasValue [HEAP_SIZE]; 
  

恕我直言，Java是不允许 new T [HEAP_SIZE ] 去做你必须做的事情。

I'm writing a simple implementation of a generic max heap. If I write

public class FastMaxHeap<T>{

  T[] data;
  int size;

  static final int HEAP_SIZE = 10000;

  @SuppressWarnings("unchecked")
  public FastMaxHeap(){
    data = (T[]) new Object[HEAP_SIZE];
  size = 0;
  }
}

it compiles. Now to actually implement the heap, i.e. write maxHeapify(), I need to be able to compare two T's. One option that a priori seems possible would be to tell the compiler that T implements Comparable. But if I type replace < T > with < T implements Comparable > the compiler complains -- how can I do this?

Alternatively, I could define a class

public class HasValue{

  int value;

  public HasValue(int value){
        this.value = value;
  }

}

and in theory I should then be able to compare two HasValue objects like x.value > y.value. But if I type

public class FastMaxHeap<T extends HasValue>{

  T[] data;
  int size;

  static final int HEAP_SIZE = 10000;

  @SuppressWarnings("unchecked")
  public FastMaxHeap(){
    data = (T[]) new Object[HEAP_SIZE];
  size = 0;
  }
}

I now get a ClassCastException. What is going on here? Java generics hurt my brain.

解决方案

In the first case T extends Object which is erased to Object at runtime.

In the second case T extends HasValue is erased to HasValue so you need to have.

data = (T[]) new HasValue[HEAP_SIZE];

IMHO It is needlessly pedantic that Java doesn't allow new T[HEAP_SIZE] to do what you have to do anyway.

这篇关于通用类中的java通用转换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！