Removing Backing Array From Strings

First, it's string.substring() not .subString().

Second, what do you mean "impossible to compare easily"? You can compare strings with .equals() easily.

public static void main(String[] args) {
    String longString = "abcdefghijklmn";
    String shortString = longString.substring(0, 3);
    System.out.println(shortString.equals(longString));
}

this code prints false, as it should.

Update:

If you call .substring() so that it produces string of the same length as original string (e.g. "abc".substring(0,2)) than it will return reference to the same string. So, .equals() in this case will return true.

How would you want to compare? There's built in method for simple comparison:

longString.subString(0, 3).compareTo(anotherString); 

Alternatively, since String is a CharSequence, something like:

for (int i=0; i<4; i++){
    if (anotherString.charAt(i) != shortString.charAt(i)) return false;
}

would work as well.

Finally, every String is constructed in backing Array, there's no way to deny it, and longString.subString(0,3) would always (except index out of bound) return a String with a 4-element Char Array.

In the event that you actually need to get rid of the backing array the following will work:

String newString = StringBuilder(oldString).toString();

This might be necessary, for example, if you are parsing strings and creating substrings and you might need to do this:

String newString = StringBuilder(oldString.substring(start,end).toString();

This creates a truly new string with a zero offset and independent backing array. Otherwise, you maintain the same backing array which, in rare cases might cause a problem for the heap because it can never be garbage collected.