“+=”和append的区别

首先感谢大佬https://segmentfault.com/a/1190000018238445?utm_source=tag-newest
java
发布于 2019-02-21
“+=”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.""????)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：

虚拟机开始是定义了一个字符串”a“将其入栈;
然后new了一个StringBuilder对象，将”a“封装到Sb对象中;
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.""????)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.""????)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。