Java中字符串常量的串联 [英] concatenation of string constants in java

问题描述

在C ++中，创建多行字符串的最佳规范方法是创建相邻字符串，并让编译器在编译时将它们连接起来，如下所示：

In C++, the best canonical way to create a multiline string of which I am aware is to create adjacent strings and let the compiler concatenate them at compile time, like this:

string s = "This is a very long string ...\n"
   " and it keeps on going...";

在Java中，我唯一知道的方法是串联：

In Java, the only way I know to do this is with concatenation:

String s = "This is a very long string ...\n" +
   " and it keeps on going...";

问题是，这是在运行时生成单个字符串，还是在编译时实际连接了Java？也？问题出现的原因是由于以下行为：

The question is, does this generate a single string at runtime, or does java actually concatenate at compile time also? The reason the question comes up is because of the following behavior:

String s1 = "abc";
String s2 = "abc";
System.out.println(s1 == s2); // this prints true, because the compiler
// generates only one "abc" object
String s3 = "a";
s3 += "bc";
System.out.println(s1 == s3); // false

推荐答案

String s3 = "a";
s3 += "bc";

与以下内容相同：

String s3 = "a";
s3 = new StringBuilder().append(s3).append("bc").toString();

因此它创建了 String 的新实例

您甚至可以尝试：

String s = null;
s += null;
System.out.println(s); // prints "nullnull"

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