如何编写比较多个参数的比较器?

Question

我试图编写一个比较器来比较类坐标的两个对象。坐标类非常简单：

public class Coordinate {
private int x, y;
public Coordinate(int x, int y) {
this.x = x;
this.y = 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;
}
}

现在，我希望比较器比较类坐标的两个实例的 x 和 y 值。这里有一个例子：
我有一个坐标 c1，它的 x = 42 和 y = 23。我的第二个坐标 c2 有 x = 23 和 y = 54。现在我把它们都放在一个 ArrayList 中，想要对列表进行排序。我想像下面这样排序：y值最低的坐标
总是排在第一位，当你有两个坐标具有相同的y值时，坐标首先是x值较低的坐标。
例：

c1 (y = 4, x = 5 ) < c2 (y = 4, x = 6) < c3 (y = 5, x = 2)  

那么我该如何为此目的编写比较器呢？
谢谢！

Answer 1

Comparator<Coordinate> c = Comparator.comparingInt(Coordinate::getY)
.thenComparingInt(Coordinate::getX);

您可以通过thenComparing和thenComparingX来构建化合物比较器。

var list = List.of(
new Coordinate(6, 4),
new Coordinate(2, 5),
new Coordinate(5, 4)
);
list.sort(c);
System.out.println(list);

代码段打印

[{y=4, x=5}, {y=4, x=6}, {y=5, x=2}]

Answer 2

• 使用比较器

import java.util.ArrayList;
import java.util.Comparator;
class Coordinate {
private int x, y;
public Coordinate(int x, int y) {
this.x = x;
this.y = 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;
}
public String toString() {
return "x = " + x + " y = " + y;
}
}
public class Temp {
public static void main(String[] args) {
ArrayList<Coordinate> A = new ArrayList<>();
A.add(new Coordinate(1, 2));
A.add(new Coordinate(2, 1));
A.add(new Coordinate(3, 2));
A.sort(new Comparator<Coordinate>() {
@Override
public int compare(Coordinate o1, Coordinate o2) {
if (o1.getY() < o2.getY()) {
return -1;
} else if (o1.getY() > o2.getY()) {
return 1;
} else {
if (o1.getX() < o2.getX()) {
return -1;
} else if (o1.getX() > o2.getX()) {
return 1;
}
return 0;
}
}
});
System.out.println(A.toString());
}
}

• 使用可比较的接口

import java.util.ArrayList;
class Coordinate implements Comparable<Coordinate> { # Notice implementing Comparable interface
private int x, y;
public Coordinate(int x, int y) {
this.x = x;
this.y = 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 int compareTo(Coordinate o) { # implementing the abstract method of Comparable interface
if (y < o.y) {
return -1; 
} else if (y > o.y) {
return 1;
} else {
if (x < o.x) {
return -1;
} else if (x > o.x) {
return 1;
}
return 0;
}
}
public String toString() {
return "x = " + x + " y = " + y;
}
}
public class Temp {
public static void main(String[] args) {
ArrayList<Coordinate> A = new ArrayList<>();
A.add(new Coordinate(1, 2));
A.add(new Coordinate(2, 1));
A.add(new Coordinate(3, 2));
A.sort(null);
System.out.println(A.toString());
}
}

输出

[x = 2 y = 1, x = 1 y = 2, x = 3 y = 2]