Java 8:目标输入

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+b，a++或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:在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;

在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).而目标类型是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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