重载+运算符

public class Contenation {
	public static void main(String[] args) {
	    String str = "Happy";
		String str2 = "I" + str + "now" + 21;
		System.out.println(str2);
	}
}

使用javap -c Contenation反编译得到的JVM字节码：
Compiled from "Contenation.java"
public class Contenation extends java.lang.Object{
public Contenation();
  Code:
   0:	aload_0
   1:	invokespecial	#1; //Method java/lang/Object."<init>":()V
   4:	return

public static void main(java.lang.String[]);
  Code:
   0:	ldc	#2; //String Happy
   2:	astore_1
   3:	new	#3; //class java/lang/StringBuilder
   6:	dup
   7:	invokespecial	#4; //Method java/lang/StringBuilder."<init>":()V
   10:	ldc	#5; //String I
   12:	invokevirtual	#6; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   15:	aload_1
   16:	invokevirtual	#6; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   19:	ldc	#7; //String now
   21:	invokevirtual	#6; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   24:	bipush	21
   26:	invokevirtual	#8; //Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
   29:	invokevirtual	#9; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;
   32:	astore_2
   33:	getstatic	#10; //Field java/lang/System.out:Ljava/io/PrintStream;
   36:	aload_2
   37:	invokevirtual	#11; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
   40:	return
}

可以发现，当我们使用+运算符时，他会生成很多临时的String对象，这些对象会在程序结束之后由垃圾回收机制回收；然后调用StringBuilder的append()方法将其拼接在一起，最后调用toString()方法输出。