常量—终态静态字段和临时终态本地变量—全部适用大写字母，并以下划线分隔各词，如MAX_CONNECTIONS。
Constants - final static fields and occasionally final local variables - are written in all uppercase with underscores separating the words, as in MAX_CONNECTIONS.
支持导入常量：这个特性使静态方法和字段能够被导入，这样在访问静态成员时就不必使用完全限定的类名。
Support for importing constants: This facility allows static methods and fields to be imported, avoiding the need to use fully qualified class names when accessing static members.
支持导入常量：这个特性使静态方法和字段能够被导入，这样在访问静态成员时就不必使用完全限定的类名。
Support for importing constants: This facility allows static methods and fields to be imported, avoiding the need to use fully qualified class names when accessing static members.
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