equals()方法和hashCode()方法简单使用

equals()一般用来比较数据的地址是否相等，针对字符串类型，如此比较没有没有问题，但是在对象之间进行比较，单纯的比较地址就有些问题，比如学生对象，地址相同，但是具体的属性也要进行比较，才能判断是否就是同一个学生

调用从object类继承的equals方法，该方法默认比较两个对象的地址

但是在现实生活中比较两个学生是不是不是同一个人，就不能用比较地址的方法来比较，还要看看具体的属性是否一样,所以需要重写equals()

public class Student extends Object {
    /**
     * 学号
     */
    private int id;
 
    /**
     * 姓名
     */
    private String name;
 
    /**
     * @return the id
     */
    public int getId() {
        return id;
    }
 
    /**
     * @param id the id to set
     */
    public void setId(int id) {
        if (id > 0) {
            this.id = id;
        } else {
            System.out.println("学号不合理!!");
        }
    }
 
    /**
     * @return the name
     */
    public String getName() {
        return name;
    }
 
    /**
     * @param name the name to set
     */
    public void setName(String name) {
        this.name = name;
    }
 
    /**
     * @param id
     * @param name
     */
    public Student(int id, String name) {
        super();
        this.id = id;
        this.name = name;
    }
 
    @Override
    public boolean equals(Object obj) {
        if (null == obj) {
            return false;
        }
        if (this == obj) {
            return true;
        }
        if (obj instanceof Student) {// 当传输的是我们需要的类型时候，进行强制类型转换
            Student student = (Student) obj;
            if (this.id == student.id && this.name.equals(student.name)) {
                return true;
            } else {
                return false;
            }
        }
 
        return false;
    }
 
}

Student student1 = new Student(1001, "qiuxie");
        Student student2 = new Student(1002, "zhouxiaomei");
        Student student3 = new Student(1001, "qiuxie");
        Student student4 = student1;
        Student student5 = null;
        Integer integer = 1;
        System.out.println("student1和student2比较结果:" + student1.equals(student2));
        System.out.println("student1和student3比较结果:" + student1.equals(student3));
        System.out.println("student1和student4比较结果:" + student1.equals(student4));
        System.out.println("student1和student5比较结果:" + student1.equals(student5));
        System.out.println("student1和integer比较结果:" + student1.equals(integer));

这里使用instanceof是因为，obj不一定是Student类型，有可能是别的类型

hashCode()返回对象的哈希码值，即内存地址的编号

重写equals()方法，同时也是需要重写hashCode()方法

假如两个对象调用equals方法相等，则调用hashCode方法结果必须相同

假如两个对象调用equals方法不相等，则调用hashCode方法结果应该不相同

为了保证hashCode()和equals方法一致，需要重写hashCode()方法

Student student1 = new Student(1001, "qiuxie");
        Student student2 = new Student(1002, "zhouxiaomei");
        Student student3 = new Student(1001, "qiuxie");
        Student student4 = student1;
        System.out.println("student1和student2比较结果:" + student1.equals(student2));
        System.out.println("student1和student3比较结果:" + student1.equals(student3));
        System.out.println("student1和student4比较结果:" + student1.equals(student4));
        System.out
                .println("student1.hashCode()=" + student1.hashCode() + " student2.hashCode()=" + student2.hashCode());
        System.out
                .println("student1.hashCode()=" + student1.hashCode() + " student3.hashCode()=" + student3.hashCode());
        System.out
                .println("student1.hashCode()=" + student1.hashCode() + " student4.hashCode()=" + student4.hashCode());

重写hashCode方法

@Override
    public int hashCode() {
        final int type = 12;
        return type * 31 + getId();
 
    }

保证了equals比较结果相同，hashCode结果相同，重写的方法避免了使用内存地址编号

下面列出基本类型和包装类型的比较

int a = 1;
       int b = 1;
       if (a == b) {
           System.out.println("相等");
       } else {
           System.out.println("不相等");
       }
 
       Integer cInteger = 2;
       Integer dInteger = 2;
 
       if (cInteger.equals(dInteger)) {
           System.out.println("包装类型相等");
       } else {
           System.out.println("包装类型不相等");
       }