这是来自Bruce Eckel的“Thinking in Java”的仿制药示例吗？错误？ [英] Is this generics example from Bruce Eckel's "Thinking in Java" wrong?

问题描述

我正在阅读Thinking in java中的泛型章节。该计划如下。

 公共类GenericWriting {
 static< T> void writeExact（List< T> list，T item）{
 list.add（item）; 
} 
静态列表< Apple> apples = new ArrayList< Apple>（）; 
静态列表< Fruit> fruit = new ArrayList< Fruit>（）; 
 static void f1（）{
 writeExact（apples，new Apple（））; 
 // writeExact（fruit，new Apple（））; //错误：------------------第1行
 //不兼容的类型：找到水果，必需Apple 
} 
 static< T> void writeWithWildcard（List<？super T> list，T item）{
 list.add（item）; 
} 
 static void f2（）{
 writeWithWildcard（apples，new Apple（））; 
 writeWithWildcard（fruit，new Apple（））; 
} 
 public static void main（String [] args）{f1（）; F2（）; } 
} 
  

下面写着writeExact（）方法使用精确的参数类型（没有通配符）。在f1（）中你可以看到这个工作正常 - 只要你把Apple放到 List< Apple> 中。但是，writeExact（）会不允许您将Apple放入列表< Fruit> ，即使您知道应该可以。

<但是当我取消注释第1行并执行它时，它工作正常。任何人都可以在我出错的地方帮助我吗？

解决方案

然而， writeExact（）不允许您将 Apple 放入列表< Fruit> ，即使你知道这应该是可能的。

这是不正确的。 writeExact 可以将 Apple 放入列表< Fruit> 。 < T> 被推断为水果和 Apple （推测）是 Fruit 的子类型。

该书包含错误。我可以确认，这将在Java 5中编译，当引入泛型时。

I am reading the generics chapter in "Thinking in java". The program is there below.

public class GenericWriting {
    static <T> void writeExact(List<T> list, T item) {
        list.add(item);
    }
    static List<Apple> apples = new ArrayList<Apple>();
    static List<Fruit> fruit = new ArrayList<Fruit>();
    static void f1() {
        writeExact(apples, new Apple());
        // writeExact(fruit, new Apple()); // Error:------------------line 1
        // Incompatible types: found Fruit, required Apple
    }
    static <T> void writeWithWildcard(List<? super T> list, T item) {
        list.add(item);
    }
    static void f2() {
        writeWithWildcard(apples, new Apple());
        writeWithWildcard(fruit, new Apple());
    }
    public static void main(String[] args) { f1(); f2(); }
}

Below it says "The writeExact( ) method uses an exact parameter type (no wildcards). In f1( ) you can see that this works fine—as long as you only put an Apple into a List<Apple>. However, writeExact( ) does not allow you to put an Apple into a List<Fruit>, even though you know that should be possible."

But when I uncomment the line 1 and execute it, it is working fine. Can anyone please help me where I went wrong?

解决方案

However, writeExact( ) does not allow you to put an Apple into a List<Fruit>, even though you know that should be possible.

This is incorrect. writeExact can put an Apple in to a List<Fruit>. <T> is inferred to be Fruit and Apple (presumably) is a subtype of Fruit.

The book contains an error. I can confirm that this will compile as of Java 5, when generics were introduced.

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