一：元素比较

（1）基本类型比较

基本类型比较：在Java中，基本类型是可以直接进行比较的

public class TestDemo {
    public static void main(String[] args) {
        // 数值类型比较
        int a = 10;
        int b = 20;
        System.out.println(a > b);
        System.out.println(a < b);

        // 字符类型比较
        char c = 'A';
        char d = 'B';
        System.out.println(c > d);
        System.out.println(c < d);

        // 布尔类型比较
        boolean e = true;
        boolean f = false;
        System.out.println(e == f);
        System.out.println(e != f);
    }
}

（2）引用类型比较

引用类型比较：在Java中，引用类型是不能采用<或>进行比较的，因为引用类型在比较时比较的是引用变量的地址

• student1 == student3结果为false是因为它们是不同的对象，虽然内容相同
• student1 == student3结果为true是因为它们是相同的对象

class Student{
    int age;
    String name;
    public Student(int age, String name){
        this.age = age;
        this.name = name;
    }
}

public class TestDemo {
    public static void main(String[] args) {
        Student student1 = new Student(17, "张三");
        Student student2 = new Student(19, "李四");
        Student student3 = new Student(17, "张三");
        Student student4 = student1;

        System.out.println(student1 < student2);//无法编译
        System.out.println(student1 == student3);
        System.out.println(student1 == student4);
    }
}

其实，对于用户自定义类型在进行比较时会默认调用equal方法（继承自Object），而上面的==之所以可以比较是因为它默认调用的也是equal方法

public boolean equals(Object obj){
	return (this==obj);
}

二：对象比较

很多情况下，我们需要根据对象中的内容来对对象进行比较，主要有如下三种方法

• 重写父类的equals方法：因为所有类都是继承自Object的，所以直接重写即可，不过只能比较相等与否

• 基于Comparable接口类的比较：需要手动实现接口，侵入性比较强，但一旦实现，每次用该类都有顺序，属于内部顺序

• 基于Comparator比较器的比较：需要实现一个比较器对象，对待比较类的侵入性弱，但对算法代码实现侵入性而强

（1）重写父类的equals方法

重写父类的equals方法虽然可行，但是它只能按照相等的方式进行比较，而不能比较大于或小于

class Student{
    int age;
    String name;
    public Student(int age, String name){
        this.age = age;
        this.name = name;
    }

    @Override
    public boolean equals(Object o){
        if(this == o){
            return  true;
        }
        //o如果是null对象或者o不是Student的子类
        if(o == null || !(o instanceof Student)){
            return false;
        }

        Student student = (Student)o;
        return this.age == student.age && this.name.equals(student.name);
    }
}

public class TestDemo {
    public static void main(String[] args) {
        Student student1 = new Student(17, "张三");
        Student student2 = new Student(19, "李四");
        Student student3 = new Student(17, "张三");

        System.out.println(student1.equals(student2));
        System.out.println(student1.equals(student3));
    }
}

（2）基于Comparable接口类的比较

Comparable：它是Java提供的泛型的比较类接口，具体实现如下

• <0：表示当前对象this小于对象o
• =0：表示当前对象this等于对象o
• >0：表示当前对象this大于对象o

public interface Comparable<E>{
	int compareTo(E 0)
}

所以对于自定义类型，可以实现Comparable接口，然后在类中重写compareTo方法，制定自己的比较规则

class Student implements Comparable<Student>{
    int age;
    String name;
    public Student(int age, String name){
        this.age = age;
        this.name = name;
    }

    @Override
    public int compareTo(Student o){
        if(o == null){
            return 1;
        }
        if(this.age == o.age && this.name.equals(o.name)){
            return 0;
        }

        return this.age - student.age;
    }
}

public class TestDemo {
    public static void main(String[] args) {
        Student student1 = new Student(17, "张三");
        Student student2 = new Student(19, "李四");
        Student student3 = new Student(17, "张三");
        System.out.println(student1.compareTo(student3));
        System.out.println(student1.compareTo(student2));
        System.out.println(student2.compareTo(student3));
    }
}

（3）基于Comparator比较器的比较

Comparator：它是Java提供的泛型接口类，使用步骤如下

• 用户自定义比较器类，实现Comparator接口
• 重写Comparator中的compare方法

import java.util.Comparator;

class Student {
    int age;
    String name;
    public Student(int age, String name){
        this.age = age;
        this.name = name;
    }
}

class StudentComparator implements Comparator<Student>{
    @Override
    public int compare(Student o1, Student o2){
        if(o1 == o2){
            return 0;
        }
        if(o1 == null){
            return -1;
        }
        if(o2 == null){
            return 1;
        }

        if(o1.age == o2.age && o1.name.equals(o2.name)){
            return 0;
        }
        return o1.age - o2.age;

    }
}

public class TestDemo {
    public static void main(String[] args) {
        Student student1 = new Student(17, "张三");
        Student student2 = new Student(19, "李四");
        Student student3 = new Student(17, "张三");
        StudentComparator studentComparator =  new StudentComparator();
        System.out.println(studentComparator.compare(student1, student3));
        System.out.println(studentComparator.compare(student1, student2));
        System.out.println(studentComparator.compare(student2, student3));
    }
}