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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(ac2); System.out.println(c1==c2); System.out.println(c1
java对象对比之comparable和comparator的区别
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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
java对象对比之comparable和comparator的区别
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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
【java对象对比之comparable和comparator的区别】java对象对比之comparable和comparator的区别
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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 {/*** 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.** The implementor must ensure sgn(x.compareTo(y)) ==* -sgn(y.compareTo(x)) for all x and y.(This* implies that x.compareTo(y) must throw an exception iff* y.compareTo(x) throws an exception.)** The implementor must also ensure that the relation is transitive:* (x.compareTo(y)> 0 & & y.compareTo(z)> 0) implies* x.compareTo(z)> 0.** Finally, the implementor must ensure that x.compareTo(y)==0* implies that sgn(x.compareTo(z)) == sgn(y.compareTo(z)), for* all z.** It is strongly recommended, but not strictly required that* (x.compareTo(y)==0) == (x.equals(y)).Generally speaking, any* class that implements the Comparable 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."** In the foregoing description, the notation* sgn(expression) designates the mathematical* signum function, which is defined to return one of -1,* 0, or 1 according to whether the value of* expression is negative, zero or positive.** @paramo the object to be compared.* @returna 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)); }}classCard implements Comparable{public int rank; public String suit; public Card(int rank,String suit){this.rank = rank; this.suit = suit; }@Overridepublic int compareTo(Card o) {if(o==null){return 1; }return rank-o.rank; }}

java对象对比之comparable和comparator的区别
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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 {/*** 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.** In the foregoing description, the notation* sgn(expression) designates the mathematical* signum function, which is defined to return one of -1,* 0, or 1 according to whether the value of* expression is negative, zero or positive.** The implementor must ensure that sgn(compare(x, y)) ==* -sgn(compare(y, x)) for all x and y.(This* implies that compare(x, y) must throw an exception if and only* if compare(y, x) throws an exception.)** The implementor must also ensure that the relation is transitive:* ((compare(x, y)> 0) & & (compare(y, z)> 0)) implies* compare(x, z)> 0.** Finally, the implementor must ensure that compare(x, y)==0* implies that sgn(compare(x, z))==sgn(compare(y, z)) for all* z.** It is generally the case, but not strictly required that* (compare(x, y)==0) == (x.equals(y)).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 hereimport 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)); }}classCard {public int rank; public String suit; public Card(int rank,String suit){this.rank = rank; this.suit = suit; }}classCardComparator implements Comparator{@Overridepublic 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; }}

java对象对比之comparable和comparator的区别
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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
方法 说明
object.equals 直接覆写即可，不过只能比较相等与否
Comparable.compareTO 需要手动实现接口，当前类之后的所有对比方式都被定义，属于内部顺序
Comparator.compare 需要实现一个比较器对象，对待比较类的侵入性弱，但对代码的侵入性强
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