Comparing Strings

== tests for Object equality, i.e. the a == b means a and b are references to the same object. Any time you specify "AAA" in your code the JVM will give you a reference to the same object. However using "new" will create a new Object each time, therefore a1 and b1 will not point to the same object, instead they will both have their own String objects with the value "AAA".

What you really want to check is that the value of a1 is equal to the value of b1. This is what the equals method is for. a1.equals(b1)

should return true.

Hope that makes sense.

JasonChoya at 2007-7-12 0:55:48 >

> !!!!!Never use constructor for String class!!!!!

And why's that? If I do, for example:

String test;

for(int iCounter=0; iCounter < 100; iCounter++)

test += String.valueOf(iCounter);

It will give me a nullpointerexception. While:

String test = new String();

for(int iCounter=0; iCounter < 100; iCounter++)

test += String.valueOf(iCounter);

will work fine. So please tell me, why shouldn't I use the constructor? Isn't it designed to be used? Otherwise, String should be an Abstract class, shouldn't it?

Nemesish3da at 2007-7-12 0:55:48 >

> !!!!!Never use constructor for String class!!!!!

Not entirely true.

String s1 = "... some very, very long string ...";

...

String s2 = new String(s1.subString(start, verySmallLength));

If we don't need s1 any more, but we do need s2 for a long time, we'll want to do new String(). If we don't, then the backing char array for s1 will be shared b2 s2, and we'll be eating up a lot of memory for just a few characters.

jverda at 2007-7-12 0:55:48 >

> @jverd, have you looked at the source code for String

> lately? I just checked my copies of JDK 1.6, 1.5 and

> 1.4.2, and in all of them, substring() uses new

> String(String) internally unless the substring is

> exactly the same as the original (in which case it

> returns the original).

In my version of 1.4.2, substring() uses this code if the substring is shorter than the original:new String(offset + beginIndex, endIndex - beginIndex, value)

And that calls this code:// Package private constructor which shares value array for speed.

String(int offset, int count, char value[]) {

this.value = value;

this.offset = offset;

this.count = count;

}

So that isn't new String(string) at all. I don't have a Java 5 JDK on this computer so I can't comment on later versions.

DrClapa at 2007-7-21 20:12:21 >

See reply 9. StringBuilder sb = new StringBuilder();

for (int iCounter = 0 ; iCounter < 100 ; iCounter++) {

sb.append(String.valueOf(iCounter);

}

String test = sb.toString()

I don't know if that was what georgemc was referring to - I wouldn't say it is an "ultra-obscure" bug, so I guess not. It is preferred over what you provided for performance reasons however.

TimRyanNZa at 2007-7-21 20:12:21 >