数组的 Arrays.asList() [英] Arrays.asList() of an array

问题描述

这种转换有什么问题?

public int getTheNumber(int[] factors) {
    ArrayList<Integer> f = new ArrayList(Arrays.asList(factors));  
    Collections.sort(f);
    return f.get(0)*f.get(f.size()-1);
}

我在阅读了中的解决方案后做了这个从数组创建 ArrayList.getTheNumber(...) 中的第二行(排序)导致以下异常:

I made this after reading the solution found in Create ArrayList from array. The second line (sorting) in getTheNumber(...) causes the following exception:

线程main"中的异常 java.lang.ClassCastException: [我无法转换为 java.lang.Comparable]

Exception in thread "main" java.lang.ClassCastException: [I cannot be cast to java.lang.Comparable]

这里有什么问题?我确实意识到可以使用 Arrays.sort() 进行排序，我只是对这个很好奇.

What is wrong here? I do realize that sorting could be done with Arrays.sort(), I'm just curious about this one.

推荐答案

让我们考虑以下简化示例:

Let's consider the following simplified example:

public class Example {
    public static void main(String[] args) {
        int[] factors = {1, 2, 3};
        ArrayList<Integer> f = new ArrayList(Arrays.asList(factors));
        System.out.println(f);
    }
}

在 println 行，这会打印类似[[I@190d11]"的内容，这意味着您实际上已经构建了一个包含 int arrays 的 ArrayList.

At the println line this prints something like "[[I@190d11]" which means that you have actually constructed an ArrayList that contains int arrays.

您的 IDE 和编译器应该警告该代码中未经检查的赋值.您应该始终使用 new ArrayList() 或 new ArrayList<>() 而不是 new ArrayList().如果你使用过它，就会因为试图将 List 传递给构造函数而出现编译错误.

Your IDE and compiler should warn about unchecked assignments in that code. You should always use new ArrayList<Integer>() or new ArrayList<>() instead of new ArrayList(). If you had used it, there would have been a compile error because of trying to pass List<int[]> to the constructor.

从 int[] 到 Integer[] 没有自动装箱，无论如何自动装箱只是编译器中的语法糖，所以在这种情况下你需要做手动复制数组:

There is no autoboxing from int[] to Integer[], and anyways autoboxing is only syntactic sugar in the compiler, so in this case you need to do the array copy manually:

public static int getTheNumber(int[] factors) {
    List<Integer> f = new ArrayList<Integer>();
    for (int factor : factors) {
        f.add(factor); // after autoboxing the same as: f.add(Integer.valueOf(factor));
    }
    Collections.sort(f);
    return f.get(0) * f.get(f.size() - 1);
}

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