这些初始化HashMap的方式有什么区别？ [英] What's the difference between these ways of initializing a HashMap?

问题描述

我为我的程序使用了一个HashMap，它工作正常，但我不明白这些HashMap的初始化之间的区别。

假设我正在实施一个字符作为键和一个整数作为值的HashMap。
$ b

  HashMap< Character，Integer> alphabet1 =新的HashMap（）; 
 HashMap<字符，整数> alphabet1 = new HashMap< Character，Integer>（）; 
 HashMap alphabet1 = new HashMap< Character，Integer>（）; 
 Map alphabet1 = new HashMap< Character，Integer>（）; 
 HashMap alphabet1 = new HashMap< Character，Integer>（）; 
 HashMap alphabet1 = new HashMap（）; 
 Map alphabet1 = new HashMap（）; 
  

解决方案

涉及 HashMap 或 Map 没有类型参数（尖括号<和>和它们之间的部分）是绘制类型，不应该使用。原始类型不是通用的，可以让你做不安全的事情。

正确的方式是

 映射< Character，Integer> alphabet1 = new HashMap< Character，Integer>（）; 
 HashMap<字符，整数> alphabet1 = new HashMap< Character，Integer>（）; 
  

第一个使用接口Map作为参考类型。 它通常更习惯和风格很好。 p>

另外一种你没有提到的方式，

  Map<字符，整数> ; alphabet1 = new HashMap<>（）; 
 HashMap<字符，整数> alphabet1 = new HashMap<>（）; 
  

这或多或少与前两种正确的方式相同。左侧参考类型的参数隐式提供给右侧的构造函数。

I used a HashMap for my program and it works fine, but I don't understand the difference between these initializations of HashMap.

Let's say I'm implementing a HashMap with a character as a key and an integer as a value. What's the difference between these?

HashMap<Character, Integer> alphabet1 = new HashMap();
HashMap<Character, Integer> alphabet1 = new HashMap<Character, Integer>();
HashMap alphabet1 = new HashMap<Character, Integer>();
Map alphabet1 = new HashMap<Character, Integer>();
HashMap alphabet1 = new HashMap<Character, Integer>();
HashMap alphabet1 = new HashMap();
Map alphabet1 = new HashMap();

解决方案

Anything involving HashMap or Map without a type argument (the angle brackets < and > and the part between them) is a raw type and shouldn't be used. A raw type is not generic and lets you do unsafe things.

The "correct" ways are

Map<Character, Integer> alphabet1 = new HashMap<Character, Integer>();
HashMap<Character, Integer> alphabet1 = new HashMap<Character, Integer>();

The first uses the interface Map as the reference type. It is generally more idiomatic and a good style.

Also another way you did not mention, using the Java 7 diamond operator

Map<Character, Integer> alphabet1 = new HashMap<>();
HashMap<Character, Integer> alphabet1 = new HashMap<>();

Which is more or less equivalent to the first two correct ways. The arguments to the reference type on the left-hand side are supplied implicitly to the constructor on the right-hand side.

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