Comparing equality between 2 strings using '=='

Question

Sorry if this seems a pretty trivial question, but it has me rather puzzled.
when comparing reference types (objects) for equality using '==' , it normally only returns true if the two operands are one and the same object.
Now say I were to test this out with the following code:
public class EqualsTest {
  public static void main(String[] args)  {
     String a = "hello";
     String b = "hello";
     System.out.println(a==a);
     System.out.println(a==b);
     System.out.println(a.equals(a));
     System.out.println(a.equals(b));
}a==b actually returns true. This seems counter to what I expected; aren't Strings a and b separate objects despite being the same value (i.e. the literal value of "hello")?
However if I were to change the two string initializations to:
     String a = new String("hello");
     String b = new String("hello");...then it works as expected.
What's the difference here? I'm puzzled...

Answer 1

Grant, thanks for your answer - I am unclear as to
the concept of this constants pool - maybe I'll do a
search on that...As both a space and time saving, when you put something in "" in your code, it gets put into the 'constant pool' section of your class file, and everywhere in your code you use the exact same string ends up referencing the same entity.
You can accomplish this with runtime Strings by using the String.intern() method - it adds the String to the CP, and lets you use == for compares. Can be useful in code where performance is more important than clarity.
Consider this sample:public class StrCheck {
public static boolean pool() {
  String a = "one";
  String b = "one";
  return (a==b);
public static boolean ctor() {
  String c = new String("two");
  String d = new String("two");
  return c==d;
}which decompiles to:ggainey@dhcp59-157 t]$ javap -c -s -verbose StrCheck
Compiled from "StrCheck.java"
public class StrCheck extends java.lang.Object
  SourceFile: "StrCheck.java"
  minor version: 0
  major version: 49
  Constant pool:
const #1 = Method       #7.#17; //  java/lang/Object."<init>":()V
const #2 = String       #18;    //  one
const #3 = class        #19;    //  java/lang/String
const #4 = String       #20;    //  two
const #5 = Method       #3.#21; //  java/lang/String."<init>":(Ljava/lang/String;)V
const #6 = class        #22;    //  StrCheck
const #7 = class        #23;    //  java/lang/Object
const #8 = Asciz        <init>;
const #9 = Asciz        ()V;
const #10 = Asciz       Code;
const #11 = Asciz       LineNumberTable;
const #12 = Asciz       pool;
const #13 = Asciz       ()Z;
const #14 = Asciz       ctor;
const #15 = Asciz       SourceFile;
const #16 = Asciz       StrCheck.java;
const #17 = NameAndType #8:#9;//  "<init>":()V
const #18 = Asciz       one;
const #19 = Asciz       java/lang/String;
const #20 = Asciz       two;
const #21 = NameAndType #8:#24;//  "<init>":(Ljava/lang/String;)V
const #22 = Asciz       StrCheck;
const #23 = Asciz       java/lang/Object;
const #24 = Asciz       (Ljava/lang/String;)V;
public StrCheck();
  Signature: ()V
  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 boolean pool();
  Signature: ()Z
  Code:
   Stack=2, Locals=2, Args_size=0
   0:   ldc     #2; //String one
   2:   astore_0
   3:   ldc     #2; //String one
   5:   astore_1
   6:   aload_0
   7:   aload_1
   8:   if_acmpne       15
   11:  iconst_1
   12:  goto    16
   15:  iconst_0
   16:  ireturn
  LineNumberTable:
   line 3: 0
   line 4: 3
   line 5: 6
public static boolean ctor();
  Signature: ()Z
  Code:
   Stack=3, Locals=2, Args_size=0
   0:   new     #3; //class java/lang/String
   3:   dup
   4:   ldc     #4; //String two
   6:   invokespecial   #5; //Method java/lang/String."<init>":(Ljava/lang/String;)V
   9:   astore_0
   10:  new     #3; //class java/lang/String
   13:  dup
   14:  ldc     #4; //String two
   16:  invokespecial   #5; //Method java/lang/String."<init>":(Ljava/lang/String;)V
   19:  astore_1
   20:  aload_0
   21:  aload_1
   22:  if_acmpne       29
   25:  iconst_1
   26:  goto    30
   29:  iconst_0
   30:  ireturn
  LineNumberTable:
   line 9: 0
   line 10: 10
   line 11: 20
}You can see the constant pool (and lots of other neat stuff) in the disassembled output. Note that "ldc" is LoaDing a Constant from the pool - in pool(), you can see it loading constant #2 twice, and then comparing the results. ctor() uses #4 twice, but loads the results of invokespecial() and compaes that.
Grant