Need some Java help

[raphael2040]

I know some of you guys know Java, and since I'm learning it in my course, I figured you guys may be able to help me with some stuff.

I missed a lecture last week, but one of the questions set in the lab session was;

"Why is it not sensible to use the = = operator for Strings? How would the results differ if you used the = = operator to test equality of the above strings?"

The given code was;

public class StringEquality
{
public static void main (String args[])
{
boolean isEqualString;
String str1, str2, str3;
str1 = new String("George");
str2 = new String("George");
str3 = str2;
isEqualString = (str1.equals(str2));
System.out.println(isEqualString);
isEqualString = (str1.equals(str3));
System.out.println(isEqualString);
isEqualString = (str2.equals(str3));
System.out.println(isEqualString);
}
}

I know that the '==' operator should not be used in the above code, but I don't know why.

So, how would I answer the set question? They told us in the lecture, but I didn't attend it. =(

Thanks in advance guys. <3

Edit;

The result I'm getting when compiling the code when I use the == operator is this;

Compiling 1 source file to C:\Users\Tetsuo\Downloads\JavaApplication1\build\classes
C:\Users\Tetsuo\Downloads\JavaApplication1\src\StringEquality2.java:10: not a statement
isEqualString == (str1.equals(str2));
C:\Users\Tetsuo\Downloads\JavaApplication1\src\StringEquality2.java:12: not a statement
isEqualString == (str1.equals(str3));
C:\Users\Tetsuo\Downloads\JavaApplication1\src\StringEquality2.java:14: not a statement
isEqualString == (str2.equals(str3));
3 errors
BUILD FAILED (total time: 0 seconds

Is that right? Or is it actually supposed to work?

[Áedán]

Because the reference you have is a reference to a string object, not the string itself perhaps?

For instance, if I did something like:

String str1a="Geo";
String str1b="rge";
String str2="George";
String str3="George";
String str1 = str1a + str1b ;

What would str1, str2 and str3 look like? As Java can re-use an existing string object, str2 and str3 are probably the same as they both reference the same one string object. However, str1 and str2 will be difference, as they'll be different string objects.

That's my understanding at least.

[Gizmo]

You're on the right track, Áedán. The == operator tests to see if the two objects being compared are the same. It is possible for two strings to have the same lexical value but point to different objects, in which case the == operator will fail, even though the two strings are identical.

[danrok]

So, in Aedan's example, Java "remembers" that str1 is composed of two objects, rather than just assign the value only?

[Gizmo]

Actually, no. The assignment of 1a+1b creates an entirely new string. While str2 and str3 both point to the same object, str1 points to an entirely new object made by pasting str1a and str1b together. The interpreter has no way of knowing (at least, not an efficient way) that str1 is now equivalent to str2 and str3 in value, so the comparison fails.

[Wolf2000me]

Use string.equals(comparedobject) or string.compareTo(tocomparestring)

Those are the ones that'll compare both strings lexographically.

String (Java 2 Platform SE 5.0))

Override the compareTo method in an Object if you want to test for equality in the Objects with defining String attributes.

[raphael2040]

Originally Posted by Gizmo

You're on the right track, Áedán. The == operator tests to see if the two objects being compared are the same. It is possible for two strings to have the same lexical value but point to different objects, in which case the == operator will fail, even though the two strings are identical.

Yeah, I figured it out a few hours after I posted it! Took some reading, though.

Thanks for your help, guys. Expect more java-related questions here soon!

=D