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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"

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();

You are adding numeric values of chars first before concatenating the result (now integer) to the string. Try debugging with print statements:

public static String doubleChar(String str) 
 String s = "";
 for (int i = 0; i < str.length(); i++) 
 System.out.println(str.charAt(i));
 System.out.println(str.charAt(i) + str.charAt(i));
 s += str.charAt(i) + str.charAt(i);
 
 return s;

The more efficient way of doing what you want is:

public static String doubleChar(String str) 
 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();

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"

You are adding the value of two characters together. Change the String concatenation from:

s += str.charAt(i) + str.charAt(i);

To:

s += str.charAt(i) + "" + str.charAt(i);

Which will ensure the characters convert to a String.

Note: This is a quick fix, and you should use StringBuilder when String concatenating inside of a loop. See the other answers for how this is done.

String.charAt() returns a char (https://docs.oracle.com/javase/7/docs/api/java/lang/String.html#charAt(int)) so you're dealing with a single character. With onechar, you can perform operations on it like this (which will print "65" which is ASCII value for the 'A' character):

System.out.println('A' + 0);

you can print the ASCII value for the next character ("B") by adding 1 to 'A', like this:

System.out.println('A' + 1);

To make your code work – so that it doubles each character – there are number of options. You could append each character one at a time:

s += str.charAt(i);
s += str.charAt(i);

or various ways of casting the operation to a string:

s += "" + str.charAt(i) + str.charAt(i);