java String “+“面试总结

String a="a";
String b="b";
String c= "a"+"b";
Code:
       0: ldc           #2                  // String a
       2: astore_1
       3: ldc           #3                  // String b
       5: astore_2
       6: ldc           #4                  // String ab
       8: astore_3
       9: return

String a="a";
String b="b";
String c= "a"+b;

 Code:
       0: ldc           #2                  // String a
       2: astore_1
       3: ldc           #3                  // String b
       5: astore_2
       6: new           #4                  // class java/lang/StringBuilder
       9: dup
      10: invokespecial #5                  // Method java/lang/StringBuilder."":()V
      13: ldc           #2                  // String a
      15: invokevirtual #6                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      18: aload_2
      19: invokevirtual #6                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      22: invokevirtual #7                  // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
      25: astore_3
      26: return

"+"连接字符串的时候，默认调用StringBuilder的append方法，最后的toString方法返回的是新对象

    @Override
    public String toString() {
        // Create a copy, don't share the array
        return new String(value, 0, count);
    }

其中的注释就告诉了我们字符数组不是共享的，创建的一个复制的对象。
唯一不符合的地方就是两个字符串连接生成字符串常量如01。