Why Does HashMap in Java Not Accept Primitive Data Types Like int and char?

Why can't I use primitive types like int and char with HashMap in Java?

public HashMap<Character, Integer> buildMap(String letters) {
    HashMap<Character, Integer> checkSum = new HashMap<>();

    for (int i = 0; i < letters.length(); ++i) {
        checkSum.put(letters.charAt(i), primes[i]);
    }

    return checkSum;
}

In Java, the HashMap class is part of the Java Collections Framework, which is designed to work with objects, not primitive data types. Consequently, using primitive types like `char` or `int` directly in a HashMap results in a compilation error. The solution is to use their respective wrapper classes: `Character` for `char` and `Integer` for `int`. This article dives deeper into why this is the case and how you can work with collections in Java effectively.

public HashMap<Character, Integer> buildMap(String letters) {
    HashMap<Character, Integer> checkSum = new HashMap<>();

    for (int i = 0; i < letters.length(); ++i) {
        checkSum.put(letters.charAt(i), primes[i]);
    }

    return checkSum;
}

Causes

• Java Collections, including HashMap, are designed to use object references rather than primitive types.
• Primitive types are not objects; therefore, they cannot be directly used in generic classes like HashMap.

Solutions

• Use wrapper classes like `Character` for `char` and `Integer` for `int` when declaring your HashMap.
• Example: `HashMap<Character, Integer>` instead of `HashMap<char, int>`.

Mistake: Attempting to use primitive type directly in the HashMap declaration (e.g., `HashMap<char, int>`).

Solution: Always use wrapper classes: `HashMap<Character, Integer>`.

Mistake: Forgetting to convert primitive values to corresponding wrapper types while storing or retrieving values from HashMap.

Solution: Ensure to box the primitive values appropriately when using HashMap.