“+=”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@H_263_406@(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。