String == operator

[Jay Pawar]

Can anyone explain the why the output is false for the following code ?

String str1 = "SCJP";
String str2 = "SC";
String str3 = "JP";
String str5 = str2 + str3; //LINE X
System.out.println(str1 == str5)

The output is false that means string created at LINE X has different bits than str1. I thought that if the same string (in our case "SCJP") is available in String constant pool, then str5 would simply be pointed to str1, that would save memory isn't it ?

Am I thinking wrong here ?

TIA

[Jeroen Wenting]

A String MAY be taken from the String constant pool, but it doesn't have to be.
Therefore the behaviour of == on Strings (like on any other object) will NOT be predictable unless you're intent on comparing actual references.

[Jay Pawar]

Jeroen,
Thanks for the reply.I tried various combinations of string comparison and here is what I observed.

Whenever the == operator is operating on string objects which perform concatenation using + operator, the result is false, else the result is true.

Guys, this is my observation feel free to point out any combination which might prove this wrong.

Below is the code I wrote to test this, hopefully my fellow ranchers might benefit from this.

public class test
{
 
    public static void main (String args[])
  {
    System.out.println("SC" + "JP" == "SC" + "JP");
    System.out.println("SCJ" + "P" == "SC" + "JP");
    System.out.println("SCJP" == "SCJP" + "");
    String str1= "SCJP";
    String str2= "SCJP"; // str2 will be pointing to str1 
 
    System.out.println(str1 == "SC" + "JP");
    System.out.println(str1 == "SCJ" + "P");
    System.out.println(str1 == "SCJP" + "");
 
    System.out.println(str1 == str2);
 
    String str3 = "SC";
    String str4 = "JP";
    String str5 = str3 + str4;
            String str6 = "SCJP"; //will point to str1 itself
    System.out.println(str1 == str3 + str4);
    System.out.println(str1 == str5);
            [B]System.out.println(str1 == str6.concat("")); //Line x[/B]
  }
}

NOTE: the line x will result in true, because the concat function in String.java will check for string length which is zero in this case and will simply return this which is str6 object itself.

[Hai Le]

i think when you concatinate Str2 and Str23, reference of Str5 is different than Str1. If you try to compare Str5 with Str1 by "==", it will result "false". but with Str1.equals(Str5), it will return true.

Plz Correct me if i'm wrong.

[Jay Pawar]

You are absolutely correct Hai Le, equals will give true

[Martin Mathis]

It doesn't work because str5 isn't a string literal and has no relation to the string literal pool.

According to this...

When a .java file is compiled into a .class file, any String literals are noted in a special way, just as all constants are. When a class is loaded (note that loading happens prior to initialization), the JVM goes through the code for the class and looks for String literals. When it finds one, it checks to see if an equivalent String is already referenced from the heap. If not, it creates a String instance on the heap and stores a reference to that object in the constant table. Once a reference is made to that String object, any references to that String literal throughout your program are simply replaced with the reference to the object referenced from the String Literal Pool.

[Barry Gaunt]

Those who are interested may like to compile and run this:

public class TestCat
{
    public static void main(String[] args)
    {
        String str1 = "SCJP";
        String str2 = "SC";
        String str3 = "JP";
        String str5 = str2 + str3; //LINE X
        System.out.println(str1 == str5);
        str5 = (str2 + str3).intern(); //LINE Y
        System.out.println(str1 == str5);
    }
}
  

Note line Y and the ouput. The intern method was discussed fairly recently in this forum.