“+=”和append的区别

“+=”和append的区别是面试中出现频率较高的一个题目了，下面我们就来分析一下这两者的区别吧。
首先看一下这段代码的结果：

String s1 = "a";
String s2 = s1 + "b";
System.out.println(s2 == "ab"); // false

输出结果是false;

javap将其反编译之后的结果如下：

public static void main(java.lang.String[]);
    Code:
       0: ldc           #2                  // String a
       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 b
      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: getstatic     #8                  // Field java/lang/System.out:Ljava/io/PrintStream;
      26: aload_2
      27: ldc           #9                  // String ab
      29: if_acmpne     36
      32: iconst_1
      33: goto          37
      36: iconst_0
      37: invokevirtual #10                 // Method java/io/PrintStream.println:(Z)V
      40: return

根据结果我们可以看到：

1. 虚拟机开始是定义了一个字符串”a“将其入栈;
2. 然后new了一个StringBuilder对象，将”a“封装到Sb对象中;
3. 之后用StringBuilder的append()方法来完成"a"和"b"的拼接;

所以这里的"ab"字符串本质上是一个StringBuilder对象，所以再去跟常量"ab"去比较的话会是false；
这样看来用"+"和append效率似乎是一样的，并没有像网上说的那样”+“操作比append()更消耗性能。

那下面我们来看一下这两者在循环中是什么结果：
首先是”+=“操作：

public static void main(String[] args) {
    String[] arr = new String[]{"a","b","c"};
    String result = "";
    for (int i = 0 ; i < arr.length; i ++) {
        result += arr[i];
    }
    System.out.println(result);
}

反编译结果如下：

public static void main(java.lang.String[]);
    Code:
       0: iconst_3
       1: anewarray     #2                  // class java/lang/String
       4: dup
       5: iconst_0
       6: ldc           #3                  // String a
       8: aastore
       9: dup
      10: iconst_1
      11: ldc           #4                  // String b
      13: aastore
      14: dup
      15: iconst_2
      16: ldc           #5                  // String c
      18: aastore
      19: astore_1
      20: ldc           #6                  // String
      22: astore_2
      23: iconst_0
      24: istore_3
      25: iload_3
      26: aload_1
      27: arraylength
      28: if_icmpge     58
                                                                                                                                                                            43,1          34%
      31: new           #7                  // class java/lang/StringBuilder
      34: dup
      35: invokespecial #8                  // Method java/lang/StringBuilder."<init>":()V
      38: aload_2
      39: invokevirtual #9                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      42: aload_1
      43: iload_3
      44: aaload
      45: invokevirtual #9                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      48: invokevirtual #10                 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
      51: astore_2
      52: iinc          3, 1
      55: goto          25
      58: getstatic     #11                 // Field java/lang/System.out:Ljava/io/PrintStream;
      61: aload_2
      62: invokevirtual #12                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
      65: return

可以看到从25: iload_3和55: goto 25两句构成了一个循环，而31: new #7 // class java/lang/StringBuilder创建StringBuilder对象语句在循环内，所以会创建多个
SB对象；

下面我们把代码改为append()实现：

public static void main(String[] args) {
        String[] arr = new String[]{"a","b","c"};
        StringBuilder result = new StringBuilder();
        for (int i = 0 ; i < arr.length; i ++) {
            result.append(arr[i]);
        }
        System.out.println(result);
    }

反编译后：

public static void main(java.lang.String[]);
    Code:
       0: iconst_3
       1: anewarray     #2                  // class java/lang/String
       4: dup
       5: iconst_0
                                                                                                                                                                            85,7          68%
       6: ldc           #3                  // String a
       8: aastore
       9: dup
      10: iconst_1
      11: ldc           #4                  // String b
      13: aastore
      14: dup
      15: iconst_2
      16: ldc           #5                  // String c
      18: aastore
      19: astore_1
      20: new           #6                  // class java/lang/StringBuilder
      23: dup
      24: invokespecial #7                  // Method java/lang/StringBuilder."<init>":()V
      27: astore_2
      28: iconst_0
      29: istore_3
      30: iload_3
      31: aload_1
      32: arraylength
      33: if_icmpge     50
      36: aload_2
      37: aload_1
      38: iload_3
      39: aaload
      40: invokevirtual #8                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      43: pop
      44: iinc          3, 1
      47: goto          30
      50: getstatic     #9                  // Field java/lang/System.out:Ljava/io/PrintStream;
      53: aload_2
      54: invokevirtual #10                 // Method java/io/PrintStream.println:(Ljava/lang/Object;)V
      57: return

可以发现在由30: iload_3和47: goto 30构成的循环体内没有new操作，而是放到了循环外部
20: new #6 // class java/lang/StringBuilder

由此得出，在循环时使用+=会创建多个StringBuilder对象，而使用append()，只会创建一个。
所以我们在平时写代码的时候一定注意，不要再循环中使用+=操作，效率很低的。