这可以返回一个整数列表:

public List<Integer> GetIListImpl() {
    return new ArrayList<Integer>();
}

但是如果我想让调用者指定泛型呢？像这样的东西，虽然在语法上我不确定该怎么做:

public List<T> GetIListImpl<T>() {
    return new ArrayList<T>();
}

用法是:

    List<String> = GetIListImpl<String>();

最佳答案

关于通用 static参数化类型的工厂方法

看起来您想编写方便的工厂方法来实例化泛型集合。

你可以写这样的通用方法:

public static <T> List<T> newArrayList() {
    return new ArrayList<T>();
}
public static <K,V> Map<K,V> newHashMap() {
    return new HashMap<K,V>();
}

然后你可以简单地写:

// absolutely type-safe!!! no compilation warnings at all!!!

List<String> names = newArrayList();

List<Integer> nums = newArrayList();

Map<String, List<String>> map = newHashMap();

请注意，在某些情况下，上述方法不一定是 static ，您可以选择省略实现 class从方法中命名并仅使用 interface名称(例如 newList 、 newMap )。

来自 Effective Java 2nd Edition 的认可

这种泛型类型推断 static工厂方法实际上得到了 Effective Java 2nd Edition 的认可；它有幸成为书中讨论的第一个项目。

以下是第 1 项的相关引述:考虑 static工厂方法而不是构造函数:

A fourth advantage of static factory methods is that they reduce the verbosity of creating parameterized type instances.

When you invoke the constructor of a parameterized class, unfortunately you must specify the type parameters even if they're obvious from context. This typically requires you to provide the type parameters twice in quick succession:

    Map<String,List<String>> m = 
        new HashMap<String,List<String>>();

This redundant specification quickly becomes painful as the length and complexity of the type parameters increase. With static factories, however, the compiler can figure out the type parameters for you. This is known as type inference. For example, suppose that HashMap provided this static factory:

    public static <K,V> HashMap<K,V> newInstance() {
        return new HashMap<K,V>();
    }

Then you could replace the wordy declaration above with this succinct alternative:

    Map<String,List<String>> m = HashMap.newInstance();

Unfortunately the standard collection implementations such as HashMap do not have static factory methods as of release 1.6, but you can put these methods in your own utility class. More importantly you can provide such static factories in your own parameterized classes.

该项目还规定了这些 static 的通用命名约定工厂方法:

• getInstance - returns an instance that is described by the parameters […]
• newInstance - Like getInstance, except it guarantees that each instance returned is distinct from all others.
• newType - Like newInstance, but used when the factory method is in a different class. Type indicates the type of object returned by the factory method.

关于显式类型参数

在大多数情况下，您不必显式提供类型参数，因为 Java 泛型类型推理系统通常可以找出您需要什么。

然而，要提供显式类型参数，语法是将其放在方法名称​​之前(而不是之后)。这是一个使用显式参数调用泛型方法 <T> List<T> emptyList() 的示例来自 java.util.Collections :

Collections.<String>emptyList();
// Collections.emptyList<String>(); // DOES NOT COMPILE

请注意，泛型方法调用的显式类型参数化的语法怪癖是您必须限定类型(如果 static )或您调用该方法的对象，甚至如果不是显式参数化，它们是否可以省略。

引用资料

• Angelika Langer's Java Generics FAQS
  • What is a generic method?
  • What is type argument inference?
  • What is explicit type argument specification?
• JLS 8.4.4 Generic Methods

附录:Guava 的集合工厂方法

需要注意的是Guava事实上已经提供了static Java Collections Framework 中类型的工厂方法:

来自主 package com.google.common.collect :

• Lists.newArrayList() , newLinkedList() , ……
• Sets.newHashSet() , newTreeSet() , newEnumSet(…) , ……
• Maps.newHashMap() , newTreeMap() , newEnumMap(…) , ……

事实上，本着Effective Java 2nd Edition推荐的精神，Guava自己的集合不提供public构造函数，而是提供 static create()工厂方法:

• HashMultiSet.create() , 一个 Multiset 实现
• TreeMultimap.create() , 一个 Multimap 实现

库的其余部分还提供了许多非常有用的功能。

关于Java:泛型语法，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/3542139/