目录

一、基本类型的比较

二、对象的比较

三、三种方式的比较

比如说：

public class TestDemo {
    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);
         boolean b1 = true;
         boolean b2 = false;
         System.out.println(b1 == b2);
         System.out.println(b1 != b2);
    }
}

二、对象的比较

1.equals()------->比较两个对象相不相同

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

    @Override
    public String toString() {
        return "Card{" +
                "rank=" + rank +
                ", suit='" + suit + ''' +
                '}';
    }
}
public class TestDemo {
    public static void main(String[] args) {
        Card card1 = new Card(1,"♥");
        Card card2 = new Card(2,"♥");
        System.out.println(card1.equals(card2));//比较的是引用
    }
}

编译并运行该代码，输出如下：

false 

 

 Java中引用类型的变量不能直接按照 > 或者 < 方式进行比较。 那为什么==可以比较？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

 但是我们希望比较一下花色和数值，如果它们都相同，说明两张牌相同，否则不同；所以，此时我们需要重写equals方法，如下代码所示：

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

    @Override
    public String toString() {
        return "Card{" +
                "rank=" + rank +
                ", suit='" + suit + ''' +
                '}';
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;//是不是同一个类型
        Card card = (Card) o;
        return rank == card.rank && Objects.equals(suit, card.suit);
    }

    @Override
    public int hashCode() {
        return Objects.hash(rank, suit);
    }
}
public class TestDemo {
    public static void main(String[] args) {
        Card card1 = new Card(1,"♥");
        Card card2 = new Card(2,"♥");
        System.out.println(card1.equals(card2));//比较的是引用
    }
}

 

2.比较大小   Comparable接口、Comparator接口......其实这些我们在面向对象编程（2）都已总结过了，这里我们再简单补充一下

class Card implements Comparable{
    public int rank; // 数值
    public String suit; // 花色
    public Card(int rank, String suit) {
        this.rank = rank;
        this.suit = suit;
    }

    @Override
    public int compareTo(Card o) {
        return this.rank - o.rank;
    }

    @Override
    public String toString() {
        return "Card{" +
                "rank=" + rank +
                ", suit='" + suit + ''' +
                '}';
    }
}
public class TestDemo {
    public static void main(String[] args) {
        PriorityQueue priorityQueue = new PriorityQueue<>();
        priorityQueue.offer(new Card(2,"♥"));
        priorityQueue.offer(new Card(1,"♥"));
        System.out.println(priorityQueue);
    }
}

编译并运行该代码，输出如下：

[Card{rank=1, suit='♥'}, Card{rank=2, suit='♥'}]

 当然Comparable接口的侵入性比较强，我们可以使用Comparator接口

class Card implements Comparable{
    public int rank; // 数值
    public String suit; // 花色
    public Card(int rank, String suit) {
        this.rank = rank;
        this.suit = suit;
    }

    @Override
    public int compareTo(Card o) {
        return this.rank - o.rank;
    }

    @Override
    public String toString() {
        return "Card{" +
                "rank=" + rank +
                ", suit='" + suit + ''' +
                '}';
    }
}
class RankComparator implements Comparator{

    @Override
    public int compare(Card o1, Card o2) {
        return o2.rank - o1.rank;
    }
}
public class TestDemo {
    public static void main(String[] args) {
        Card card1 = new Card(1,"♥");
        Card card2 = new Card(2,"♥");
        RankComparator rankComparator = new RankComparator();
        PriorityQueue priorityQueue = new PriorityQueue<>();
        priorityQueue.offer(card1);
        priorityQueue.offer(card2);
        System.out.println(priorityQueue);
    }
}

 另一种写法（1）：匿名内部类

class Card implements Comparable{
    public int rank; // 数值
    public String suit; // 花色
    public Card(int rank, String suit) {
        this.rank = rank;
        this.suit = suit;
    }

    @Override
    public int compareTo(Card o) {
        return this.rank - o.rank;
    }

    @Override
    public String toString() {
        return "Card{" +
                "rank=" + rank +
                ", suit='" + suit + ''' +
                '}';
    }
}
public class TestDemo {
    public static void main(String[] args) {
        Card card1 = new Card(1,"♥");
        Card card2 = new Card(2,"♥");
        PriorityQueue priorityQueue = new PriorityQueue<>(new Comparator() {
            @Override
            public int compare(Card o1, Card o2) {
                return o2.rank - o1.rank;
            }
        });//匿名内部类
        priorityQueue.offer(card1);
        priorityQueue.offer(card2);
        System.out.println(priorityQueue);
    }
}

 另一种写法（2）：lambda表达式

class Card implements Comparable{
    public int rank; // 数值
    public String suit; // 花色
    public Card(int rank, String suit) {
        this.rank = rank;
        this.suit = suit;
    }

    @Override
    public int compareTo(Card o) {
        return this.rank - o.rank;
    }

    @Override
    public String toString() {
        return "Card{" +
                "rank=" + rank +
                ", suit='" + suit + ''' +
                '}';
    }
}
public class TestDemo {
    public static void main(String[] args) {
        Card card1 = new Card(1,"♥");
        Card card2 = new Card(2,"♥");
        PriorityQueue priorityQueue = new PriorityQueue<>((x,y)->{return x.rank- y.rank;});//lambda表达式（可读性差）
        priorityQueue.offer(card1);
        priorityQueue.offer(card2);
        System.out.println(priorityQueue);
    }
}

三、三种方式的比较