java对象对比之comparable和comparator的区别

一、元素的比较

1.1 基本类型的比较

java中的基本类型的对象是可以进行比较的
如

public static void main(String[] args){
        int a = 10;
        int b = 20;
        System.out.println(a>b);
        System.out.println(a==b);
        System.out.println(a<b);

        char c1 = 'A';
        char c2 = 'B';
        System.out.println(c1>c2);
        System.out.println(c1==c2);
        System.out.println(c1<c2);

        boolean b1 = true;
        boolean b2 =false;
        System.out.println(b1==b2);
        System.out.println(b1!=b2);
    }

1.2 对象的比较

public class Main{
    public static void main(String[] args){
       Card c1 = new Card(1,"♠");
        Card c2 = new Card(2,"♠");
        Card c3 = c1;
        System.out.println(c1==c2);
        System.out.println(c1==c3);
//        System.out.println(c1>c2);  编译报错
//        System.out.println(c1<c2);  编译报错
    }

}
class  Card{
    public int rank;
    public String suit;
     public Card(int rank,String suit){
         this.rank = rank;
         this.suit = suit;
     }
}

可以看出在进行相等比较时，是可以进行比较的，但进行大于或小于比较就不行了
这是因为对于用户实现自定义类型，都默认继承自Object类，而Object类中提供了equal方法，而==默认情况下调用的就是equal方法，但是该方法的比较规则是：没有比较引用变量引用对象的内容，而是直接比较引用变量的地址，但有些情况下该种比较就不符合题意。

二、对象的比较

有些情况下，需要比较的是对象中的内容，比如：向优先级队列中插入某个对象时，需要对按照对象中内容来调整堆，那该如何处理呢？

2.1 覆写基类的equal

一般覆写 equals 的套路就是上面演示的

1.如果指向同一个对象，返回 true

2.如果传入的为 null，返回 false

3.如果传入的对象类型不是 Card，返回 false

4.按照类的实现目标完成比较，例如这里只要花色和数值一样，就认为是相同的牌

5.注意下调用其他引用类型的比较也需要 equals，例如这里的 suit 的比较
覆写基类equal的方式虽然可以比较，但缺陷是：equal只能按照相等进行比较，不能按照大于、小于的方式进行比较。

2.2 基于Comparable接口的比较

Comparble是JDK提供的泛型的比较接口类，源码实现具体如下：

public interface Comparable<T> {
    /**
     * Compares this object with the specified object for order.  Returns a
     * negative integer, zero, or a positive integer as this object is less
     * than, equal to, or greater than the specified object.
     *
     * <p>The implementor must ensure <tt>sgn(x.compareTo(y)) ==
     * -sgn(y.compareTo(x))</tt> for all <tt>x</tt> and <tt>y</tt>.  (This
     * implies that <tt>x.compareTo(y)</tt> must throw an exception iff
     * <tt>y.compareTo(x)</tt> throws an exception.)
     *
     * <p>The implementor must also ensure that the relation is transitive:
     * <tt>(x.compareTo(y)&gt;0 &amp;&amp; y.compareTo(z)&gt;0)</tt> implies
     * <tt>x.compareTo(z)&gt;0</tt>.
     *
     * <p>Finally, the implementor must ensure that <tt>x.compareTo(y)==0</tt>
     * implies that <tt>sgn(x.compareTo(z)) == sgn(y.compareTo(z))</tt>, for
     * all <tt>z</tt>.
     *
     * <p>It is strongly recommended, but <i>not</i> strictly required that
     * <tt>(x.compareTo(y)==0) == (x.equals(y))</tt>.  Generally speaking, any
     * class that implements the <tt>Comparable</tt> interface and violates
     * this condition should clearly indicate this fact.  The recommended
     * language is "Note: this class has a natural ordering that is
     * inconsistent with equals."
     *
     * <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.
     *
     * @param   o the object to be compared.
     * @return  a negative integer, zero, or a positive integer as this object
     *          is less than, equal to, or greater than the specified object.
     *
     * @throws NullPointerException if the specified object is null
     * @throws ClassCastException if the specified object's type prevents it
     *         from being compared to this object.
     */
    public int compareTo(T o);
}

可以看到在Comparable接口中只实现了一个方法 compareTo,因此我们在实现自定义比较时，在类的定义中实现Comparable接口即可，然后在类中重写compareTo方法

public class Main{
    public static void main(String[] args){
        Card c1 = new Card(1,"♠");
        Card c2 = new Card(2,"♠");
        Card c3 = c1;
        System.out.println(c1.compareTo(c2));
        System.out.println(c1.compareTo(c3));
        System.out.println(c2.compareTo(c3));

    }

}
class  Card implements Comparable<Card>{
    public int rank;
    public String suit;
     public Card(int rank,String suit){
         this.rank = rank;
         this.suit = suit;
     }

    @Override
    public int compareTo(Card o) {
        if(o==null){
            return 1;
        }
        return rank-o.rank;
    }
}

当前值比要比较值小则输出-1；当前值与要比较值相等则输出0；
当前值比要比较值大输出1；

2.3 基于比较器Comparator的比较

首先了解一下Comparator接口

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.
     */
    int compare(T o1, T o2);

当然还有许多comparator实现的自定义比较方法，但这里我只贴出需要自己实现的方法compare；
接下来看看comparator的用法
当使用comparator时，如果要使用自定义的比较方式需要实现comparator接口，并且覆写compare方法；因此需要自己构造一个比较器类实现comparator接口，然后利用我们自定义的比较器进行比较即可；
下面是一个应用实例

// write your code here
import java.util.*;
import java.lang.*;

public class Main{
    public static void main(String[] args){
        Card c1 = new Card(1,"♠");
        Card c2 = new Card(2,"♠");
        Card c3 = c1;
        CardComparator cardComparator = new CardComparator();
        System.out.println(cardComparator.compare(c1,c2));
        System.out.println(cardComparator.compare(c1,c3));
        System.out.println(cardComparator.compare(c2,c3));
    }
}
class  Card {
    public int rank;
    public String suit;
     public Card(int rank,String suit){
         this.rank = rank;
         this.suit = suit;
     }
}
class  CardComparator implements Comparator<Card>{

    @Override
    public int compare(Card o1, Card o2) {
        if (o1==o2){
            return 0;
        }
        if (o1==null)return -1;
        if (o2==null)return 1;
        return o1.rank-o2.rank;
    }
}

Comparator属于java.util包中泛型接口类，使用时必须导入相关的包；
我们将Comparator中的compare方法重写，就可以对需要进行对比的对象进行对比并返回结果。

2.4 几种不同的compare对比

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