java - Java 编译器是否包含字符串常量折叠？

Question

我发现 Java supports constant folding of primitive types ，但是 String 呢？是吗？

示例

如果我创建以下源代码

out.write(""
        + "<markup>"
        + "<nested>"
        + "Easier to read if it is split into multiple lines"
        + "</nested>"
        + "</markup>"
        + "");

编译代码中包含什么？

合并版本？ out.write("<markup><nested>Easier to read if it is split into multiple lines</nested></markup>");

还是效率较低的运行时串联版本？ out.write(new StringBuilder("").append("<markup>").append("<nested>").append("Easier to read if it is split into multiple lines").append("</nested>").append("</markup>").append(""));

Answer 1

这是一个简单的测试:

public static void main(final String[] args) {
    final String a = "1" + "2";
    final String b = "12";        

    System.out.println(a == b);
}

输出:

true

所以，是的，编译器会折叠。