I have a problem comparing strings..please help

Programming Questions

sha89red

I have a problem comparing strings..please help

in Programming Questions • 8 months ago

I tried a simple program like below to just for test :

String a = "a";
String b = "a";
if(a == b){
print(true);
}

and its working. But when i tried to read a String from a text file and compare it, it doesnt work :

String read[] = loadStrings("locationOfTheFile"); //let say that we have 3 lines each with 2 columns,
//the String I would like to compare is at the
//1st column
String answer = "why"; //compare with this
for(int i=0; i<read.length; i++){
if( why == splitTokens(read[i])[0]){
println(true);
} else {
println(false);
}
}

I got all 3 lines as false but its actually got the same words "why"..could someone help??

Replies(11)

tubutas

Re: I have a problem comparing strings..please help

8 months ago

In java you use String1.equals(string2)

The reasoning behind this IIRC is because in java the string class is nothing more than an array of characters and == means that the two variables are pointing to the same object and even though ['H','e','l','l','o'] looks the same as ['H','e','l','l','o'], it is not the same object so it'll return false.

sha89red

Re: Re: I have a problem comparing strings..please help

8 months ago

thanks a lot for the help and fast reply too tubutas..

sha89red

Re: I have a problem comparing strings..please help

8 months ago

but could someone explain why the first code could be done just by using == ?

tubutas

Re: I have a problem comparing strings..please help

8 months ago

but could someone explain why the first code could be done just by using == ?

Idk it doesn't seem like it should work. Or maybe because the string is one character long it converts "a"(the string) into 'a'(the character) and you can use == for characters.

sha89red

Re: Re: I have a problem comparing strings..please help

8 months ago

thanks for the explanation.. maybe your right..

GoToLoop

Re: I have a problem comparing strings..please help

8 months ago

String a = "xyz";
String b = "xyz";
println(a == b);
exit();

Even literal values occupy space within RAM.
So, the literal " xyz" has an address memory!
And I believe it uses 6 bytes, since each char primitive data-type is 2 bytes each!

When String a = "xyz"; is issued, a = address memory of " xyz" + reference to char[] I believe.

What happens is that when Java compiler sees the 2nd " xyz"
and realizes it has the same content as the 1st one;
rather than allocating another memory portion for the same thing, it reuses it again for String b!

Thus, both variables a & b has the same reference memory address.
And in this case, directly comparing them results true!!!

Well, that's my theory on it anyways!

P.S.: As some may notice, my point-of-view is from a C/C++ programmer!

PhiLho

Re: I have a problem comparing strings..please help

8 months ago

That's right, GoToLoop. It is named string interning.
In the first case, the compiler sees all the strings at compile time and can find they are the same. So, to save space, since strings are immutable anyway, it just make each variable to point to the same string in the binary.
In the second case, one string has been built from an external source, so the trick doesn't work and the strings occupy different locations.

GoToLoop

Re: I have a problem comparing strings..please help

8 months ago

Nice I've got it right! ^_^

And I've got an extra tip to save memory:
When using literal fractional numbers, always suffix them w/ an f.

The reason for it is that Java stores fractional literals as double data-type,
which uses 8 bytes of RAM, rather than 4 bytes as float!

float a = 5.5; <- 5.5 uses 8 bytes
float a = 5.5f; <- 5.5f uses 4 bytes

PhiLho

Re: I have a problem comparing strings..please help

8 months ago

I am sorry, but the last trick doesn't work...
If a is declared as a float, its value will be stored as a float.
Perhaps the bytecode will hold the double value, but that won't bloat the .class file anyway...
Actually, I think the first expression won't compile in classical Java, you will have an error because of the loss of precision. You have either to add the suffix or to cast to force the precision loss.
Except in Processing: the pre-processor will take all float literals without suffix and will force the F suffix on them.

GoToLoop

Re: I have a problem comparing strings..please help

8 months ago

Wow! I had no idea that Processing's pre-processor would suffix w/ an f
every literal number which had a decimal point in it and was not suffixed yet!

I only wished it could go further and include #define & #if as C/C++ enjoy for decades!

Now, I was aware that after the assigning operation, the literal would be stored as float.
That float-type variable has only 4 bytes to spare, of course!

What I've meant is that the literal would hang in the byte-code as an 8-byte double!
But since Processing auto-suffix them to be 4 bytes, now I have 1 less worry when coding!

sha89red

Re: I have a problem comparing strings..please help

8 months ago

though i dont think i fully understand, thanks for the explanation everyone..

Top Reply