Simple and Helpful:

OOPS Concepts: Overload and Override

HashMap.Contains: Best Feature of Java 8

What are wrapper classes in Java?

What is the difference between equals() and == in Java?

What is a singleton class in Java and how can we make a class singleton?

Count the number of repeated characters in a String.

java HashMap<Character, Integer> my_map = new HashMap<Character, Integer>(); char[] str_array = charstring.toCharArray();

for (char abc : str_array) { if (my_map.containsKey(abc)) { my_map.put(abc, my_map.get(abc) + 1); } else { my_map.put(abc, 1); } } // end of char for

for (Map.Entry entry : my_map.entrySet()) { System.out.println(entry.getKey() + " " + entry.getValue()); // print (char "space" Integer) <-- comes from the map! }

String charstring = "rrrttsr";