String replace() returns extra space in Java

Question

Consider:

System.out.println(new String(new char[10]).replace("\0", "hello"));

has output:

hellohellohellohellohellohellohellohellohellohello 

but:

System.out.println(new String(new char[10]).replace("", "hello")); 

has output:

hello hello hello hello hello hello hello hello hello hello

Where are these extra spaces coming from?

Answer 1

It is not a space. It is how your IDE/console shows \0 character which new char[10] is filled with by default.

You are not replacing \0 with anything, so it stays in string. Instead with .replace("", "hello") you are replacing only empty string "". Important thing is that Java assumes that "" exists at:

• start of the string,
• end of string,
• and between each of the characters

since we can get "abc" with:

"abc" = "" + "a" + "" + "b" + "" + "c" + ""`;
      //^          ^          ^          ^

Now .replace("", "hello") replaces each of those "" with "hello", so for string of length 10 it will place additional 11 hellos (not 10), without modifying \0, which will be shown at your output like spaces.

---

Maybe this will be easier to grasp:

System.out.println("aaa".replace("", "X"));

• lets represent each "" with as |. We will get "|a|a|a|" (notice that there are 4 |)
• so replacing "" with X will result in "XaXaXaX" (but in your case instead of a your console will print \0 using character which will look like space)

Answer 2

Short version

\0 represents character NUL, it does not equals empty string "".

Long version

1. When you try to create a String with empty char[10],:

   String input = new String(new char[10]);
   

   this String will contains 10 NUL character:

   |NUL|NUL|NUL|NUL|NUL|NUL|NUL|NUL|NUL|NUL|
   

2. When you call input.replace("\0", "hello"), the NUL value(\0) will be replaced by hello:

   |hello|hello|hello|hello|hello|hello|hello|hello|hello|hello|
   

3. When you call input.replace("", "hello"), the NUL value will not be replaced since it does not match empty string "":

   |hello|NUL|hello|NUL|hello|NUL|hello|NUL|hello|NUL|hello|NUL|hello|NUL|hello|NUL|hello|NUL|hello|NUL|hello|
   

Answer 3

Explanation

You are using the method String#replace(CharSequence target, CharSequence replacement) (documentation).

If invoked with an empty target character sequence replace("", replacement) it will not replace the elements in the source, but insert the replacement before every character.

This is because "" matches the positions between the characters, not the characters themselves. So every position between will be replaced, i.e. replacement is inserted.

Example:

"abc".replace("", "d") // Results in "dadbdcd"

Your string contains only the default value of char at every position, it is

\0\0\0\0\0\0\0\0\0\0

using the method thus results in:

hello\0hello\0hello\0hello\0hello\0hello\0hello\0hello\0hello\0hello\0

---

Display

Your console probably displayed the character \0 as whitespace, while it's actually not a whitespace but \0.

If I try out your code in a different console, I get:

Confirming that the characters are indeed not spaces but something different (i.e. \0).