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问题描述

在阅读文章 状态Lambda 我来到了目标输入主题,我对以下段落有些困惑:

While reading the article State of the Lambda I came to the topic Target Typing and I'm getting a bit confused with the following paragraph:

此方法的含义是相同的lambda表达式可以 在不同的上下文中具有不同的类型:

An implication of this approach is that the same lambda expression can have different types in different contexts:

Callable<String> c = () -> "done";

PrivilegedAction<String> a = () -> "done";

在第一种情况下,lambda表达式() -> "done"表示一个 Callable的实例.在第二种情况下,相同的表达式 表示PrivilegedAction的实例.

In the first case, the lambda expression () -> "done" represents an instance of Callable. In the second case, the same expression represents an instance of PrivilegedAction.

编译器负责推断每个lambda的类型 表达.它使用在以下情况下期望的类型: 表达出现;这种类型称为目标类型. λ 表达式只能出现在目标类型为 功能界面.

The compiler is responsible for inferring the type of each lambda expression. It uses the type expected in the context in which the expression appears; this type is called the target type. A lambda expression can only appear in a context whose target type is a functional interface.

您能否以简单的方式向我解释与引用的段落有关的这些要点:

Can you explain me these points in relation with the quoted paragraph in a simple way:

  1. 目标类型
  2. 上下文

如果您还提供代码段,我将不胜感激.

I will really appreciate it if you also provide code snippets.

推荐答案

上下文

上下文是在代码中使用表达式的方式.这不仅仅是lambda表达式-它是任何表达式,例如a+ba++Math.random().

Context

Context is the way an expression is used within the code. It's not just lambda expressions - it's any expression, like a+b, a++ or Math.random().

可能的上下文示例:

  • 分配:采用表达式a+b.如果将其分配给变量,则会在分配上下文中使用它:

  • Assignment: take the expression a+b. If you assign it to a variable, it is used in an assignment context:

c = a+b;

  • 方法或构造函数的参数:.这是当您将其传递给某个方法调用时:

  • Argument to a method or constructor:. This is when you pass it to some method call:

    System.out.println(a+b);
    

  • 返回值:return语句中使用表达式时:

  • Return value: When you are using the expression in a return statement:

    return a+b;
    

  • 数组索引:当您的表达式是数组索引时:

  • Index to an array: When your expression is the index of an array:

    x[a+b] = 3;
    

  • 目标类型是在给定上下文中预期的类型.例如,如果您将方法定义为:

    The target type is the type expected in the given context. For example, if you have a method defined as:

    public int myMethod() { ... }
    

    ,则其主体中return语句中的任何表达式都应具有类型int.所以,如果你有这个:

    then any expression in a return statement in its body is expected to have the type int. So if you have this:

    return a+b;
    

    myMethod内部,预计a+b将解析为int或可分配给int的内容.

    inside myMethod, it's expected that a+b will resolve to an int or something that's assignable to an int.

    现在,假设您具有此方法:

    Now, suppose you have this method:

    public void anotherMethod( double d );
    

    然后,当您调用它并传递一个表达式作为参数时,该表达式的类型应为double.像这样的呼叫:

    Then when you call it, and pass an expression as an argument, that expression is expected to be of type double. So a call like:

    anotherMethod(a+b);
    

    期望a+b解析为double.这就是它的目标类型.

    expects a+b to resolve to a double. That's its target type.

    在声明中:

    Callable<String> c = () -> "done";
    

    表达式是lambda表达式() -> "done".它在赋值上下文中使用(已分配给c).而目标类型Callable<String>,因为这是将任何内容分配给c时所期望的.

    the expression is the lambda expression () -> "done". It is used in an assignment context (it is assigned to c). And the target type is Callable<String> because that's what is expected when you assign anything to c.

    有关更正式的讨论,请参考 Java语言规范,第5章.

    For a more formal discussion, refer to the Java Language Specification, Chapter 5.

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    08-29 06:16