Java SE 10语言更新：局部变量类型推断增强可读性

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JDK10-Java Platform, Standard Edition (Java SE) 的最新语言更新文档（E91239-01，发布于2018年3月）关注了Java SE 10引入的重要特性，特别是Local-Variable Type Inference（局部变量类型推断）。这一功能旨在提高代码可读性和减少冗余，使得程序员在编写Java程序时无需显式指定局部变量的类型，而是通过上下文自动推断。在Java SE 10之前，当创建像URL、URLConnection和BufferedReader这样的对象时，程序员需要明确地为每个变量指定类型，如`URL url = new URL("http://www.oracle.com/");`，这可能导致代码显得冗余且不易理解。为了简化这种过程，Java SE 10引入了新的标识符`var`，允许开发者在初始化时省略类型声明，编译器会根据上下文自动推断出变量的类型。例如，上述代码可以被重写为： ```java var url = new URL("http://www.oracle.com/"); var conn = url.openConnection(); var reader = new BufferedReader( new InputStreamReader(conn.getInputStream())); ``` `var`不是一个关键字，而是一个保留的类型名称，这意味着现有的代码中如果已经使用`var`作为变量、方法或包名，不会与这个新功能冲突。这个改变有助于减少编程中的类型声明，使代码更加简洁，并提升开发者的编码效率。此外，Java SE 10的语言更新还包括其他改进，但本摘要重点在于介绍Local-Variable Type Inference这一核心变更，它标志着Java语言朝着更加智能和易读的方向发展。

Java Platform, Standard Edition

Java Language Updates

Release 10

E91239-01

March 2018

Java Language Changes for Java SE 10

Java SE 10 introduces support for inferring the type of local variables from the context,

which makes code more readable and reduces the amount of required boilerplate

code.

Local-Variable Type Inference

Code such as that shown in the following example seems redundant and makes the

code harder to read.

URL url = new URL("http://www.oracle.com/");

URLConnection conn = url.openConnection();

Reader reader = new BufferedReader(

new InputStreamReader(conn.getInputStream()));

A new identifier named

var

is now available for local variables with non-null initializers.

Using this identifier, the type of the variable is inferred from the context. The code from

the previous example can be rewritten as shown in the following example:

var url = new URL("http://www.oracle.com/");

var conn = url.openConnection();

var reader = new BufferedReader(

new InputStreamReader(conn.getInputStream()));

var

is a reserved type name, not a keyword, which means that existing code that uses

var

as a variable, method, or package name is not affected. However, code that uses

var

as a class or interface name is affected and the class or interface needs to be

renamed.

var

can be used for the following types of variables:

• Local variable declarations with initializers

• Enhanced for-loop indexes

• Index variables declared in traditional for loops

• Try-with-resources variable

The following code shows examples using

var

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