用于if条件的Java Lambda表达式 - 这里不需要 [英] Java Lambda Expression for if condition - not expected here

问题描述

考虑if条件需要评估数组或List的情况。一个简单的例子：检查所有元素是否都为真。但我正在寻找通用方式来做这件事

Consider the case where an if condition needs to evaluate an array or a List. A simple example: check if all elements are true. But I'm looking for generic way to do it

通常我会这样做：

boolean allTrue = true;
for (Boolean bool : bools){
    if (!bool) {
        allTrue = false;
        break;
     }
}
if (allTrue){
     // do Something
}

但现在我想把它隐藏到我的if条件中。我尝试使用Lambda Expressions，但它不起作用：

But now I'd like to hide it into my if condition. I tried using Lambda Expressions for this, but it's not working:

if (() -> {
    for (Boolean bool : bools)
        if (!bool)
            return false;
    return true;
}){
      // do something
}

如果这样可行，我可以做更复杂的事情，比如

If this were working I could do something more complicated like

if (() -> {
   int number = 0;
   for (MyObject myobject : myobjects)
       if (myObject.getNumber() != 0)
           numbers++;
   if (numbers > 2) 
       return false;
   return true;
 }{
     //do something
 }

是否有更好的方法，只是语法错误？

Is there a better way to do it is it just a syntax error?

UPDATE
我不是在谈论布尔数组，而是寻找一种通用的方法来实现它。

UPDATE I'm not talking about the boolean array, rather looking for a generic way to achieve that.

推荐答案

你可以写一下，例如 List< Boole > ：

if (!list.stream().allMatch(x -> x)) {
    // not every member is true
}

或：

if (list.stream().anyMatch(x -> !x)) {
    // at least one member is false
}

如果你有一个数组 of booleans，然后使用 Arrays.stream（）来获取它的流。

If you have an array of booleans, then use Arrays.stream() to obtain a stream out of it instead.

更一般地说，对于 Stream 提供（通用）类型的元素 X ，你必须提供 Predicate<？超级X> 到。{all，any} Match（）（完整谓词，或lambda或方法引用 - - 很多事情都去了）。这些方法的返回值是不言自明的 - 我认为。

More generally, for a Stream providing elements of (generic) type X, you have to provide a Predicate<? super X> to .{all,any}Match() (either a "full" predicate, or a lambda, or a method reference -- many things go). The return value of these methods are self explanatory -- I think.

现在， count 服从某个谓词的元素，你有 .count（），你可以与 .filter（）结合使用 - 还将谓词作为参数。例如，检查你的长度大于5的 List< String> 中是否有超过2个元素：

Now, to count elements which obey a certain predicate, you have .count(), which you can combine with .filter() -- which also takes (whatever is) a Predicate as an argument. For instance checking if you have more than 2 elements in a List<String> whose length is greater than 5 you'd do:

if (list.stream().filter(s -> s.length() > 5).count() > 2L) {
    // Yup...
}

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