Java表达式的常量折叠

Java语言规范15.28规定了Java的常量表达式可以表示原始类型或者字符串；它们不但可以由纯粹的字面量构成，还可以包含能在编译时确定结果的运算，包括+、-、~、!、*、/、%、<<、>>、>>>、<、>、<=、>=、==、!=、&、|、^、&&、||、? :，还有指向上述类型的常量（final变量）的表达式。

所以下面的abc不会在运行时创建多个String，而是编译时就已经合并为一个String了。

public class JavaConstantExpression {
    public static void main(String[] args) {
        final long maxLong = Long.MAX_VALUE;
        // results in a single String constant
        String abc = "the integer " + (true ? maxLong : 0) + " is big";
    }
}

编译的.class文件：

Compiled from "JavaConstantExpression.java"
public class JavaConstantExpression extends java.lang.Object
  SourceFile: "JavaConstantExpression.java"
  minor version: 0
  major version: 50
  Constant pool:
const #1 = Method       #4.#13; //  java/lang/Object."<init>":()V
const #2 = String       #14;    //  the integer 9223372036854775807 is big
const #3 = class        #15;    //  JavaConstantExpression
const #4 = class        #16;    //  java/lang/Object
const #5 = Asciz        <init>;
const #6 = Asciz        ()V;
const #7 = Asciz        Code;
const #8 = Asciz        LineNumberTable;
const #9 = Asciz        main;
const #10 = Asciz       ([Ljava/lang/String;)V;
const #11 = Asciz       SourceFile;
const #12 = Asciz       JavaConstantExpression.java;
const #13 = NameAndType #5:#6;//  "<init>":()V
const #14 = Asciz       the integer 9223372036854775807 is big;
const #15 = Asciz       JavaConstantExpression;
const #16 = Asciz       java/lang/Object;

{
public JavaConstantExpression();
  Code:
   Stack=1, Locals=1, Args_size=1
   0:   aload_0
   1:   invokespecial   #1; //Method java/lang/Object."<init>":()V
   4:   return
  LineNumberTable:
   line 1: 0


public static void main(java.lang.String[]);
  Code:
   Stack=1, Locals=4, Args_size=1
   0:   ldc     #2; //String the integer 9223372036854775807 is big
   2:   astore_3
   3:   return
  LineNumberTable:
   line 5: 0
   line 6: 3


}

而下面代码的比较也会相等：

public class StringConstant {
    public static void main(String[] args) {
        String a = "123" + "456";
        String b = "12" + 3456;
        System.out.println(a == b); // true
    }
}

即便混搭了字符串与数字的运算也没关系。