Comparable接口源码分析

1 package java.lang;
2 import java.util.*;

 1 /**
 2  * This interface imposes a total ordering on the objects of each class that
 3  * implements it.  This ordering is referred to as the class's <i>natural
 4  * ordering</i>, and the class's <tt>compareTo</tt> method is referred to as
 5  * its <i>natural comparison method</i>.<p>
 6  *
 7  * Lists (and arrays) of objects that implement this interface can be sorted
 8  * automatically by {@link Collections#sort(List) Collections.sort} (and
 9  * {@link Arrays#sort(Object[]) Arrays.sort}).  Objects that implement this
10  * interface can be used as keys in a {@linkplain SortedMap sorted map} or as
11  * elements in a {@linkplain SortedSet sorted set}, without the need to
12  * specify a {@linkplain Comparator comparator}.<p>
13  *
14  * The natural ordering for a class <tt>C</tt> is said to be <i>consistent
15  * with equals</i> if and only if <tt>e1.compareTo(e2) == 0</tt> has
16  * the same boolean value as <tt>e1.equals(e2)</tt> for every
17  * <tt>e1</tt> and <tt>e2</tt> of class <tt>C</tt>.  Note that <tt>null</tt>
18  * is not an instance of any class, and <tt>e.compareTo(null)</tt> should
19  * throw a <tt>NullPointerException</tt> even though <tt>e.equals(null)</tt>
20  * returns <tt>false</tt>.<p>
21  *
22  * It is strongly recommended (though not required) that natural orderings be
23  * consistent with equals.  This is so because sorted sets (and sorted maps)
24  * without explicit comparators behave "strangely" when they are used with
25  * elements (or keys) whose natural ordering is inconsistent with equals.  In
26  * particular, such a sorted set (or sorted map) violates the general contract
27  * for set (or map), which is defined in terms of the <tt>equals</tt>
28  * method.<p>
29  *
30  * For example, if one adds two keys <tt>a</tt> and <tt>b</tt> such that
31  * {@code (!a.equals(b) && a.compareTo(b) == 0)} to a sorted
32  * set that does not use an explicit comparator, the second <tt>add</tt>
33  * operation returns false (and the size of the sorted set does not increase)
34  * because <tt>a</tt> and <tt>b</tt> are equivalent from the sorted set's
35  * perspective.<p>
36  *
37  * Virtually all Java core classes that implement <tt>Comparable</tt> have natural
38  * orderings that are consistent with equals.  One exception is
39  * <tt>java.math.BigDecimal</tt>, whose natural ordering equates
40  * <tt>BigDecimal</tt> objects with equal values and different precisions
41  * (such as 4.0 and 4.00).<p>
42  *
43  * For the mathematically inclined, the <i>relation</i> that defines
44  * the natural ordering on a given class C is:<pre>
45  *       {(x, y) such that x.compareTo(y) &lt;= 0}.
46  * </pre> The <i>quotient</i> for this total order is: <pre>
47  *       {(x, y) such that x.compareTo(y) == 0}.
48  * </pre>
49  *
50  * It follows immediately from the contract for <tt>compareTo</tt> that the
51  * quotient is an <i>equivalence relation</i> on <tt>C</tt>, and that the
52  * natural ordering is a <i>total order</i> on <tt>C</tt>.  When we say that a
53  * class's natural ordering is <i>consistent with equals</i>, we mean that the
54  * quotient for the natural ordering is the equivalence relation defined by
55  * the class's {@link Object#equals(Object) equals(Object)} method:<pre>
56  *     {(x, y) such that x.equals(y)}. </pre><p>
57  *
58  * This interface is a member of the
59  * <a href="{@docRoot}/../technotes/guides/collections/index.html">
60  * Java Collections Framework</a>.
61  *
62  * @param <T> the type of objects that this object may be compared to
63  *
64  * @author  Josh Bloch
65  * @see java.util.Comparator
66  * @since 1.2
67  */

这一接口会对实现了它的类施加一个整体的顺序.这一顺序被认为是类的自然顺序,类的比较方法compareTo()也被认为是自然比较方法

实现这一接口的对象中,List类对象使用Collections.sort方法实现自动排序(升序),数组使用Arrays.sort()方法实现升序排序.实现这一接口的对象在有序Map中被用来作为Key进行排序的;在有序Set中,是作为set集合中的元素排序的.而使用这些方法时,我们并不需要指定比较器comparator  

对于类C的任意变量e1和e2,当且仅当e1.compareTo(e2) == 0的和e1.equals(e2)有相同的返回值时,类的自然排序才能被认为是和equals方法的 结果保持一致的.
注意:虽然e.equals(null)返回值为false,但是null不是任何类的实例,所以如果调用方法e.compareTo(null)应该抛出异常NullPointerException

