String Based

Counting

Base Code

void count(String str) {
    int count = 0;
    char ch;
    for (int i = 0; i < str.length(); i++) {
        ch = str.charAt(i);
        if ({condition here}) {
            count++; // or count = count + 1
        }
    }
    System.out.println(count);
}

Pick & Concatenate (Join)

Here, we'll pick a character if it satisfies a particular condition and add it to another string which will be printed out later.

Base Code

void pickAndConcat(String str) {
    String s = "";
    char ch;
    for (int i = 0; i < str.length(); i++) {
        ch = str.charAt(i);
        if ({condition here}) {
            s += ch;
        }
    }
    System.out.println(s);
}

Pick & Remove (Join)

Here, we'll pick a character if it satisfies a particular condition. If it does not, then it is added to another string else not.

Base Code

void pickAndConcat(String str) {
    String s = "";
    char ch;
    for (int i = 0; i < str.length(); i++) {
        ch = str.charAt(i);
        if (!{condition here}) {
            s += ch;
        }
    }
    System.out.println(s);
}

Here we've used the ! (not) operator which basically flips the boolean.

The Conditions

You'll have to substitute these conditions in place of {condition here}

1. Digits:

   Character.isDigit(ch)
   

2. Spaces: ch == ' ' or Character.isWhitespace(ch) if you want to include all types of whitespaces.
3. Vowels:

   ch == 'a' || ch == 'e' || ch == 'i' || ch == 'o' || ch == 'u'
   

4. Consonants:

   (ch != 'a' || ch != 'e' || ch != 'i' || ch != 'o' || ch != 'u') && Character.isLetter(ch)
   

   Note: Character.isLetter is important as the supplied character may not be a letter.

5. Uppercase:

   Character.isUpperCase(ch)
   

6. Lowercase:

   Character.isLowerCase(ch)
   

String & ASCII Based

1. Printing character ASCII table. The objective is to print each character of a string along with its corresponding ASCII value.

   Solution

   void printASCIITable(String s) {
        for (int i = 0; i < s.length(); i++) {
            System.out.println(s.charAt(i) + "\t" + (int)(s.charAt(i)));
        }
   }
   

2. Total of ASCII values of all characters.

   Solution

   int totalASCII(String s) {
        int total = 0;
        for (int i = 0; i < s.length(); i++) {
            total += (int)(s.charAt(i));
        }
        return total;
   }
   

3. Concat odd ASCII Characters

   We'll concat all odd ASCII value characters in one new string

   Solution

   String oddASCIIStr(String s) {
        String new_string = "";
        for (int i = 0; i < s.length(); i++) {
            if ((int)(s.charAt(i)) % 2 != 0) {
                new_string += s.charAt(i);
            }
        }
        return new_string;
   }
   

First & Last

1. Converting the first and last character of an given string to lowercase and others to upper case.

   Solution

   String firstLastLower(String str) {
       String s = Character.toLowerCase(str.charAt(0)); // first lower
       s += s.substring(1, s.length() - 1).toUpperCase(); // the middle
       s += Character.toLowerCase(s.charAt(s.length() - 1));
       return s;
   }
   

   One liner:

   String firstLastLower(String str) {
       return Character.toLowerCase(str.charAt(0)) +
                   s.substring(1, s.length() - 1).toUpperCase() +
                   Character.toLowerCase(s.charAt(s.length() - 1));
   }
   

   Note that this is error-prone as it does not check whether string is empty or not.