### 編譯器對字符串的優化 * String通過"+"拼接時如果拼接的對象是常量，則在編譯期被合并優化； ``` String st = "a" + "b" + "c"; // 或者final String s = "a";編譯器認為它是不可變的常量會自動合并優化 javap out==============> Code: stack=1, locals=2, args_size=1 0: ldc #19 // String abc 2: astore_1 3: return ``` * Stirng通過"+"拼接的對象是變量，則編譯器通過StringBuilder進行append ``` public static void main(String[] args){ String name = "luoyobao"; String sb = name + "hello"; } javap -v AppTest.class==> public static void main(java.lang.String[]); descriptor: ([Ljava/lang/String;)V flags: ACC_PUBLIC, ACC_STATIC Code: stack=2, locals=3, args_size=1 0: ldc #2 // String luoyobao 2: astore_1 3: new #3 // class java/lang/StringBuilder 6: dup 7: invokespecial #4 // Method java/lang/StringBuilder."<init>":()V 10: aload_1 11: invokevirtual #5 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder; 14: ldc #6 // String hello 16: invokevirtual #5 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder; 19: invokevirtual #7 // Method java/lang/StringBuilder.toString:()Ljava/lang/String; 22: astore_2 23: return LineNumberTable: line 5: 0 line 6: 3 line 7: 23 LocalVariableTable: Start Length Slot Name Signature 0 24 0 args [Ljava/lang/String; 3 21 1 name Ljava/lang/String; 23 1 2 sb Ljava/lang/String; ```