Comparable与Comparator浅析

转载自 http://blog.csdn.net/u013256816/article/details/50899416

今天博主在翻阅TreeMap的源码，发现其键必须是实现Comparable或者Comparator的接口时产生了一些兴趣，比如在TreeMap中的put方法分别对Comparable和Comparator接口分别进行处理。那么疑问就来了，Comparable和Comparator接口的区别是什么，Java中为什么会存在两个类似的接口？

  Comparable和Comparator接口都是用来比较大小的，首先来看一下Comparable的定义：

<code class="hljs cs has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"> package java.lang;
import java.util.*;
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">interface</span> Comparable<T> {
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> <span class="hljs-title" style="box-sizing: border-box;">compareTo</span>(T o);
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li></ul>

  Comparator的定义如下：

<code class="hljs cs has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">package java.util;
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">interface</span> Comparator<T> {
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> compare(T o1, T o2);
    boolean equals(Object obj);
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li></ul>

  Comparable对实现它的每个类的对象进行整体排序。这个接口需要类本身去实现（这句话没看懂？没关系，接下来看个例子就明白了）。若一个类实现了Comparable 接口，实现 Comparable 接口的类的对象的 List 列表 ( 或数组)可以通过 Collections.sort（或 Arrays.sort）进行排序。此外，实现 Comparable 接口的类的对象 可以用作 “有序映射 ( 如 TreeMap)” 中的键或 “有序集合 (TreeSet)” 中的元素，而不需要指定比较器。 
  举例（类Person1实现了Comparable接口）

<code class="hljs axapta has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">package collections;

<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-class" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">class</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">Person1</span> <span class="hljs-inheritance" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">implements</span></span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">Comparable</span><<span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">Person1</span>>
{</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> age;
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> String name;

    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> Person1(String name, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> age)
    {
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>.name = name;
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>.age = age;
    }
    @Override
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> compareTo(Person1 o)
    {
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>.age-o.age;
    }
    @Override 
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> String toString()
    {
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> name+<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">":"</span>+age;
    }
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li></ul>

  可以看到Person1实现了Comparable接口中的compareTo方法。实现Comparable接口必须修改自身的类，即在自身类中实现接口中相应的方法。 
  测试代码：

<code class="hljs php has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">        Person1 person1 = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> Person1(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"zzh"</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">18</span>);
        Person1 person2 = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> Person1(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"jj"</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">17</span>);
        Person1 person3 = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> Person1(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"qq"</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">19</span>);

        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">List</span><Person1> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">list</span> = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> ArrayList<>();
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">list</span>.add(person1);
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">list</span>.add(person2);
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">list</span>.add(person3);

        System.out.println(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">list</span>);
        Collections.sort(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">list</span>);
        System.out.println(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">list</span>);</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li></ul>

  输出结果：

<code class="hljs json has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">[zzh:<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">18</span>, jj:<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">17</span>, qq:<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">19</span>]
[jj:<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">17</span>, zzh:<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">18</span>, qq:<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">19</span>]</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li></ul>

  如果我们的这个类无法修改，譬如String，我们又要兑取进行排序，当然String中已经实现了Comparable接口，如果单纯的用String举例就不太形象。对类自身无法修改这就用到了Comparator这个接口（策略模式）。

<code class="hljs java has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> <span class="hljs-class" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">class</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">Person2</span>
{</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> age;
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> String name;

    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-title" style="box-sizing: border-box;">Person2</span>(String name, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> age)
    {
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>.name = name;
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>.age = age;
    }

    <span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@Override</span> 
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> String <span class="hljs-title" style="box-sizing: border-box;">toString</span>()
    {
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> name+<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">":"</span>+age;
    }

    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//getter and setter方法省略....</span>
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li></ul>

  如类Person2，这个类已经固定，无法进行对其类自身的修改，也修饰词final了，你也别想继承再implements Comparable，那么此时怎么办呢？在类的外部使用Comparator的接口。如下测试代码：

