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)&gt;0) &amp;&amp; (compare(y, z)&gt;0))</tt> implies
     * <tt>compare(x, z)&gt;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：

    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
     */