equals()方法的标准重写

最新推荐文章于 2024-08-27 12:19:07 发布

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最新推荐文章于 2024-08-27 12:19:07 发布

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本文链接：https://blog.youkuaiyun.com/qq_42034068/article/details/88881067

本文阐述了《CoreJava》中equals()方法的设计原则：自反性、对称性、传递性、一致性和空值处理。通过Java代码示例展示了如何正确实现这些原则，并提供了完整的测试用例。

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《core java》给出的 equals() 方法需要满足的原则

It is reflexive: For any non-null reference x, x.equals(x) should return true. 自反性
It is symmetric: For any references x and y, x.equals(y) should return true if and
only if y.equals(x) returns true. 对称性
It is transitive: For any references x, y, and z, if x.equals(y) returns true and
y.equals(z) returns true, then x.equals(z) should return true. 传递性
It is consistent: If the objects to which x and y refer haven’t changed, then
repeated calls to x.equals(y) return the same value. 一致性/不变性
For any non-null reference x, x.equals(null) should return false. 空值false
These rules are certainly reasonable. You wouldn’t want a library

标准重写

 @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        BaseStudent that = (BaseStudent) o;
        return Objects.equals(name, that.name);
    }

完整测试‘

import java.util.Objects;

public class EqualsTest {
    public static void main(String[] args) {
        // Student ISA BaseStudent
        System.out.println(BaseStudent.class.getClassLoader().getClass() == Student.class.getClassLoader().getClass());

        BaseStudent baseStudent = new Student("小明",10);
        Student student = new Student("小明",99);
        Student otherStudent = new Student("小明",19);

        // 自反性
        System.out.println(student.equals(student));
        // 对称性
        System.out.println(student.equals(baseStudent));
        System.out.println(baseStudent.equals(student));
        // 传递性
        // 空返回false
        System.out.println(student.equals(null));

    }
}

class BaseStudent {
    private String name;
    private Integer age;

    public BaseStudent() {
    }

    public BaseStudent(String name, Integer age) {
        this.name = name;
        this.age = age;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        BaseStudent that = (BaseStudent) o;
        return Objects.equals(name, that.name);
    }

}

class Student extends BaseStudent{
    private String name;
    private Integer age;

    public Student() {
    }

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

}