将对象投射到< T>型 [英] Casting Objects to <T>-type

问题描述

我将List接口实现为将数据存储在< T> 类型数组中的类。由于集合将对象存储为 Object ，所以这引发了将Collection作为参数的方法的问题。我如何将 Object 转换为 T ？简单转换不起作用。

  class MyCollection< T>实现List< T> {
 T [] tab; 
 
 MyCollection（）{
 this.tab =（T []）new Object [10]; 
 
 
 public boolean addAll（Collection c）{
 for（Object o：c）{
 o =（T）o; 
 for（int i = 0; i< tab.length; i ++）{
 tab [i] = o; 
} 
} 
} 
 
} 
  

$ p $ public boolean addAll（Collection< ;? extends T> c）


code>

但失败：

public boolean retainAll（Collection <？> c），因为我无法在这里更改Collection类型：/ b /

  public boolean retainAll（Collection<？> c）{
布尔结果= false; （T t：c）{
} 
返回结果为
; 
} 
  

解决方案

这是正确的做法吗？

是的，那是正确的做法。这就是界面的定义（除了界面使用 E ，但没关系）。

<请注意，你的 addAll 应该返回一个布尔值。此外，你不需要投入你已经实现的 addAll 。改变你的循环：

  for（T o：c）{...} 
  

并且您的 retainAll 应该也可以，只要您返回 boolean 。

编辑：

$ c> retainAll 实现，不需要遍历传入的集合<> 并转换为 T 。考虑迭代选项卡支持数组，并查看每个实例是否包含在传入的 Collection <？>> ç。如果由于某种原因，您绝对需要使用 c 中的项目作为 T s，您可以投射。

I'm implementing List interface to a class which stores data in a <T> type array. This raises problems with methods that take Collection as parameters, since collection stores it's objects as Object. How can I transform Object to T ? Simple casting doesn't work.

class MyCollection<T> implements List<T>{
    T[] tab;

    MyCollection() {
        this.tab = (T[])new Object[10];
    }

    public boolean addAll(Collection c){
        for(Object o : c){
            o = (T)o;
            for(int i = 0; i < tab.length; i++){
                tab[i] = o;
            }
        }
    }

}

I'm trying :

public boolean addAll(Collection<? extends T> c)

but it fails with :

public boolean retainAll(Collection<?> c) since I cannot change the Collection type here :/

public boolean retainAll(Collection<?> c){
    boolean result = false;
    for(T t : c){
    }
    return result;
}

解决方案

Is it the right approach ?

Yes, that is correct approach. That is what the interface is defined as (except that the interface uses E, but that doesn't matter).

Note that your addAll should return a boolean. Also, you dont need to cast in addAll that you have implemented. Change your loop instead:

   for(T o : c){...}

And your retainAll should be fine as well, as long as you return a boolean.

EDIT:

For your retainAll implementation, there shouldn't be a need to iterate over the passed in Collection<?> and cast to a T. Consider iterating over your tab backing array and seeing if each instance is contained in the passed in Collection<?> c. If for some reason you absolutely need to use the items within c as Ts, you can cast.

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