Java Academy · Lesson

Maps & Frequency Counting

Map basics (put/get/remove), entry iteration, word-frequency counter, and TreeMap for sorted view.

Lesson 1 of 38 steps

Maps & Frequency Counting is a free Java Academy lesson on CoddyKit. This is lesson 1 of 3. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Java Academy learning path, and your progress syncs across the web and the CoddyKit app. The Java Academy course includes 3 lessons in total.

What is a Map?

A Map stores key→value pairs. Good choices:

HashMap: fast, no order.
TreeMap: keys sorted.
LinkedHashMap: remembers insertion order.

Common operations

Main operations:

put(k,v), get(k), containsKey(k), remove(k)
Size: size()
Loop: entrySet() for key and value.

Code: Map basics

Keys are unique. Reusing a key replaces its value. Use entrySet() to loop over entries.

public class Main {
  public static void main(String[] args) {
    java.util.Map<String, Integer> scores = new java.util.HashMap<>();
    scores.put("Ada", 90);
    scores.put("Bob", 75);
    scores.put("Ada", 95); // replaces old value

    System.out.println("Bob's score=" + scores.get("Bob"));
    System.out.println("Has Ada? " + scores.containsKey("Ada"));
    scores.remove("Bob");

    for (java.util.Map.Entry<String, Integer> e : scores.entrySet()) {
      System.out.println(e.getKey() + " -> " + e.getValue());
    }
  }
}

Frequency idea

Word count: Loop through words and update with getOrDefault: map.put(w, map.getOrDefault(w,0)+1).

Code: word frequency

getOrDefault makes counting short. Each word adds +1 to its current count.

public class Main {
  public static void main(String[] args) {
    String[] words = { "to", "be", "or", "not", "to", "be" };
    java.util.Map<String,Integer> counter = new java.util.HashMap<>();

    for (String w : words) {
      int newVal = counter.getOrDefault(w, 0) + 1;
      counter.put(w, newVal);
    }
    System.out.println(counter);
  }
}

Code: TreeMap sorted

TreeMap sorts keys automatically. A quick way to print results in alphabetical order.

public class Main {
  public static void main(String[] args) {
    java.util.Map<String,Integer> counts = new java.util.HashMap<>();
    counts.put("banana",2);
    counts.put("apple",3);
    counts.put("cherry",1);

    java.util.Map<String,Integer> sorted = new java.util.TreeMap<>(counts);
    for (java.util.Map.Entry<String,Integer> e : sorted.entrySet()) {
      System.out.println(e.getKey() + " -> " + e.getValue());
    }
  }
}

HashMap check

Quick check: Which statement about HashMap is correct?

Answer it in the interactive lesson

Recap

Recap: Maps store key→value pairs. You learned put/get/remove, word counting with getOrDefault, and sorted views with TreeMap.

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Frequently Asked Questions

Is the “Maps & Frequency Counting” lesson free?

Yes — the full text of “Maps & Frequency Counting” is free to read here on the web. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Java Academy course, upgrade to CoddyKit PRO. The Java Academy course includes 3 lessons in total.

What will I learn in “Maps & Frequency Counting”?

Map basics (put/get/remove), entry iteration, word-frequency counter, and TreeMap for sorted view. You practise Java Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Java Academy?

No prior experience is required. Java Academy on CoddyKit is structured for beginners through advanced learners, so you can start here or from the beginning and move at your own pace. This is lesson 1 of 3.

How long does the “Maps & Frequency Counting” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Java Academy lesson?

Yes. Every Java Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

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