下面进一步探讨，什么样的string + 表达式会被编译器当成常量表达式？
String b = "a" + "b";
这个String + String被正式是ok的,那么string + 基本类型呢？
String a = "a1";
String b = "a" + 1;
System.out.println((a == b)); //result = true
String a = "atrue";
String b = "a" + true;
System.out.println((a == b)); //result = true
String a = "a3.4";
String b = "a" + 3.4;
System.out.println((a == b)); //result = true
可见编译器对string + 基本类型是当成常量表达式直接求值来优化的。
事实的真假还是用Javap看看:
0: ldc #16; //String a1
2: astore_1
3: ldc #16; //String a1
5: astore_2
6: getstatic #18; //Field java/lang/System.out:Ljava/io/PrintStream;
9: aload_1
10: aload_2
11: if_acmpne 18
14: iconst_1
15: goto 19
18: iconst_0
19: invokevirtual #24; //Method java/io/PrintStream.println:(Z)V
22: return
看看 0: ldc #16; //String a1和 0: ldc #16; //String a1，知道作者的分时正确的。
接着作者又分析了下 string+string对象,作者分析如下：
String a = "ab";
String bb = "b";
String b = "a" + bb;
System.out.println((a == b)); //result = false
这个好理解，"a" + bb中的bb是变量，不能进行优化。这里很很好的解释了为什么3的观点不正确，如果String+String的操作是在运行时进行的，则会产生新的对象，而不是直接从jvm的string池中获取。
还是javap看看代码：
0: ldc #16; //String ab
2: astore_1
3: ldc #18; //String b
5: astore_2
6: new #20; //class java/lang/StringBuilder
9: dup
10: ldc #22; //String a
12: invokespecial #24; //Method java/lang/StringBuilder."<init>":(Ljava/lang/String;)V
15: aload_2
16: invokevirtual #27; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
19: invokevirtual #31; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;
22: astore_3
23: getstatic #35; //Field java/lang/System.out:Ljava/io/PrintStream;
26: aload_1
27: aload_3
28: if_acmpne 35
31: iconst_1
32: goto 36
35: iconst_0
36: invokevirtual #41; //Method java/io/PrintStream.println:(Z)V
39: return
来看看 0: ldc #16; //String ab 说明 变量a直接从常量池中取出ab字符串，在看看
3: ldc #18; //String b
5: astore_2
6: new #20; //class java/lang/StringBuilder
9: dup
10: ldc #22; //String a
可以知道变量b是通过new出一个StringBuilder来赋值给变量b的。这进一步证明了作者的说法。
最后看看作者分析的最后一种情况，原文：
把bb作为常量变量：
String a = "ab";
final String bb = "b";
String b = "a" + bb;
System.out.println((a == b)); //result = true
竟然又是true，编译器的优化好厉害啊，呵呵，考虑下面这种情况：
String a = "ab";
final String bb = getBB();
String b = "a" + bb;
System.out.println((a == b)); //result = false
private static String getBB() {
return "b";
}
先看看常量变量的javap的结果：
0: ldc #16; //String ab
2: astore_1
3: ldc #18; //String b
5: astore_2
6: ldc #16; //String ab
8: astore_3
9: getstatic #20; //Field java/lang/System.out:Ljava/io/PrintStream;
12: aload_1
13: aload_3
14: if_acmpne 21
17: iconst_1
18: goto 22
21: iconst_0
22: invokevirtual #26; //Method java/io/PrintStream.println:(Z)V
25: return
看看 0: ldc #16; //String ab 和 6: ldc #16; //String ab 通过优化后变量a，b是指向相同的常量空间。
再用javap分析下通过静态方法获得的：
0: ldc #16; //String ab
2: astore_1
3: invokestatic #18; //Method getBB:()Ljava/lang/String;
6: astore_2
7: new #22; //class java/lang/StringBuilder
10: dup
11: ldc #24; //String a
13: invokespecial #26; //Method java/lang/StringBuilder."<init>":(Ljava/lang/String;)V
16: aload_2
17: invokevirtual #29; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
20: invokevirtual #33; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;
23: astore_3
24: getstatic #36; //Field java/lang/System.out:Ljava/io/PrintStream;
27: aload_1
28: aload_3
29: if_acmpne 36
32: iconst_1
33: goto 37
36: iconst_0
37: invokevirtual #42; //Method java/io/PrintStream.println:(Z)V
40: return
分析：0: ldc #16; //String ab 变量a的值直接指向常量池中的‘ab’，