我们强烈建议(尽管并不是必须的):自然排序应该和equals结果保持一致(这是因为自然排序用到了compare方法,这里的意思是需要满足关系:  e1.compareTo(e2) == 0的和e1.equals(e2)有相同的返回值).这是因为没有明确比较器的有序set(和有序map) (什么叫没有明确比较器?对于TreeSet和TreeMap,都有多个实例构造函数,而其中有一个无参构造函数,就指定了比较器comparator = null;同时,  这也说明了,如果你想在建立有序set或者有序map时就指定它的排序方法,那么可以给构造函数传入一个比较器参数即可.),如果自然排序不能和equals方法 保持一致,那么它们会表现出一些诡异的行为.而且,这样的有序set(或者map)和equals中通用规范是矛盾的。 

举个例子:如果向一个没有明确比较器的有序set中添加2个值a和b(a.equals(b)值为false,而 a.compareTo(b) == 0 值为true(a==b)), 那么第二次的add操作会失败,因为从有序set的角度看,a和b是等值的.

实质上,所有实现了Comparable接口的java核心类,都满足自然排序的要求.唯一的例外类是:BigDecimal类.它的自然排序要求是:值相等而精度是不等的. 所以,精度不同但值相同的两个BigDecimal对象,它们的equals方法返回值应该为true,而compare()方法应该返回0: 

类C上面的等价关系,自然排序是对类C上面的元素整体的一个排序,这符合compareTo的通用规范. 当我们说一个类的自然排序是和equals结果保持一致.那就意味着自然排序是由这个类的equals方法定义的等价关系.

 1 public interface Comparable<T> {
 2 /**
 3      * Compares this object with the specified object for order.  Returns a
 4      * negative integer, zero, or a positive integer as this object is less
 5      * than, equal to, or greater than the specified object.
 6      *
 7      * <p>The implementor must ensure <tt>sgn(x.compareTo(y)) ==
 8      * -sgn(y.compareTo(x))</tt> for all <tt>x</tt> and <tt>y</tt>.  (This
 9      * implies that <tt>x.compareTo(y)</tt> must throw an exception iff
10      * <tt>y.compareTo(x)</tt> throws an exception.)
11      *
12      * <p>The implementor must also ensure that the relation is transitive:
13      * <tt>(x.compareTo(y)&gt;0 &amp;&amp; y.compareTo(z)&gt;0)</tt> implies
14      * <tt>x.compareTo(z)&gt;0</tt>.
15      *
16      * <p>Finally, the implementor must ensure that <tt>x.compareTo(y)==0</tt>
17      * implies that <tt>sgn(x.compareTo(z)) == sgn(y.compareTo(z))</tt>, for
18      * all <tt>z</tt>.
19      *
20      * <p>It is strongly recommended, but <i>not</i> strictly required that
21      * <tt>(x.compareTo(y)==0) == (x.equals(y))</tt>.  Generally speaking, any
22      * class that implements the <tt>Comparable</tt> interface and violates
23      * this condition should clearly indicate this fact.  The recommended
24      * language is "Note: this class has a natural ordering that is
25      * inconsistent with equals."
26      *
27      * <p>In the foregoing description, the notation
28      * <tt>sgn(</tt><i>expression</i><tt>)</tt> designates the mathematical
29      * <i>signum</i> function, which is defined to return one of <tt>-1</tt>,
30      * <tt>0</tt>, or <tt>1</tt> according to whether the value of
31      * <i>expression</i> is negative, zero or positive.
32      *
33      * @param   o the object to be compared.
34      * @return  a negative integer, zero, or a positive integer as this object
35      *          is less than, equal to, or greater than the specified object.
36      *
37      * @throws NullPointerException if the specified object is null
38      * @throws ClassCastException if the specified object's type prevents it
39      *         from being compared to this object.
40      */
41     public int compareTo(T o);
42 }

 

将当前对象a和指定对象b进行比较.
a>b:返回正数;
a=b:返回0;
a<b:返回负数;

实现这一接口的类必须保证对于任意的x和y,都应该满足的条件是:sgn(x.compareTo(y)) == -sgn(y.compareTo(x))  这意味如果y.compareTo(x)抛出异常,则x.compareTo(y)必须抛出异常. 

实现这个接口的类还必须保证:联系是具有传递性的:  如果: x.compareTo(y)>=0 && y.compareTo(z)>=0成立,则:x.compareTo(z)>=0也成立. 

最后,实现这个的类必须保证:对于任意的z,如果x.compareTo(y)==0成立,则sgn(x.compareTo(z)) == sgn(y.compareTo(z))也成立.

我们强烈建议,但并非严格约定:x.compareTo(y)==0和x.equals(y)的返回值一致. 通常,任何实现了Comparable接口但违反了这一条件的类应该明确的表明这一事实. 作为这一事实的提醒语言,它可以这样写: 注意:这个类的自然排序并没有和equals结果保持一致.

在前面的表述中,用到的符号sgn是数学函数signum的一种表达式,它定义了:
sgn的参数为负数,返回-1;
sgn的参数为正数,返回1;
sgn的参数为0,返回0.