标签：vat obj str getc 实体类 class void log add
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;//对比值 } }
以上代码重写了equals。
@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)); } }
这个时候输出是true
@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)); } }
a.equals(b) is true points.contains(b) is false
这里就是因为没有重写hashcode导致的。没有重写，默认使用的是父类的hashcode，每个对象各不相同（hashset的实现暂时先不谈）
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;//对比值 } @Override public int hashCode() { return Objects.hash(x, y); } }
a.equals(b) is true points.contains(b) is true
同样重写hashcode后下面的代码也能输出正常的值0000
HashMap<Point, String> map = new HashMap<>(); map.put(a, "00000"); log.info(map.get(b));
如果不重新，则会输出null
Java 重写equals的时候为什么一定要重写hashcode-一个例子
标签：vat obj str getc 实体类 class void log add
原文地址：https://www.cnblogs.com/Brake/p/12716839.html
