在JAVA中使用Lambda表达式进行循环 [英] For Loop with Lambda Expression in JAVA
问题描述
为什么当我使用以下代码时,我得到 IndexOutOfBoundsException
代码:
列表<整数> ints = Stream.of(21,22,32,42,52).collect(Collectors.toList());
System.out.print(list:); ((i) - > {
System.out.print(ints.get(i-1)+);
});
ints.forEach
我的错误堆栈:
在Agent.lambda $ main $ 1(Agent.java:33)
在Agent $$ Lambda $ 8 / 980546781.accept(Unknown Source)
at java.util.ArrayList.forEach(ArrayList.java:1234)
at Agent.main(Agent.java:32)
列表:Java结果:1
但是当我将列表更改为一位数字时,一切正常
Code:
List< Integer> ints = Stream.of(2,8,7,4,3).collect(Collectors.toList());
System.out.print(list:); ((i) - > {
System.out.print(ints.get(i-1)+);
});
ints.forEach
输出:
2 8 7 4 3
认为原因很明显。
pre $ $ $ $ $ $ ints.forEach((i) - > {
System.out。 print(ints.get(i-1)+);
});
大致转换为:
for(Integer i:ints){
System.out.println(ints.get(i - 1)+);
$ / code>
这将导致 IndexOutOfBoundsException
i 是指每个列表的元素,并且这些元素中的每一个 - 1都会给出一个清晰的索引 / em>越界。对于你的第一个例子, i
将会是 21
,它给出一个 21 - 1的索引== 20
,它超出了你创建的列表。
示例:
List< Integer> ints = Stream.of(21,22,32,42,52).collect(Collectors.toList());
最终会让
$ $
$ >因此,当你运行这个:
ints.forEach((i) - > {
System.out。 print(ints.get(i-1)+);
});
计算机取第一个元素,并尝试执行lambda:
执行System.out.print(ints.get(i-1)+);:
第一个元素是21
21 - 1 == 20
ints.get(20) - > IndexOutOfBoundsException
第二个例子:
列表与LT;整数> ints = Stream.of(2,8,7,4,3).collect(Collectors.toList());
变成
ints == [2,8,7,4,3]
运行这个:
ints.forEach((i) - > {
System.out.print(ints。 get(i-1)+);
});
计算机通过元素并尝试执行lambda:
执行System.out.print(ints.get(i-1)+);:
第一个元素是2
2 - 1 == 1
ints.get(1) - > 8
打印8
执行System.out.print(ints.get(i-1)+);:
第二个元素是8
8 - 1 == 7
ints.get(7) - > IndexOutOfBoundsException
显然,第二个例子中的代码是不是你实际上有什么。我怀疑你实际上的代码是:
List< Integer> ints = Stream.of(2,8,7,4,3).collect(Collectors.toList());
System.out.print(list:); $(b)$ b $ ints.forEach((i) - > {
System.out.print(i +);
^^^^^^^< - 这是不同的
});
与您发布的内容完全不同。
Why when I use following code I get IndexOutOfBoundsException
Code:
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) + " ");
});
My error stack:
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
but When I change my list to one digit numbers everything is fine
Code:
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) + " ");
});
output:
2 8 7 4 3
I think the reason is pretty clear.
ints.forEach((i) -> {
System.out.print(ints.get(i-1) + " ");
});
Translates approximately to:
for (Integer i : ints) {
System.out.println(ints.get(i - 1) + " ");
}
Which will cause IndexOutOfBoundsException
s because i
refers to the elements of each list, and each of those elements - 1 will give an index that is clearly out of bounds. For your first example, i
will be 21
, which gives an index of 21 - 1 == 20
, which is out of bounds for the list you created.
Example:
List<Integer> ints = Stream.of(21,22,32,42,52).collect(Collectors.toList());
will end up so that
ints == [21, 22, 32, 42, 52]
So when you run this:
ints.forEach((i) -> {
System.out.print(ints.get(i-1) + " ");
});
The computer takes the first element and tries to execute the body of the lambda:
Execute System.out.print(ints.get(i-1) + " ");:
First element is 21
21 - 1 == 20
ints.get(20) --> IndexOutOfBoundsException
And for your second example:
List<Integer> ints = Stream.of(2,8,7,4,3).collect(Collectors.toList());
becomes
ints == [2, 8, 7, 4, 3]
So when you run this:
ints.forEach((i) -> {
System.out.print(ints.get(i-1) + " ");
});
The computer goes through the elements and tries to execute the body of the lambda:
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
So evidently the code in your second example is not what you actually have. I suspect that the code you actually have is:
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
});
Which is entirely different than what you posted.
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