Java的Collections.rotate（）与数组不起作用 [英] Java Collections.rotate() with an array doesn't work
问题描述
我有以下Java code：
I have the following Java code:
import java.util.Arrays;
import java.util.Collections;
public class Test {
public static void main(String[] args) {
int[] test = {1,2,3,4,5};
Collections.rotate(Arrays.asList(test), -1);
for(int i = 0; i < test.length; i++) { System.out.println(test[i]); }
}
}
欲阵列被旋转，但是输出我得到的是
I want the array to be rotated, but the output I get is
1
2
3
4
5
这是为什么？
和是否有其他解决方案？
And is there an alternative solution?
编辑：
所以这个作品：
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
public class Test {
public static void main(String[] args) {
int[] test = {1,2,3,4,5};
List<Integer> testList = new ArrayList<Integer>();
for(int i = 0; i < test.length; i++) { testList.add(test[i]); }
Collections.rotate(testList, -1);
for(int i = 0; i < test.length; i++) { System.out.println(testList.get(i)); }
}
}
但Arrays.asList应该返回，当写入，复制更改数组列表。有什么办法来解决，而无需手动操作，从数组转换列出？
But Arrays.asList is supposed to return a list that when written to, copies the changes to the array. Is there any way to fix this without manually doing the conversion from array to list?
我（想我）不能浪费那么多的CPU时间和内存进行转换。
I (think that I) can't afford to waste that much CPU time and memory to do the conversion.
推荐答案
这是一个棘手的问题：是的， asList 备份的列表它与数组形式返回，并更改列表将通写到数组。但是，由于 T ... 如何可变参数与原始类型在这种情况下，一个数组进行交互，你实际上有1个元素创建列表！
This is a tricky problem: yes, asList backs the List it returns with the array, and changes to the List will "write-through" to the array. However, due to how varargs of T... interacts with an array of primitive type in this case, you're actually creating a list with 1 element!
int[] test = {1,2,3,4,5};
System.out.println(Arrays.asList(test).size());
// prints "1"
让我们尝试不同的东西：
Let's try something different:
int[] test = {1,2,3,4,5};
List<Integer> list = Arrays.asList(test);
// "Type mismatch: cannot convert from List<int[]> to List<Integer>"
正如你所见，你打算一个 INT [] 不工作方式的可变参数，编译器提供了一个错误。 Arrays.asList 实际上返回一个1元列表＆LT; INT []＆GT; ，而不是5元列表与LT;整数方式＆gt;
As you see, the varargs with an int[] doesn't work the way you intended, and the compiler gives an error. Arrays.asList actually returns a 1-element List<int[]> instead of a 5-element List<Integer>.
使用整数[] 而不是 INT [] 按预期工作：
Using Integer[] instead of int[] works as expected:
Integer[] test = {1,2,3,4,5};
Collections.rotate(Arrays.asList(test), -1);
System.out.println(Arrays.toString(test));
// prints "[2, 3, 4, 5, 1]"
更多解释
的完整签名 asList 是的 ＆LT; T＆GT;清单＆LT; T＆GT; Arrays.asList（T ...一） 。需要注意的是 T 不能 INT 在这种情况下，出于同样的原因，你不能有一个列表与LT; INT＆GT; 在Java中： T 必须是引用类型
More explanation
The full signature of asList is <T> List<T> Arrays.asList(T... a). Note that T can't be int in this case, for the same reason why you can't have a List<int> in Java: T needs to be a reference type.
考虑下面的代码片段：
System.out.println(Arrays.asList(1,2,3));
// prints "[1, 2, 3]"
这里发生的是，每个 INT 是盒装入整数和可变参数机制作品和 asList 创建了3个元素的列表。现在考虑下面的形式：
What happens here is that each int is boxed into an Integer, and the varargs mechanism "works" and asList creates a list of 3 elements. Now consider the following form instead:
System.out.println(Arrays.asList(new int[] { 1,2,3 }));
// prints "[[I@xxxxxx]"
现在的参数 asList 是 INT [] 。 T 不能是 INT ，因此， T ... 可变参数机制失败，而 asList 只得到一个元素，它是一个 INT [] ，而不是 INT 值本身。
Now the argument to asList is an int[]. T can't be an int, therefore, the T... varargs mechanism "fails", and asList only gets one element, and it's an int[], instead of the int values themselves.
现在考虑这种形式：
System.out.println(Arrays.asList(new Integer[] { 1,2,3 }));
// prints "[1, 2, 3]"
现在因为整数[] 是 T ... ， asList 获取3元素的预期。
Now since Integer[] is a T..., asList gets 3 elements as expected.
- Java:基本数据类型的数组没有autobox
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INT []不autobox到整数[]
- Java: Array of primitive data types does not autobox
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int[]does not autobox to anInteger[]
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