比較器Comparator的底層結構淺談
阿新 • • 發佈:2018-12-23
查API可知,介面 Comparator<T>存在於java.util包中,可以知道此介面所處的包是非常實用,我們的比較器見名知意,肯定是用於比較的,在Java1.6的API中有這麼一句話:public interface Comparator<T>強行對某個collection 進行整體排序 的比較函式,這句話讀起來有點繞口,但我們可以知道這個介面是針對於collection的並且對其元素進行比較。我們來看看Comparator<T>的底層:
public interface Comparator<T> { /** * Compares its two arguments for order. Returns a negative integer, * zero, or a positive integer as the first argument is less than, equal * to, or greater than the second.<p> * * In the foregoing description, the notation * <tt>sgn(</tt><i>expression</i><tt>)</tt> designates the mathematical * <i>signum</i> function, which is defined to return one of <tt>-1</tt>, * <tt>0</tt>, or <tt>1</tt> according to whether the value of * <i>expression</i> is negative, zero or positive.<p> * * The implementor must ensure that <tt>sgn(compare(x, y)) == * -sgn(compare(y, x))</tt> for all <tt>x</tt> and <tt>y</tt>. (This * implies that <tt>compare(x, y)</tt> must throw an exception if and only * if <tt>compare(y, x)</tt> throws an exception.)<p> * * The implementor must also ensure that the relation is transitive: * <tt>((compare(x, y)>0) && (compare(y, z)>0))</tt> implies * <tt>compare(x, z)>0</tt>.<p> * * Finally, the implementor must ensure that <tt>compare(x, y)==0</tt> * implies that <tt>sgn(compare(x, z))==sgn(compare(y, z))</tt> for all * <tt>z</tt>.<p> * * It is generally the case, but <i>not</i> strictly required that * <tt>(compare(x, y)==0) == (x.equals(y))</tt>. Generally speaking, * any comparator that violates this condition should clearly indicate * this fact. The recommended language is "Note: this comparator * imposes orderings that are inconsistent with equals." * * @param o1 the first object to be compared. * @param o2 the second object to be compared. * @return a negative integer, zero, or a positive integer as the * first argument is less than, equal to, or greater than the * second. * @throws NullPointerException if an argument is null and this * comparator does not permit null arguments * @throws ClassCastException if the arguments' types prevent them from * being compared by this comparator. */
我們可以清晰的看到,底層是基於sgn函式進行計算的,由於sgn(x)函式在x大於0的時候為1,等於0的時候為0,小於0的時候為-1,因此能夠保證比較的三種狀態(大於、等於、小於)。
Comparator的常見功能是用於排序,但是這個介面不僅僅用於排序,因為API中清楚第介紹了它的兩種方法:compare和equals,如下表所示:
int | compare(T o1, T o2) | 比較用來排序的兩個引數 |
boolean | equals(object obj) | 指示某個其他物件是否“等於”此 Comparator |
很清晰地可見,這兩個方法的引數,但是我們並不知道它們是如何作用的,過程是怎麼樣的,下面我們就來看看這兩個方法在底層是怎麼實現的。
對於int compare(T o1, T o2)而言,進入compare的底層可以發現:
int compare(T o1, T o2); /** * Indicates whether some other object is "equal to" this * comparator. This method must obey the general contract of * {@link Object#equals(Object)}. Additionally, this method can return * <tt>true</tt> <i>only</i> if the specified object is also a comparator * and it imposes the same ordering as this comparator. Thus, * <code>comp1.equals(comp2)</code> implies that <tt>sgn(comp1.compare(o1, * o2))==sgn(comp2.compare(o1, o2))</tt> for every object reference * <tt>o1</tt> and <tt>o2</tt>.<p> * * Note that it is <i>always</i> safe <i>not</i> to override * <tt>Object.equals(Object)</tt>. However, overriding this method may, * in some cases, improve performance by allowing programs to determine * that two distinct comparators impose the same order. * * @param obj the reference object with which to compare. * @return <code>true</code> only if the specified object is also * a comparator and it imposes the same ordering as this * comparator. * @see Object#equals(Object) * @see Object#hashCode() */
對於equals(object obj)而言,進入equals(object obj)的底層可以發現:
boolean equals(Object obj);
/**
* Returns a comparator that imposes the reverse ordering of this
* comparator.
*
* @return a comparator that imposes the reverse ordering of this
* comparator.
* @since 1.8
*/