对Java字符串使用'=='而不是.equals [英] Using '==' instead of .equals for Java strings

问题描述

可能重复：

什么使得参考比较（==）适用于Java中的某些字符串？

我知道这是之前的问题，但是尽管建议使用 .equals（）而不是 == 比较运算符，我发现 == 始终有效：

I know this has been asked before, but in spite of recommendations to use .equals() instead of the == comparison operator, I found that == works all the time:

String s1 = "Hello";
String s2 = "Hello";
System.out.println(s1 == s2); // true

任何人都可以给我一个的例子== 运算符失败？

Can anyone give me an example of the == operator failing?

推荐答案

这是因为你很幸运。 Java中的 == 运算符检查引用相等：如果指针相同，则返回true。它不检查内容是否相等。在编译时找到的相同字符串将折叠为单个 String 实例，因此它适用于 String 文字，但不能使用在运行时生成的字符串。

This is because you're lucky. The == operator in Java checks for reference equality: it returns true if the pointers are the same. It does not check for contents equality. Identical strings found at compile-time are collapsed into a single String instance, so it works with String literals, but not with strings generated at runtime.

例如，Foo==Foo可能有效，但Foo== new String（Foo）不会，因为 new String（Foo）创建一个新的 String 实例，并打破任何可能的指针相等。

For instance, "Foo" == "Foo" might work, but "Foo" == new String("Foo") won't, because new String("Foo") creates a new String instance, and breaks any possible pointer equality.

更重要的是，大多数 在实际程序中处理的字符串是运行时生成的。文本框中的用户输入是运行时生成的。通过套接字接收的消息是运行时生成的。从文件读取的东西是运行时生成的。因此，如果要检查是否使用等于方法，而不是 == 运算符，这一点非常重要内容平等。

More importantly, most Strings you deal with in a real-world program are runtime-generated. User input in text boxes is runtime-generated. Messages received through a socket are runtime-generated. Stuff read from a file is runtime-generated. So it's very important that you use the equals method, and not the == operator, if you want to check for contents equality.

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