HashMap with only most recent entries

Tanu Garg :

I recently had an interview where the interviewer asked me to create a HashMap that has a maximum of 7 key/value pairs. If an 8th key/value pair is added, the first key/value pair should be removed and the eighth inserted to replace it, etc.

What's a good strategy for solving this problem?

shash678 :

Make a data structure using LinkedHashMap and override removeEldestEntry i.e. something like this:

import java.util.LinkedHashMap;

class CustomHashMap extends LinkedHashMap<Integer, Integer> {
    private int capacity;

    public CustomHashMap(int capacity) {
        super(capacity, 0.75F, true);
        this.capacity = capacity;
    }

    public int get(int key) {
        return super.getOrDefault(key, -1);
    }

    public void put(int key, int value) {
        super.put(key, value);
    }

    @Override
    protected boolean removeEldestEntry(Map.Entry<Integer, Integer> eldest) {
        return size() > capacity; 
    }
}

---

Alternatively, if you are not allowed to use standard libraries or you are using a language that does not have an ordered dictionary structure like Java/Python you can use a Hashtable + and a DoubleEndedLinkedList that you can define yourself and achieve the same thing or use a Deque:

• Time complexity: O(1) for both put and get.
• Space complexity: O(capacity).

Although you have to write a lot more code.

---

Generic version as per @Holger's request:

import java.util.LinkedHashMap;
import java.util.Map;

class CustomHashMap<K, V> extends LinkedHashMap<K, V> {
    private int capacity;

    public CustomHashMap(int capacity) {
        super(capacity, 0.75F, true);
        this.capacity = capacity;
    }

    @Override
    protected boolean removeEldestEntry(Map.Entry<K, V> eldest) {
        return size() > capacity;
    }
}

Example Usage:

class Main {
    public static void main(String[] args) {
        CustomHashMap map = new CustomHashMap(3);
        map.put(1, null);
        map.put(2, null);
        map.put(3, null);
        map.put(4, null);
        System.out.println(map.keySet());
    }
}

Output:

[2, 3, 4]