Java 8 lambdas执行 [英] Java 8 lambdas execution
问题描述
如何在Java 8中执行此类操作?
How can I do something like this in Java 8?
boolean x = ((boolean p)->{return p;}).apply(true);
现在我收到以下错误:
此表达式的目标类型必须是功能接口
The target type of this expression must be a functional interface
推荐答案
根据JLS 第15.27节:
如果lambda表达式出现在除分配上下文之外的某个位置的程序中,那么这是一个编译时错误( §5.2),调用上下文(第5.3节)或转换上下文(第5.5节)。
It is a compile-time error if a lambda expression occurs in a program in someplace other than an assignment context (§5.2), an invocation context (§5.3), or a casting context (§5.5).
也可以在 返回
声明。
It is also possible to use a lambda expression in a return
statement.
然后我们可以用四种不同的方式重写你的例子:
We can then rewrite your example in four different ways:
-
通过创建作业上下文:
By creating an assignment context:
Function<Boolean, Boolean> function = p -> p;
boolean x = function.apply(true);
通过创建调用上下文:
By creating an invocation context:
foobar(p -> p);
private static void foobar(Function<Boolean, Boolean> function) {
boolean x = function.apply(true);
}
通过创建投射上下文:
By creating a casting context:
boolean x = ((Function<Boolean, Boolean>) p -> p).apply(true);
使用 return
声明:
boolean x = function().apply(true);
private static Function<Boolean, Boolean> function() {
return p -> p;
}
此外,在此简单的例子,整个lambda表达式可以重写为:
Also, in this simple example, the whole lambda expression can be rewritten as:
UnaryOperator<Boolean> function = UnaryOperator.identity();
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