“+=”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:
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 common.AppendAndAdd(); Code: 0: aload_0 1: invokespecial #1 // Method java/lang/Object."<init>":()V 4: return 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。