Java HashMap的工作原理

面试的时候经常会遇见诸如：“java中的HashMap是怎么工作的”，“HashMap的get和put内部的工作原理”这样的问题。本文将用一个简单的例子来解释下HashMap内部的工作原理。首先我们从一个例子开始，而不仅仅是从理论上，这样，有助于更好地理解，然后，我们来看下get和put到底是怎样工作的。

我们来看个非常简单的例子。有一个”国家”(Country)类，我们将要用Country对象作为key，它的首都的名字（String类型）作为value。下面的例子有助于我们理解key-value对在HashMap中是如何存储的。

1.?Country.java

<span class="hljs-keyword">long population;

<span class="hljs-function"><span class="hljs-keyword"><span class="hljs-function"><span class="hljs-keyword">public<span class="hljs-function"> <span class="hljs-title"><span class="hljs-function"><span class="hljs-title">Country<span class="hljs-params"><span class="hljs-function"><span class="hljs-params">(String name,<span class="hljs-keyword"><span class="hljs-function"><span class="hljs-params"><span class="hljs-keyword">long<span class="hljs-function"><span class="hljs-params"> population)<span class="hljs-function"> {

<span class="hljs-keyword">super();

<span class="hljs-keyword">this.name = name;

<span class="hljs-keyword">this.population = population;

}

<span class="hljs-function"><span class="hljs-keyword"><span class="hljs-function"><span class="hljs-keyword">public<span class="hljs-function"> String <span class="hljs-title"><span class="hljs-function"><span class="hljs-title">getName<span class="hljs-params"><span class="hljs-function"><span class="hljs-params">()<span class="hljs-function"> {

<span class="hljs-keyword">return name;

}

<span class="hljs-function"><span class="hljs-keyword"><span class="hljs-function"><span class="hljs-keyword">public<span class="hljs-function"> <span class="hljs-keyword"><span class="hljs-function"><span class="hljs-keyword">void<span class="hljs-function"> <span class="hljs-title"><span class="hljs-function"><span class="hljs-title">setName<span class="hljs-params"><span class="hljs-function"><span class="hljs-params">(String name)<span class="hljs-function"> {

<span class="hljs-keyword">this.name = name;

}

<span class="hljs-function"><span class="hljs-keyword"><span class="hljs-function"><span class="hljs-keyword">public<span class="hljs-function"> <span class="hljs-keyword"><span class="hljs-function"><span class="hljs-keyword">long<span class="hljs-function"> <span class="hljs-title"><span class="hljs-function"><span class="hljs-title">getPopulation<span class="hljs-params"><span class="hljs-function"><span class="hljs-params">()<span class="hljs-function"> {

<span class="hljs-keyword">return population;

}

<span class="hljs-function"><span class="hljs-keyword"><span class="hljs-function"><span class="hljs-keyword">public<span class="hljs-function"> <span class="hljs-keyword"><span class="hljs-function"><span class="hljs-keyword">void<span class="hljs-function"> <span class="hljs-title"><span class="hljs-function"><span class="hljs-title">setPopulation<span class="hljs-params"><span class="hljs-function"><span class="hljs-params">(<span class="hljs-keyword"><span class="hljs-function"><span class="hljs-params"><span class="hljs-keyword">long<span class="hljs-function"><span class="hljs-params"> population)<span class="hljs-function"> {

<span class="hljs-keyword">this.population = population;

}
<span class="hljs-comment">// If length of name in country object is even then return 31(any random number) and if odd then return 95(any random number).

<span class="hljs-comment">// This is not a good practice to generate hashcode as below method but I am doing so to give better and easy understanding of hashmap.

<span class="hljs-meta">@Override

<span class="hljs-function"><span class="hljs-keyword"><span class="hljs-function"><span class="hljs-keyword">public<span class="hljs-function"> <span class="hljs-keyword"><span class="hljs-function"><span class="hljs-keyword">int<span class="hljs-function"> <span class="hljs-title"><span class="hljs-function"><span class="hljs-title">hashCode<span class="hljs-params"><span class="hljs-function"><span class="hljs-params">()<span class="hljs-function"> {

<span class="hljs-keyword">if(<span class="hljs-keyword">this.name.length()%<span class="hljs-number">2==<span class="hljs-number">0)

<span class="hljs-keyword">return <span class="hljs-number">31;

<span class="hljs-keyword">else

<span class="hljs-keyword">return <span class="hljs-number">95;

}

<span class="hljs-meta">@Override

<span class="hljs-function"><span class="hljs-keyword"><span class="hljs-function"><span class="hljs-keyword">public<span class="hljs-function"> <span class="hljs-keyword"><span class="hljs-function"><span class="hljs-keyword">boolean<span class="hljs-function"> <span class="hljs-title"><span class="hljs-function"><span class="hljs-title">equals<span class="hljs-params"><span class="hljs-function"><span class="hljs-params">(Object obj)<span class="hljs-function"> {

                                                
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