StringBuilder Append, StringBuffer's append and string str = "a" + "B" difference?

Everyone knows that string+string will spend extra system resources, roughly because string is an immutable class, and each step returns a new string variable, taking up space and time.
In fact, I do not understand this, we look at the bottom of the string+ implementation.

Test case

 Public Static voidMain (string[] args) {String a= "a"; StringBuilder b=NewStringBuilder ("B"); StringBuffer C=NewStringBuffer ("C"); LongStar =System.currenttimemillis ();  for(inti=0;i<200000;i++) {a+ = "a"; }        LongEnd =System.currenttimemillis (); System.out.println ("String:" + (end-star)); Star=System.currenttimemillis ();  for(inti=0;i<200000;i++) {b.append ("B"); } End=System.currenttimemillis (); System.out.println ("StringBuilder:" + (end-star)); Star=System.currenttimemillis ();  for(inti=0;i<200000;i++) {c.append (C); } End=System.currenttimemillis (); System.out.println ("StringBuffer:" + (end-star)); }

Test results

string:17735
Stringbuilder:6
Stringbuffer:7

Test Conclusion
String+ does take up too much resources and is inefficient to handle. StringBuilder newer synchronization methods than StringBuffer, with improved performance.
Cause analysis
I use Javap-verbose to view the compiled class file discovery:

 Public Static voidMain (java.lang.string[]); Flags:acc_public, acc_static code:stack=6, locals=9, args_size=1 0:ldc #16//String a2: Astore_13:New#18//class Java/lang/stringbuilder6: DUP7:LDC #20//String b9:invokespecial #22//Method Java/lang/stringbuilder.<init> ":(ljava/lang/string;) V 12: astore_213:New#25//class Java/lang/stringbuffer16: DUP17:LDC #27//String C19:invokespecial #29//Method java/lang/stringbuffer. "Init> ":(ljava/lang/string;) V 22: Astore_323:invokestatic #30//Method java/lang/system.currentImemillis: () J26:lstore 4 28: Iconst_029:istore 6 31:Goto57 34:New#18//class Java/lang/stringbuilder37: DUP38: Aload_139:invokestatic #36//Method java/lang/string.valueof(Ljava/lang/object;) ljava/lang/String; 42:invokespecial #22//Method Java/lang/stringbuilder.<init> ":(ljava/lang/string;) V 45:LDC #16//String a47:invokevirtual #42//Method Java/lang/stringbuilder.Ppend: (ljava/lang/string;) ljava/lang/StringBuilder; 50:invokevirtual #46//Method Java/lang/stringbuilder.Ostring: () ljava/lang/String; 53: Astore_154:iinc 6, 1 57:iload 6 59:LDC #50//int 10000061:IF_ICMPLT 64:invokestatic #30//Method java/lang/system.currentImemillis: () J67:lstore 6 69:getstatic #51//Field Java/lang/system.out:ljav/io/PrintStream; 72:New#18//class Java/lang/stringbuilder75: DUP76:LDC #55//string string:78:invokespecial #22//Method Java/lang/stringbuilder.<init> ":(ljava/lang/string;) V 81:lload 6 83:lload 4 85: Lsub86:invokevirtual #57//Method Java/lang/stringbuilder.Ppend: (J) ljava/lang/StringBuilder; 89:invokevirtual #46//Method Java/lang/stringbuilder.Ostring: () ljava/lang/String; 92:invokevirtual #60//Method Java/io/printstream.prinLN: (ljava/lang/String;) V95:invokestatic #30//Method java/lang/system.currentImemillis: () J98:lstore 4 100: Iconst_0101:istore 8 103:Goto116 106: Aload_2107:LDC #20//String b109:invokevirtual #42//Method Java/lang/stringbuilder.Ppend: (ljava/lang/string;) ljava/lang/StringBuilder; 112: Pop113:iinc 8, 1 116:iload 8 118:LDC #50//int 100000120:IF_ICMPLT 106 123:invokestatic #30//Method java/lang/system.currentImemillis: () J126:lstore 6 128:getstatic #51//Field Java/lang/system.out:ljav/io/PrintStream; 131:New#18//class Java/lang/stringbuilder134: DUP135:LDC #65//String StringBuilder:137:invokespecial #22//Method Java/lang/stringbuilder.<init> ":(ljava/lang/string;) V 140:lload 6 142:lload 4 144: Lsub145:invokevirtual #57//Method Java/lang/stringbuilder.Ppend: (J) ljava/lang/StringBuilder; 148:invokevirtual #46//Method Java/lang/stringbuilder.Ostring: () ljava/lang/String; 151:invokevirtual #60//Method Java/io/printstream.prinLN: (ljava/lang/String;) V154:invokestatic #30//Method java/lang/system.currentImemillis: () J157:lstore 4 159: Iconst_0160:istore 8 162:Goto175 165: Aload_3166:LDC #27//String C168:invokevirtual #67//Method java/lang/stringbuffer.aPend: (ljava/lang/string;) ljava/lang/StringBuffer; 171: Pop172:iinc 8, 1 175:iload 8 177:LDC #50//int 100000179:IF_ICMPLT 165 182:invokestatic #30//Method java/lang/system.currentImemillis: () J185:lstore 6 187:getstatic #51//Field Java/lang/system.out:ljav/io/PrintStream; 190:New#18//class Java/lang/stringbuilder193: DUP194:LDC #70//String StringBuffer:196:invokespecial #22//Method Java/lang/stringbuilder.<init> ":(ljava/lang/string;) V 199:lload 6 201:lload 4 203: Lsub204:invokevirtual #57//Method Java/lang/stringbuilder.Ppend: (J) ljava/lang/StringBuilder; 207:invokevirtual #46//Method Java/lang/stringbuilder.Ostring: () ljava/lang/String; 210:invokevirtual #60//Method Java/io/printstream.prinLN: (ljava/lang/String;) V213:return

Judging from the compiled code, the exact operation of string+ is:
New StringBuilder ()
New String.valueof ()
Stringbuilder.<init>
Stringbuilder.append ()
Stringbuilder.tostring ()
and the exact operation of StringBuilder is:
Stringbuilder.append ()
The exact operation of StringBuffer () is:
Stringbuffer.append

Transfer from http://alqm1314-126-com.iteye.com/blog/1932879

StringBuilder Append, StringBuffer's append and string str = "a" + "B" difference?