Arrays.asList（）。包括（）双VS双阵列 [英] Arrays.asList().contains() on Double vs double arrays

问题描述

我有一个双阵列 doubleArray1 。我尝试了 Arrays.asList（）。包括（）如下图所示。

I have a double array doubleArray1. I tried a Arrays.asList().contains() operation as shown below

double doubleArray1 [] = {1D,2D,3D};
if(Arrays.asList(doubleArray1).contains(1D)) {
    System.out.println("hello-1");
}

它不打印任何东西。然后，我做了它双阵列

It does not print anything. Then I made it a Double array

Double doubleArray1 [] = {1D,2D,3D};        
if(Arrays.asList(doubleArray1).contains(1D)) {
    System.out.println("hello-1");
}

它打印 HELLO-1 。

有人能解释为什么这种差异？

Could someone explain why this difference?

推荐答案

您第一次调用 Arrays.asList 实际上返回一个列表＆LT;双[]＆GT; - 它的自动装箱的说法，因为双[] 不是 T [] ...仿制药不允许原始类型的类型参数。

Your first call to Arrays.asList is actually returning a List<double[]> - it's autoboxing the argument, because a double[] isn't a T[]... generics don't allow for primitive types as type arguments.

如果你想转换一个双[] 到列表＆LT;双＆GT; ，你要么需要做的它手动或使用第三方库做。例如：

If you want to convert a double[] into a List<Double>, you either need to do it manually or use a third-party library to do it. For example:

public List<Double> toList(double[] doubles) {
    List<Double> list = new ArrayList<>(doubles.length);
    for (double x : doubles) {
        list.add(x);
    }
    return list;
}

请注意，不像 Arrays.asList 随后到阵列的任何更改将的不的在列表反之亦然体现 - 这是一个副本，而不是视图。

Note that unlike Arrays.asList any subsequent changes to the array will not be reflected in the list or vice versa - it's a copy, not a view.

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