The problem in your code is that String.replace replaces each target character present in the String, and not only the one you want.

You could work directly on an array of chars instead of on the String:

public static String toTitleCase(String phrase) {

    // convert the string to an array 
    char[] phraseChars = phrase.toCharArray();

    for (int i = 0; i < phraseChars.length - 1; i++) {
        if(phraseChars[i] == ' ') {
            phraseChars[i+1] = Character.toUpperCase(phraseChars[i+1]);
        }
    }

    // convert the array to string
    return String.valueOf(phraseChars);
}

It's replacing all t, try below code.

It will help you.

 String phrase="this is a test";
            for (int i=0; i<phrase.length(); i++) {
                if(phrase.substring(i,i+1).equals(" ")) {
                    System.out.println(phrase.substring(i+1,i+2));
                    phrase = phrase.replace(phrase.substring(i,i+2),phrase.substring(i,i+2).toUpperCase());
                }
            }
System.out.println(phrase);

Use streams (or split) to split your string into parts, don't do it manually using substring.

Try below code

        String test = "this is a test";

        UnaryOperator<String> capitalize = str ->
                str.substring(0,1).toUpperCase() + str.substring(1).toLowerCase();

        String result =
                Stream.of(
                        test.split(" ")
                ).map(capitalize)
                .collect(
                        Collectors.joining(" ")
                );

        System.out.println(result);

Output: This Is A Test

When you replace a substring it will replace the each occurrence of that substring - which is not necessarily the one you are trying to replace. This is why it is replacing letters inside words.

Switching to a StringBuilder here to poke individual characters. Note that we don't traverse the entire String because there is no next-character to capitalize at the last character.

public String toTitleCase(String phrase) {
  StringBuilder sb = new StringBuilder(phrase);
  for (int index = 0 ; index < phrase.length - 1 ; ++index) {
    if (sb.charAt(index) == ' ') {
      sb.setCharAt(index + 1, Character.toUppercase(sb.charAt(index + 1)));
    }
  }
  return sb.toString();
}

If a letter is first in any word, it will be replaced everywhere. In your case, all t,i and a will be uppercase.
Taking example for is. It is find a space before. Than in if body, what actually happen:

phrase = phrase.replace("i","I"); 

And all i are replaced with I.
String class cannot replace at a specific position.

You have to options:

• using StringBuilder which can replace at a specific position.

String toTitleCase(String phrase) {
        StringBuilder sb= new StringBuilder(phrase);
        for (int i=0; i<phrase.length(); i++) {
            if(i==0 || phrase.charAt(i-1)==' ') {
                sb.replace(i,i+1,phrase.substring(i,i+1).toUpperCase());
            }
        }
        return sb.toString();
    }

• or with stream, which is the method I prefer because is one-line. This way you don't preserve white-spaces( multiple consecutive white-spaces will be replaced with only one space), but usually you want this.

Arrays.asList(phrase.split("\\s+")).stream().map(x->x.substring(0,1).toUpperCase()+x.substring(1)).collect(Collectors.joining(" "));