英文:
How does Java execute the Collections.contains() method, Could anyone pls show some links to this?
问题
遇到了contains()方法的问题，不确定为什么在这两种情况下表现不同，如下面的代码片段所示：
import java.awt.*;import java.util.LinkedHashSet;import java.util.Set;public class SetDemo01 {public static void main(String[] args) {Set<Point> set = new LinkedHashSet<>();Point a = new Point(0, 1);Point b = new Point(0, 1);set.add(a);System.out.println(set.contains(b));Set<Coordinate> set02 = new LinkedHashSet<>();Coordinate c = new Coordinate(0, 1);Coordinate d = new Coordinate(0, 1);set02.add(c);System.out.println(set02.contains(d));}}class Coordinate {int x;int y;Coordinate(int x, int y) {this.x = x;this.y = y;}}
控制台输出：
truefalse
现在我知道我需要重写equals()和hashCode()方法，但能否有人请出示参考：在应用contains()方法时，会执行equals()，并比较hashCode。
英文:
Met an issue with the contains() method, not sure why it behaves differently in the two scenarios, as the following code snippet shows:
import java.awt.*;import java.util.LinkedHashSet;import java.util.Set;public class SetDemo01 {public static void main(String[] args) {Set<Point> set = new LinkedHashSet<>();Point a = new Point(0, 1);Point b = new Point(0, 1);set.add(a);System.out.println(set.contains(b));Set<Coordinate> set02 = new LinkedHashSet<>();Coordinate c = new Coordinate(0, 1);Coordinate d = new Coordinate(0, 1);set02.add(c);System.out.println(set02.contains(d));}}class Coordinate {int x;int y;Coordinate (int x, int y) {this.x = x;this.y = y;}}
The console prints:
truefalse
Now I knew I need to override the equals() & hashCode() methods, but could someone pls show the reference for this: when apply the contains() method, the equals() is run, and the 'hashCode' is compared.
答案1
得分: 4
如果你像这样声明你的类，你将得到你期望的输出：
class Coordinate {int x;int y;Coordinate (int x, int y) {this.x = x;this.y = y;}@Overridepublic boolean equals(Object o) {if (this == o) return true;if (o == null || getClass() != o.getClass()) return false;Coordinate that = (Coordinate) o;return x == that.x && y == that.y;}@Overridepublic int hashCode() {return Objects.hash(x, y);}}
如果不重写 equals 方法，contains 方法将只检查两个对象是否是相同的引用。
请也参考：https://stackoverflow.com/questions/2265503/why-do-i-need-to-override-the-equals-and-hashcode-methods-in-java
英文:
If you declare your class like that, you would have your expected output:
class Coordinate {int x;int y;Coordinate (int x, int y) {this.x = x;this.y = y;}@Overridepublic boolean equals(Object o) {if (this == o) return true;if (o == null || getClass() != o.getClass()) return false;Coordinate that = (Coordinate) o;return x == that.x &&y == that.y;}@Overridepublic int hashCode() {return Objects.hash(x, y);}}
Without overriding the equals method the contains method will only look if the two objects are the same reference.
Please have also a look at: https://stackoverflow.com/questions/2265503/why-do-i-need-to-override-the-equals-and-hashcode-methods-in-java
通过集体智慧和协作来改善编程学习和解决问题的方式。致力于成为全球开发者共同参与的知识库，让每个人都能够通过互相帮助和分享经验来进步。
评论