How to concatenate chars rather than add the numeric values

Question

How to connect char number with char number? My code is:

String room = "901";
// I want to a new String "01" so i do this

String roomNum = room.charAt(1) + room.charAt(2); // it's error

String roomNum = h.charAt(0).concat(h.charAt(1)); // error too

When I use String.valueOf() it gives me an ASCII.

In Java a char is equivalent to a number between 0 - 65535. Adding two chars does not give a String, but a new number. — Adriaan Koster, Oct 01 '20 at 16:10

Giorgi Tsiklauri · Answer 1 · 2020-10-02T04:56:42.307

String room = "901";
//I want to a new String "01":

Use:

room.subString(1);

String#subString(int) returns substring starting with int and ending with the last character.

For more subString overloads, have a look at String API.

If you want to concatenate two chars, you can do:

String room = "901";

char a = room.charAt(1);
char b = room.charAt(2);

String result = String.valueOf(a).concat(String.valueOf(b));

Otherwise, String roomNum = 'a'+'b'; will not compile, as the result of adding Java chars, shorts, or bytes is an int.

Beware, that char represents an Unicode Character (in Java, 16-bits each). So, each char value, under the hood, is encoded by numeric value, which is stored as a hexadecimal, decimal or other radix-system number. This is the main reason, why arithmetic operation on char value promotes to int.

Thank you so much! i did not expect that method at that time :( — Thanyatorn First, Oct 01 '20 at 15:41

Davide Lorenzo MARINO · Answer 2 · 2020-10-01T16:19:11.860

2

While using substring will solve the problem (as in the answer of @Giorgi Tsiklauri), this doesn't point to the real hidden question posted that is:

Why a concatenation between chars is not the same of a concatenation of two strings of size 1 ?

That happens because the + symbol doesn't work as a concatenation in this context.

When you apply the + operator on chars a conversion to int is done before the operation. This operation is called Binary Numeric Promotion. From the second point in the JLS:

Widening primitive conversion (§5.1.2) is applied to convert either or both operands as specified by the following rules:

If either operand is of type double, the other is converted to double.

Otherwise, if either operand is of type float, the other is converted to float.

Otherwise, if either operand is of type long, the other is converted to long.

Otherwise, both operands are converted to type int.

So if you want to sum the string value of the 0 char and the string value of the 1 char you explicitly need to convert them in string as follow:

String room = "901";

String roomNumber = String.valueof(room.charAt(1)) + String.valueof(room.charAt(2));

If you do that the + operator is considered a concatenation between strings and not as a sum between int.

edited Oct 01 '20 at 16:19

answered Oct 01 '20 at 15:42

Davide Lorenzo MARINO

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*While using substring will solve the problem (as in the answer of @Giorgi Tsiklauri), this doesn't point to the real hidden question posted that is:* - I'm not sure I agree with you. OP clearly states "*I want to a new String "01"*", and `subString(int)` is a way to choose in this case (which was also accepted by OP). Apart of that, I also provide additional information which you can read a bit down. So, I'm not sure why you think that `subString()` does not solve/answer the problem, when OP thinks it does, and it actually does - considering what is the OP's question. – Giorgi Tsiklauri Oct 01 '20 at 16:22
Where have you read the question you cite - *Why a concatenation between chars is not the same of a concatenation of two strings of size 1 ?* ? – Giorgi Tsiklauri Oct 01 '20 at 16:23
@GiorgiTsiklauri because the title says "How to concatenate chars...", not how to extract the last two chars – Davide Lorenzo MARINO Oct 02 '20 at 06:50
1. That is an edited title of the question; 2. Even with the old title, question has never asked *why char+char evaluates to int?*. OP wanted to take the substring, which you could guess should you have read the question carefully. He also found my answer helpful and that doubles the justification of what I say.. I think you're just making up an interpretation of the question, answering something that has never been asked; 3. Moreover: as I said, I also provided an additional information which just explains the point you refer to.. but seems like you didn't pay attention to that as well. – Giorgi Tsiklauri Oct 02 '20 at 08:00

How to concatenate chars rather than add the numeric values

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