使用Java数组中元素的索引 [英] Index of an element in an array using Java

问题描述

以下Java代码返回-1.我认为应该返回3.

The following code in Java return -1. I thought that it should return 3.

int[] array = {1,2,3,4,5,6}; 
System.out.println(Arrays.asList(array).indexOf(4));

能帮我了解一下此功能的工作原理吗.

Can you help me understand how this function works.

谢谢

推荐答案

在Java 5之前，Arrays.asList用于接受Object[].当将泛型和varargs引入该语言时，它更改为

Before Java 5, Arrays.asList used to accept an Object[]. When generics and varargs were introduced into the language this was changed to

public static <T> List<T> asList(T... arr)

在您的示例中，T不能为int，因为int是原始类型.不幸的是，签名匹配的T等于int[]，这是一种引用类型.结果是您最终得到的是包含数组而不是整数的List的List.

In your example, T cannot be int because int is a primitive type. Unfortunately, the signature matches with T equal to int[] instead, which is a reference type. The result is that you end up with a List containing an array, not a List of integers.

在Java 4中，由于int[]不是Object[]，因此不会编译您的代码.不编译比产生奇怪的结果更可取.在有效Java中，乔什·布洛赫(Josh Bloch)说，将asList改装为varargs方法是一个错误.

In Java 4, your code would not have compiled because an int[] is not an Object[]. Not compiling is preferable to producing a strange result, and in Effective Java, Josh Bloch says retrofitting asList to be a varargs method was a mistake.

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