2021-04-18 10:56  阅读(75)
文章分类:Java 基础教程 文章标签:JavaJava 教程
©  原文作者:w3cschool 原文地址:https://www.w3cschool.cn/java/java-class-instance.html

Java面向对象设计 - Java Object.Equals方法

以下代码显示如何实现equals()和hashCode()方法

    class Point {
      private int x;
      private int y;
    
      public Point(int x, int y) {
        this.x = x;
        this.y = y;
      }
    
      /* implement the equals() method */
      public boolean equals(Object otherObject) {
        // Are the same?
        if (this == otherObject) {
          return true;
        }
    
        // Is otherObject a null reference?
        if (otherObject == null) {
          return false;
        }
    
        // Do they belong to the same class?
        if (this.getClass() != otherObject.getClass()) {
          return false;
        }
    
        // Get the reference of otherObject in a SmartPoint variable
        Point otherPoint = (Point) otherObject;
    
        // Do they have the same x and y co-ordinates
        boolean isSamePoint = (this.x == otherPoint.x && this.y == otherPoint.y);
    
        return isSamePoint;
      }
    
      /*
       * implement hashCode() method of the Object class, which is a requirement
       * when you implement equals() method
       */
      public int hashCode() {
        return (this.x + this.y);
      }
    }
    
    public class Main {
      public static void main(String[] args) {
        Point pt1 = new Point(10, 10);
        Point pt2 = new Point(10, 10);
        Point pt3 = new Point(12, 19);
        Point pt4 = pt1;
    
        System.out.println("pt1 == pt1: " + (pt1 == pt1));
        System.out.println("pt1.equals(pt1): " + pt1.equals(pt1));
    
        System.out.println("pt1 == pt2: " + (pt1 == pt2));
        System.out.println("pt1.equals(pt2): " + pt1.equals(pt2));
    
        System.out.println("pt1 == pt3: " + (pt1 == pt3));
        System.out.println("pt1.equals(pt3): " + pt1.equals(pt3));
    
        System.out.println("pt1 == pt4: " + (pt1 == pt4));
        System.out.println("pt1.equals(pt4): " + pt1.equals(pt4));
      }
    }
    

上面的代码生成以下结果。

2021041810511_1.png

注意

这里是equals()方法的实现的规范。假设x,y和z是三个对象的非空引用。

  • 自反性。表达式x.equals(x)应该返回true。
  • 对称性。如果x.equals(y)返回true,y.equals(x)必须返回true。
  • 传递性。如果x.equals(y)返回true,y.equals(z)返回true,则x.equals(z)必须返回true。
  • 一致性。如果x.equals(y)返回true,它应该保持返回true,直到x或y的状态被修改。如果x.equals(y)返回false,它应该保持返回false,直到x或y的状态被修改。
  • 与空引用的比较:任何类的对象不应等于空引用。表达式x.equals(null)应始终返回false。
  • 与hashCode()方法的关系:如果x.equals(y)返回true,x.hashCode()必须返回与y.hashCode()相同的值。
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