java: Arrays.sort() 与 lambda 表达式

Question

我想使用 Arrays.sort 方法按长度对数组 months 中的 String 元素进行排序。有人告诉我here ，可以使用 lambda 表达式而不是创建实现 Comparator 的新类。以完全相同的方式执行，但它不起作用。

import java.util.Arrays;
import java.util.Comparator;

public class MainClass {
public static void main(String[] args)
{

    String[] months = {"January","February","March","April","May","June","July","August","September","October","December"};

    System.out.println(Arrays.toString(months)); //printing before


    //neither this works:
    Arrays.sort(months, 
            (a, b) -> Integer.signum(a.length() - b.length())   
    );

    //nor this:
    Arrays.sort(months, 
            (String a, String b) -> { return Integer.signum(a.length() - b.length()) }; 
    );


    System.out.println(Arrays.toString(months)); //printing after
}
}

Answer 1

最干净的方法是:

Arrays.sort(months, Comparator.comparingInt(String::length));

或者，使用静态导入:

Arrays.sort(months, comparingInt(String::length));

---

然而，这也可以，但更冗长:

Arrays.sort(months,
            (String a, String b) -> a.length() - b.length());

或更短:

Arrays.sort(months, (a, b) -> a.length() - b.length());

---

最后一个:

Arrays.sort(months, 
    (String a, String b) -> { return Integer.signum(a.length() - b.length()) }; 
);

有 ; 放错地方了 - 它应该是:

Arrays.sort(months, 
    (String a, String b) -> { return Integer.signum(a.length() - b.length()); }
);