“+=”和append的区别

短命女 2022-03-07 13:37 202阅读 0赞

“+=”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。