<code class="hljs cs has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">        Person2 p1 = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> Person2(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"zzh"</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">18</span>);
        Person2 p2 = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> Person2(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"jj"</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">17</span>);
        Person2 p3 = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> Person2(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"qq"</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">19</span>);
        List<Person2> list2 = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> ArrayList<Person2>();
        list2.add(p1);
        list2.add(p2);
        list2.add(p3);
        System.<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">out</span>.println(list2);
        Collections.sort(list2,<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> Comparator<Person2>(){

            @Override
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> <span class="hljs-title" style="box-sizing: border-box;">compare</span>(Person2 o1, Person2 o2)
            {
                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span>(o1 == <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">null</span> || o2 == <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">null</span>)
                    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>;
                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> o1.getAge()-o2.getAge();
            }

        });
        System.<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">out</span>.println(list2);</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li></ul>

  输出结果：

<code class="hljs json has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">[zzh:<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">18</span>, jj:<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">17</span>, qq:<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">19</span>]
[jj:<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">17</span>, zzh:<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">18</span>, qq:<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">19</span>] </code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li></ul>

  这里（public static <T> void sort(List<T> list, Comparator<? super T> c) ）采用了内部类的实现方式，实现compare方法，对类Person2的list进行排序。 
  再譬如博主遇到的真实案例中，需要对String进行排序，且不区分大小写，我们知道String中的排序是字典排序，譬如：A a D排序之后为A D a，这样显然不对，那么该怎么办呢？同上（下面代码中的list是一个String的List集合）：

<code class="hljs java has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">        Collections.sort(list, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> Comparator<String>()
        {
            <span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@Override</span>
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> <span class="hljs-title" style="box-sizing: border-box;">compare</span>(String o1, String o2)
            {
                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span>(o1 == <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">null</span> || o2 == <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">null</span>)
                    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>;
                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> o1.toUpperCase().compareTo(o2.toUpperCase());
            }
        });</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li></ul>

  这样就可以实现不区分大小进行排序String的集合了，是不是很方便~

  细心的同学可能会有疑问，明明在Comparator接口中定义了两个方法，为什么继承的时候只实现了一个方法，难道要颠覆我对Java接口常识的理解了嚒？ 
  实际上，我们知道当一个类没有显式继承父类的时候，会有一个默认的父类，即java.lang.Object，在Object类中有一个方法即为equals方法，所以这里并不强制要求实现Comparator接口的类要实现equals方法，直接调用父类的即可，虽然你显式的实现了equals()方法 will be a better choice~

  在《Effective Java》一书中，作者Joshua Bloch推荐大家在编写自定义类的时候尽可能的考虑实现一下Comparable接口，一旦实现了Comparable接口，它就可以跟许多泛型算法以及依赖于改接口的集合实现进行协作。你付出很小的努力就可以获得非常强大的功能。 
  事实上，Java平台类库中的所有值类都实现了Comparable接口。如果你正在编写一个值类，它具有非常明显的内在排序关系，比如按字母顺序、按数值顺序或者按年代顺序，那你就应该坚决考虑实现这个接口。 
  compareTo方法不但允许进行简单的等同性进行比较，而且语序执行顺序比较，除此之外，它与Object的equals方法具有相似的特征，它还是一个泛型。类实现了Comparable接口，就表明它的实例具有内在的排序关系，为实现Comparable接口的对象数组进行排序就这么简单： Arrays.sort(a); 
  对存储在集合中的Comparable对象进行搜索、计算极限值以及自动维护也同样简单。列如，下面的程序依赖于String实现了Comparable接口，它去掉了命令行参数列表中的重复参数，并按字母顺序打印出来：

<code class="hljs cs has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">class</span> WordList{
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">static</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">main</span>(String args[]){
        Set<String> s = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> TreeSet<String>();
        Collections.addAll(s,args);
        System.<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">out</span>.println(s);
    }
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li></ul>

  Comparable 是排序接口；若一个类实现了 Comparable 接口，就意味着 “该类支持排序”。而 Comparator 是比较器；我们若需要控制某个类的次序，可以建立一个 “该类的比较器” 来进行排序。 
  前者应该比较固定，和一个具体类相绑定，而后者比较灵活，它可以被用于各个需要比较功能的类使用。可以说前者属于 “静态绑定”，而后者可以 “动态绑定”。 
  我们不难发现：Comparable 相当于 “内部比较器”，而 Comparator 相当于 “外部比较器”。