How to modify characters in a string in Java?

Question

Let's say we have string t.

Why does the following not work:

for (int i = 0; i < t.length; t++)
{
    t.charAt(i)+=3;   
}

Answer 1

You didn't mention your programming language.

I assume you are using C#. In C# you cant alternate a string. You can only create new ones. Therefore t.charAt is readonly and cant be alternated with your += operation. If you want to do somehting like this, use a char array or the StringBuilder class.

Edit: .NET doesn't have a method string.charAt() so it seems you are using another programming language. But i think its the same problem.

Answer 2

Because charAt returns a value, not a reference. The only language I'm aware of where you can assign to the return value of a function is C++ and there you can only do it if the function returns a reference.

You can do char c = t.charAt(3); c += 3; however this will only change the value of the variable c, it will not change t.

In order to change t you'd need something like t.setCharAt(3, t.charAt(3)+3); or t[3] += 3;, however since in most modern OO languages (like Java or C#) strings are immutable methods like setCharAt don't exist.

Answer 3

As Marks mentioned, you didn't mention your programming language. For most programming languages, you cannot assign to the return value of a function (and += is an assignment). C++ is an exception, but given the name of your method (charAt), it looks like you're using Java or perhaps C#. To set the character, you will need to use some kind of a setChar method, if strings can be mutated. In both Java and C#, strings are immutable, so they cannot be changed once they have been created. Your only choice is to create a new string containing the transformed characters.

So, there are several reasons, assuming Java or C#, why the code doesn't work:

• You can't assign to a return value (in C-parlance, a function call is not an l-value, meaning it cannot appear on the left-hand side of an assignment).
• Strings are immutable (canot have their characters changed once they have been created). There are a variety of reasons for this, one of which being that it's expensive to do it safely when your string is Unicode encoded in either UTF-8 or UTF-16.

Answer 4

If you're talking about Java, it's because the += operator is an assignment operator and you'd be trying to assign a value to a value returned from a method call.

Instead of trying to manipulate the String directly in this way, get a character array and manipulate that. Then create a new String with the resulting array.

Answer 5

The String class is immutable -> you can't modify a string instance.

You'll see that string methods doesn't modify the current instance but only creates a new string.

For exemple string.toLowerCase() will keep string the same and return a new string instance...

And other reasons, it's not correct because charAt returns a primitive type It's like writing something like: 3 = 5+2 Do you mean the new value of 3 is 7 ? So then when you do 3+1 it returns 8? :D

Answer 6

As others have said, Java Strings are immutable, and for good reason. I try to make as many of my classes immutable as possible for these reasons. In my opinion, one of the most compelling of these is that they are thread-safe without locking.