为什么StringBuilder有String？ [英] Why StringBuilder when there is String?

问题描述

我第一次遇到 StringBuilder ，并且因为Java已经有一个非常强大的 String 类而感到惊讶允许追加。

I just encountered StringBuilder for the first time and was surprised since Java already has a very powerful String class that allows appending.

为什么第二个字符串类？

我在哪里可以了解更多关于 StringBuilder ？

Where can I learn more about StringBuilder?

推荐答案

字符串不允许附加。您在 String 上调用的每个方法都会创建一个新对象并将其返回。这是因为 String 是不可变的 - 它不能改变其内部状态。

String does not allow appending. Each method you invoke on a String creates a new object and returns it. This is because String is immutable - it cannot change its internal state.

另一方面 StringBuilder 是可变的。当你调用 append（..）时，它会改变内部char数组，而不是创建一个新的字符串对象。

On the other hand StringBuilder is mutable. When you call append(..) it alters the internal char array, rather than creating a new string object.

因此更有效：

StringBuilder sb = new StringBuilder();
for (int i = 0; i < 500; i ++) {
    sb.append(i);
}

而不是 str + = i ，这将创建500个新的字符串对象。

rather than str += i, which would create 500 new string objects.

请注意，在示例中我使用循环。正如helios在评论中指出的那样，编译器会自动将 String d = a + b + c 等表达式转换为

Note that in the example I use a loop. As helios notes in the comments, the compiler automatically translates expressions like String d = a + b + c to something like

String d = new StringBuilder(a).append(b).append(c).toString();

另请注意另外还有 StringBuffer 到 StringBuilder 。不同之处在于前者具有同步方法。如果将其用作局部变量，请使用 StringBuilder 。如果它可能被多个线程访问，请使用 StringBuffer （这是罕见的）

Note also that there is StringBuffer in addition to StringBuilder. The difference is that the former has synchronized methods. If you use it as a local variable, use StringBuilder. If it happens that it's possible for it to be accessed by multiple threads, use StringBuffer (that's rarer)

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