performance - What are the differences of explicit and implicit instantiating String class in java -

i have been told creating string instance this

string s = new string("don't this"); // explicit 

has performance problem since creates 2 instance of string on double quoted phrase "don't this" , 1 new string() constructor!

today had time test self created 2 classes:

public class string1 { public static void main(string[] args) {     string s = new string("hello");     system.out.println(s); } }  public class string2 { public static void main(string[] args) {     string s = "hello";     system.out.println(s); } } 

here output of javap:

c:\jav>javap string1 compiled "string1.java" public class string1 extends java.lang.object{     public string1();     public static void main(java.lang.string[]); }  c:\jav>javap string2 compiled "string2.java" public class string2 extends java.lang.object{     public string2();     public static void main(java.lang.string[]); } 

seems same -c flag outputs deferent.

c:\jav>javap -c string1 compiled "string1.java" public class string1 extends java.lang.object{ public string1();   code:   0:   aload_0   1:   invokespecial   #1; //method java/lang/object."<init>":()v   4:   return  public static void main(java.lang.string[]);   code:   0:   new     #2; //class java/lang/string   3:   dup   4:   ldc     #3; //string hello   6:   invokespecial   #4; //method java/lang/string."<init>":(ljava/lang/string;)v   9:   astore_1   10:  getstatic       #5; //field java/lang/system.out:ljava/io/printstream;   13:  aload_1   14:  invokevirtual   #6; //method java/io/printstream.println:(ljava/lang/string;)v   17:  return  }   c:\jav>javap -c string2 compiled "string2.java" public class string2 extends java.lang.object{ public string2();   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 hello   2:   astore_1   3:   getstatic       #3; //field java/lang/system.out:ljava/io/printstream;   6:   aload_1   7:   invokevirtual   #4; //method java/io/printstream.println:(ljava/lang/string;)v   10:  return  } 

so here questions :) first "ldc", astore_1 etc ? there documentation describing those? second javac can't figure out these 2 sentences equal??

wikipedia has convenient summary of possible java bytecode instructions. also, full picture, it's better use javap -v, see entire content of file, including constant pool:

classfile /.../string1.class   last modified 02/05/2013; size 458 bytes   md5 checksum e3c355bf648c7441784ffc6b9765ba4d   compiled "string1.java" public class string1   sourcefile: "string1.java"   minor version: 0   major version: 51   flags: acc_public, acc_super constant pool:    #1 = methodref          #8.#17         //  java/lang/object."<init>":()v    #2 = class              #18            //  java/lang/string    #3 = string             #19            //  hello    #4 = methodref          #2.#20         //  java/lang/string."<init>":(ljava/l ang/string;)v    #5 = fieldref           #21.#22        //  java/lang/system.out:ljava/io/prin tstream;    #6 = methodref          #23.#24        //  java/io/printstream.println:(ljava /lang/string;)v    #7 = class              #25            //  string1    #8 = class              #26            //  java/lang/object    #9 = utf8               <init>   #10 = utf8               ()v   #11 = utf8               code   #12 = utf8               linenumbertable   #13 = utf8               main   #14 = utf8               ([ljava/lang/string;)v   #15 = utf8               sourcefile   #16 = utf8               string1.java   #17 = nameandtype        #9:#10         //  "<init>":()v   #18 = utf8               java/lang/string   #19 = utf8               hello   #20 = nameandtype        #9:#27         //  "<init>":(ljava/lang/string;)v   #21 = class              #28            //  java/lang/system   #22 = nameandtype        #29:#30        //  out:ljava/io/printstream;   #23 = class              #31            //  java/io/printstream   #24 = nameandtype        #32:#27        //  println:(ljava/lang/string;)v   #25 = utf8               string1   #26 = utf8               java/lang/object   #27 = utf8               (ljava/lang/string;)v   #28 = utf8               java/lang/system   #29 = utf8               out   #30 = utf8               ljava/io/printstream;   #31 = utf8               java/io/printstream   #32 = utf8               println {   public string1();     flags: acc_public     code:       stack=1, locals=1, args_size=1          0: aload_0          1: invokespecial #1                  // method java/lang/object."<init> ":()v          4: return       linenumbertable:         line 1: 0    public static void main(java.lang.string[]);     flags: acc_public, acc_static     code:       stack=3, locals=2, args_size=1          0: new           #2                  // class java/lang/string          3: dup          4: ldc           #3                  // string hello          6: invokespecial #4                  // method java/lang/string."<init> ":(ljava/lang/string;)v          9: astore_1         10: getstatic     #5                  // field java/lang/system.out:ljav a/io/printstream;         13: aload_1         14: invokevirtual #6                  // method java/io/printstream.prin tln:(ljava/lang/string;)v         17: return       linenumbertable:         line 3: 0         line 4: 10         line 5: 17 } 

and it's clear ldc loads constant.

regarding question why javac doesn't bother these optimizations - it's because entire optimization done on java deferred runtime, different compiler runs: jit compiler, compiles java bytecode native machine code. javac make effort optimize "common" cases, it's far aggressiveness of jitter.