Question

我有以下程序

 List<String> numbers = Arrays.asList("10", "68", "97", "9", "21", "12");

Collections.sort(numbers, (a, b) -> (b + a).compareTo(a + b));

嗨

如何重写此代码

（b + a）.compareTo（a + b）

到比较器

先谢谢您

Answer 1

Comparator.comparing（）结构旨在用于具有多个字段的对象，因此您可以提取所需的字段用作键。此方法接受用于提取可比排序键的功能作为参数。

但是，对列表进行排序不需要。
由于只有您的String值，没有其他可作为键的模糊字段。

如果要在（a + b）.compareTo（b + a）

上对此列表进行排序

List<String> numbers = Arrays.asList("10", "68", "97", "9", "21", "12");

然后使用标准比较器编写代码：

numbers.sort((a, b) -> (a+b).compareTo(b+a));

和使用Compartor.comparing的代码：

numbers.sort(Comparator.comparing((String s) -> s, (a, b) -> (a+b).compareTo(b+a)));

都将输出：

[ 10, 12, 21, 68, 97, 9 ]

但是正如您所看到的，在List 上这是不必要的，并且最终会重复代码。

如果不清楚，下面是 Comparator.comparing
的正确使用示例假设我们有此类

public class Car {
    private String name;
    private String type;
    private int tires;

    public Car(String name, String type, int tires) {
        this.name = name;
        this.type = type;
        this.tires= tires;
    }

    public String getName() {
        return name;
    }
    public String getType() {
        return type;
    }
    public int getTires() {
        return tires;
    }
}

还有汽车清单

List<Car> carsList = new ArrayList<Car>();
carsList.add(new Car("Audi A3", "Hatchback", 4));
carsList.add(new Car("Tyrerell P34", "Formula One", 6));
carsList.add(new Car("1932 Morgan Aero 2-Seater Sports", "Sports", 3));
carsList.add(new Car("Avtoros Shaman", "All terrain", 8));

然后我们可以像这样对List 进行排序

// By the type
carsList.sort(Comparator.comparing(Car::getType));

// By the number of tires
carsList.sort(Comparator.comparingInt(Car::getTires));

// By the number of tires in reverse order
carsList.sort(Comparator.comparingInt(Car::getTires).reversed());                   

// First by the type and then by the number of tires in reverse order
carsList.sort(Comparator.comparing(Car::getType).thenComparing(Car::getTires).reversed());

Answer 2

您正在相互比较字符串，以便对列表进行排序。

如果您使用a+b进行排序，则意味着比较的结果将始终为您提供相同的结果

使用以下代码

numbers.sort((a, b) -> a.compareTo(b));

Answer 3

以下是一些当您有复杂的对象并且要根据其属性进行排序时，Comparator.comparing会很有帮助。下面，我根据字符串的长度对字符串进行了排序。

List<String> numbers = Arrays.asList("10", "68", "97", "9", "21", "12");

Collections.sort(numbers, Comparator.naturalOrder()); //[10, 12, 21, 68, 9, 97]

Collections.sort(numbers, Comparator.reverseOrder()); //[97, 9, 68, 21, 12, 10]

numbers.sort(Comparator.comparing(String::length)); //[9, 97, 68, 21, 12, 10]

numbers.sort(Comparator.comparing(String::intern)); //[10, 12, 21, 68, 9, 97]

numbers.sort(Comparator.comparing((String num) -> num, (a, b) -> (b+a).compareTo(a+b)));// [9, 97, 68, 21, 12, 10]

    List<Movie> movies = Arrays.asList(
    new Movie("Lord of the rings"),
    new Movie("Back to the future"),
    new Movie("Carlito's way"),
    new Movie("Pulp fiction"));

    movies.sort(Comparator.comparing(Movie::getTitle));

参考：https://reversecoding.net/java-8-comparator-how-to-sort-a-list/

https://docs.oracle.com/javase/8/docs/api/java/util/Comparator.html#comparing-java.util.function.Function-

如何编写Comparator.comparing

3 个答案: