CODEWITHSUNDEEP
java
Thanyatorn First
Thanyatorn First

Reputation: 33

How to concatenate chars rather than add the numeric values

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.

Upvotes: 2

Views: 540

Answers (2)

Giorgi Tsiklauri
Giorgi Tsiklauri

Reputation: 11138

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.

Upvotes: 4

Davide Lorenzo MARINO
Davide Lorenzo MARINO

Reputation: 26946

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.

Upvotes: 2

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