實體類

class Point {
    private int x;
    private int y;

    public int getX() {
        return x;
    }

    public void setX(int x) {
        this.x = x;
    }

    public int getY() {
        return y;
    }

    public void setY(int y) {
        this.y = y;
    }

    @Override
    public boolean equals(Object obj) {
        if (this == obj)//ref相等
        {
            return true;
        }
        if (obj == null || this.getClass() != obj.getClass()) {
            return false;
        }
        Point that = (Point) obj;
        return x == that.x && y == that.y;//對比值
    }
}

以上代碼重寫了equals。

比較

@Slf4j
public class EqualsHashDemo {


    public static void compare() {
        Point a = new Point();
        a.setX(1);
        a.setY(2);

        Point b = new Point();
        b.setX(1);
        b.setY(2);

        log.info("a.equals(b) is " + a.equals(b));

    }
}

這個時候輸出是true

把對象裝入hashset進行比較

@Slf4j
public class EqualsHashDemo {


    public static void compare() {
        Point a = new Point();
        a.setX(1);
        a.setY(2);

        Point b = new Point();
        b.setX(1);
        b.setY(2);

        log.info("a.equals(b) is " + a.equals(b));

        HashSet<Point> points=new HashSet<>();
        points.add(a);

        log.info("points.contains(b) is " + points.contains(b)); 

    }
}

輸出

 a.equals(b) is true
 points.contains(b) is false

這裡就是因為沒有重寫hashcode導致的。沒有重寫，預設使用的是父類的hashcode，每個對象各不相同（hashset的實現暫時先不談）

重寫hashcode

class Point {
    private int x;
    private int y;

    public int getX() {
        return x;
    }

    public void setX(int x) {
        this.x = x;
    }

    public int getY() {
        return y;
    }

    public void setY(int y) {
        this.y = y;
    }

    @Override
    public boolean equals(Object obj) {
        if (this == obj)//ref相等
        {
            return true;
        }
        if (obj == null || this.getClass() != obj.getClass()) {
            return false;
        }
        Point that = (Point) obj;
        return x == that.x && y == that.y;//對比值
    }

    @Override
    public int hashCode() {
        return Objects.hash(x, y);
    }
}

再次運行

 a.equals(b) is true
 points.contains(b) is true

同樣重寫hashcode後下麵的代碼也能輸出正常的值0000

  HashMap<Point, String> map = new HashMap<>();
        map.put(a, "00000");
        log.info(map.get(b));

如果不重新，則會輸出null