论String操作时间复杂度 Java综合

面试题
以下类中，method1和method2的时间复杂度一样吗？为什么。

public class Test {
	public static final int MB = 1024 * 1024;
	public static final long TIMES = 500000000L;

	public static void main(String[] args) throws InterruptedException {
		method1();
		method2();
	}
//从字节码来看，method1重复new StringBuilder
然后append，显然不高效。这里排除jit优化。
	public static void method1() {
		String s = "";
		for (int i = 0; i < 100; i++) {
			s += "a";
		}

		System.out.println(s);
	}

	public static void method2() {
		// //b
		StringBuilder sb = new StringBuilder();
		for (int i = 0; i < 100; i++) {
			sb.append("a");
		}
		System.out.println(sb.toString());
	}

}

我当时的回答是应该一样，因为我实在找不出说明他们不一样的地方。但是面试官这么问肯定有原因的，我只是从效率方面来分析，比如说method1会通过StringBuilder来做，没有后面method2好。后来面试官纠正了提问，说从时间复杂度来考虑。我直接跟他说了，我只能看出是一样的，可能method2会更好些。面试官好像不是很满意。

回来我从源码着手看，感觉不靠谱，后来看到有人分析用字节码来看。可以看到编译器怎么翻译的。经过证实，跟我的想法一样。字节码如下：

Compiled from "Test.java"
public class Test extends java.lang.Object{
public static final int MB;

public static final long TIMES;

public Test();
  Code:
   0:	aload_0
   1:	invokespecial	#16; //Method java/lang/Object."<init>":()V
   4:	return

public static void main(java.lang.String[])   throws java.lang.InterruptedException;
  Code:
   0:	invokestatic	#27; //Method method1:()V
   3:	invokestatic	#30; //Method method2:()V
   6:	return

public static void method1();
  Code:
   0:	ldc	#35; //String 
   2:	astore_0
   3:	iconst_0
   4:	istore_1
   5:	goto	31
   8:	new	#37; //class java/lang/StringBuilder
   11:	dup
   12:	aload_0
   13:	invokestatic	#39; //Method java/lang/String.valueOf:(Ljava/lang/Object;)Ljava/lang/String;
   16:	invokespecial	#45; //Method java/lang/StringBuilder."<init>":(Ljava/lang/String;)V
   19:	ldc	#48; //String a
   21:	invokevirtual	#50; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   24:	invokevirtual	#54; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;
   27:	astore_0
   28:	iinc	1, 1
   31:	iload_1
   32:	bipush	100
   34:	if_icmplt	8
   37:	getstatic	#58; //Field java/lang/System.out:Ljava/io/PrintStream;
   40:	aload_0
   41:	invokevirtual	#64; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
   44:	return

public static void method2();
  Code:
   0:	new	#37; //class java/lang/StringBuilder
   3:	dup
   4:	invokespecial	#73; //Method java/lang/StringBuilder."<init>":()V
   7:	astore_0
   8:	iconst_0
   9:	istore_1
   10:	goto	23
   13:	aload_0
   14:	ldc	#48; //String a
   16:	invokevirtual	#50; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   19:	pop
   20:	iinc	1, 1
   23:	iload_1
   24:	bipush	100
   26:	if_icmplt	13
   29:	getstatic	#58; //Field java/lang/System.out:Ljava/io/PrintStream;
   32:	aload_0
   33:	invokevirtual	#54; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;
   36:	invokevirtual	#64; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
   39:	return

}