Comparable介面源碼分析_ZenDei技術網路在線

這一介面會對實現了它的類施加一個整體的順序.這一順序被認為是類的自然順序,類的比較方法compareTo()也被認為是自然比較方法實現這一介面的對象中,List類對象使用Collections.sort方法實現自動排序(升序),數組使用Arrays.sort()方法實現升序排序.實現這一介面的對象 ...

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  */

這一介面會對實現了它的類施加一個整體的順序.這一順序被認為是類的自然順序,類的比較方法compareTo()也被認為是自然比較方法

實現這一介面的對象中,List類對象使用Collections.sort方法實現自動排序(升序),數組使用Arrays.sort()方法實現升序排序.實現這一介面的對象在有序Map中被用來作為Key進行排序的;在有序Set中,是作為set集合中的元素排序的.而使用這些方法時,我們並不需要指定比較器comparator

對於類C的任意變數e1和e2,當且僅當e1.compareTo(e2) == 0的和e1.equals(e2)有相同的返回值時,類的自然排序才能被認為是和equals方法的結果保持一致的.
註意:雖然e.equals(null)返回值為false,但是null不是任何類的實例,所以如果調用方法e.compareTo(null)應該拋出異常NullPointerException

我們強烈建議(儘管並不是必須的):自然排序應該和equals結果保持一致(這是因為自然排序用到了compare方法,這裡的意思是需要滿足關係: e1.compareTo(e2) == 0的和e1.equals(e2)有相同的返回值).這是因為沒有明確比較器的有序set(和有序map) (什麼叫沒有明確比較器?對於TreeSet和TreeMap,都有多個實例構造函數,而其中有一個無參構造函數,就指定了比較器comparator = null;同時, 這也說明瞭,如果你想在建立有序set或者有序map時就指定它的排序方法,那麼可以給構造函數傳入一個比較器參數即可.),如果自然排序不能和equals方法保持一致,那麼它們會表現出一些詭異的行為.而且,這樣的有序set(或者map)和equals中通用規範是矛盾的。

舉個例子:如果向一個沒有明確比較器的有序set中添加2個值a和b(a.equals(b)值為false,而 a.compareTo(b) == 0 值為true(a==b)), 那麼第二次的add操作會失敗,因為從有序set的角度看,a和b是等值的.

實質上,所有實現了Comparable介面的java核心類,都滿足自然排序的要求.唯一的例外類是:BigDecimal類.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兩個BigDecimal對象,它們的equals方法返回值應該為true,而compare()方法應該返回0:

類C上面的等價關係,自然排序是對類C上面的元素整體的一個排序,這符合compareTo的通用規範. 當我們說一個類的自然排序是和equals結果保持一致.那就意味著自然排序是由這個類的equals方法定義的等價關係.

 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 }

將當前對象a和指定對象b進行比較.
a>b:返回正數;
a=b:返回0;
a<b:返回負數;

實現這一介面的類必須保證對於任意的x和y,都應該滿足的條件是:sgn(x.compareTo(y)) == -sgn(y.compareTo(x)) 這意味如果y.compareTo(x)拋出異常,則x.compareTo(y)必須拋出異常.

實現這個介面的類還必須保證:聯繫是具有傳遞性的: 如果: x.compareTo(y)>=0 && y.compareTo(z)>=0成立,則:x.compareTo(z)>=0也成立.

最後,實現這個的類必須保證:對於任意的z,如果x.compareTo(y)==0成立,則sgn(x.compareTo(z)) == sgn(y.compareTo(z))也成立.

我們強烈建議,但並非嚴格約定:x.compareTo(y)==0和x.equals(y)的返回值一致. 通常,任何實現了Comparable介面但違反了這一條件的類應該明確的表明這一事實. 作為這一事實的提醒語言,它可以這樣寫: 註意:這個類的自然排序並沒有和equals結果保持一致.

在前面的表述中,用到的符號sgn是數學函數signum的一種表達式,它定義了:
sgn的參數為負數,返回-1;
sgn的參數為正數,返回1;
sgn的參數為0,返回0.