对于JAVA中具有Lambda表达式的循环

Question

为什么我使用下面的代码 IndexOutOfBoundsException

码:

    List<Integer> ints = Stream.of(21,22,32,42,52).collect(Collectors.toList());
    System.out.print("the list: ");
    ints.forEach((i) -> {
        System.out.print(ints.get(i-1) + " ");
    });

我的错误堆栈:

Exception in thread "main" java.lang.IndexOutOfBoundsException: Index: 11, Size: 5
    at java.util.ArrayList.rangeCheck(ArrayList.java:638)
    at java.util.ArrayList.get(ArrayList.java:414)
    at Agent.lambda$main$1(Agent.java:33)
    at Agent$$Lambda$8/980546781.accept(Unknown Source)
    at java.util.ArrayList.forEach(ArrayList.java:1234)
    at Agent.main(Agent.java:32)
the list: Java Result: 1

但是当我将列表更改为一位数时,一切都很好

码:

    List<Integer> ints = Stream.of(2,8,7,4,3).collect(Collectors.toList());
    System.out.print("the list: ");
    ints.forEach((i) -> {
        System.out.print(ints.get(i-1) + " ");
    });

输出:

2 8 7 4 3 

Answer 1

更简单:

String s = Stream.of(21,22,32,42,52)
                 .collect(Collectors.joining(" "));

Answer 2

我认为原因很清楚.

ints.forEach((i) -> {
    System.out.print(ints.get(i-1) + " ");
});

翻译大致到:

for (Integer i : ints) {
    System.out.println(ints.get(i - 1) + " ");
}

这将导致IndexOutOfBoundsExceptions因为i引用每个列表的元素,并且每个元素 - 1将给出明显超出范围的索引.对于您的第一个示例,i将21提供索引21 - 1 == 20,该索引超出您创建的列表的范围.

例:

List<Integer> ints = Stream.of(21,22,32,42,52).collect(Collectors.toList());

最终会这样结束

ints == [21, 22, 32, 42, 52]

所以当你运行这个:

ints.forEach((i) -> {
    System.out.print(ints.get(i-1) + " ");
});

计算机获取第一个元素并尝试执行lambda的主体:

Execute System.out.print(ints.get(i-1) + " ");:
    First element is 21
    21 - 1 == 20
    ints.get(20) --> IndexOutOfBoundsException

而对于你的第二个例子:

List<Integer> ints = Stream.of(2,8,7,4,3).collect(Collectors.toList());

变

ints == [2, 8, 7, 4, 3]

所以当你运行这个:

ints.forEach((i) -> {
    System.out.print(ints.get(i-1) + " ");
});

计算机遍历元素并尝试执行lambda的主体:

Execute System.out.print(ints.get(i-1) + " ");:
    First element is 2
    2 - 1 == 1
    ints.get(1) --> 8
    Print 8
Execute System.out.print(ints.get(i-1) + " ");:
    Second element is 8
    8 - 1 == 7
    ints.get(7) --> IndexOutOfBoundsException

显然,你的第二个例子中的代码并不是你实际拥有的代码.我怀疑你实际拥有的代码是:

List<Integer> ints = Stream.of(2,8,7,4,3).collect(Collectors.toList());
System.out.print("the list: ");
ints.forEach((i) -> {
    System.out.print(i + " ");
                     ^^^^^^^ <-- this is different
});

这与你发布的完全不同.

Answer 3

forEach贾瓦多克说

对Iterable的每个元素执行给定操作,直到处理完所有元素或操作抛出异常为止.

所以i在

(i) -> {System.out.print(ints.get(i-1) + " ");}

是每个元素List.如果减去1的任何元素大于或等于5(大小List),则尝试获取超出范围的元素.