Why does my String turn into Integers instead of letters after I add characters with +? [duplicate]

Question

So this is the task: Given a string, return a string where for every char in the original, there are two chars.

And I don't understand why its output are numbers instead of letters, I tried doesn't work?

public String doubleChar(String str) {   String s = "";   for(int i=0; i<str.length(); i++){   s +=  str.charAt(i) + str.charAt(i);      }     return s; } 

Expected :

doubleChar("The") → "TThhee"

doubleChar("AAbb") → "AAAAbbbb"

Output:

doubleChar("The") → "168208202"

doubleChar("AAbb") → "130130196196"

Answer 1

Why does it output integers?

The + operator is overloaded in Java to perform String concatenation only for Strings, not chars.

From the Java Spec:

If the type of either operand of a + operator is String, then the operation is string concatenation.

Otherwise, the type of each of the operands of the + operator must be a type that is convertible (§5.1.8) to a primitive numeric type, or a compile-time error occurs.

In your case, char is converted to its primitive value (int), then added.

Instead, use StringBuilder.append(char) to concatenate them into a String.

If performance is not a concern, you could even do:

char c = 'A'; String s = "" + c + c; 

and s += "" + c + c;

That will force the + String concatenation operator because it starts with a String (""). The Java Spec above explains with examples:

The + operator is syntactically left-associative, no matter whether it is determined by type analysis to represent string concatenation or numeric addition. In some cases care is required to get the desired result. For example [...]

1 + 2 + " fiddlers" is "3 fiddlers"

but the result of:

"fiddlers " + 1 + 2 is "fiddlers 12"

Answer 2

In Java the char primitive type is basically just a numeric value that maps to a character, so if you add two char values together they produce a number and not another char (and not a String) so you end up with an int as you're seeing.

To fix this you can use the Character.toString(char) method like this:

s += Character.toString(str.charAt(i)) + Character.toString(str.charAt(i)) 

But this is all fairly inefficient because you're doing this in a loop and so string concatenation is producing a lot of String objects needlessly. More efficient is to use a StringBuilder and its append(char) method like this:

StringBuilder sb = new StringBuilder(str.length() * 2); for (int i = 0; i < str.length(); ++i) {     char c = str.charAt(i);     sb.append(c).append(c); } return sb.toString();