Added LFU cache implementation

src/main/java/com/caching/LFUCache.java

+package com.caching;
+
+import java.util.HashMap;
+import java.util.NoSuchElementException;
+import java.util.TreeMap;
+
+/**
+ * Your LFUCache object can be instantiated and called as such: LFUCache
+ * lfuCache = new LFUCache(capacity); lfuCache.put(key,value); int param_1 =
+ * lfuCache.get(key);
+ */
+class LFUCache<T> {
+    // internal Node to store cache element
+    private class Node {
+        int key;
+        T value;
+        int freq;
+        Node next;
+        Node pre;
+
+        public Node(int key, T value, int freq) {
+            this.key = key;
+            this.value = value;
+            this.freq = freq;
+            next = pre = null;
+        }
+
+        public String toString() {
+            return " Key: " + key + "Value: " + value + "Freq: " + freq;
+        }
+
+    }
+
+    // internal Doubly Linked List to store cache nodes
+    private class DLL {
+        Node head;
+        Node tail;
+        int len;
+
+        public DLL() {
+            head = new Node(-1, null, -1);
+            tail = new Node(-1, null, -1);
+            head.next = tail;
+            tail.pre = head;
+            len = 0;
+        }
+
+        public void addToHead(Node node) {
+            node.next = head.next;
+            head.next.pre = node;
+            head.next = node;
+            node.pre = head;
+            len++;
+        }
+
+        public void deleteNode(Node node) {
+            node.pre.next = node.next;
+            node.next.pre = node.pre;
+            len--;
+        }
+
+    }
+
+    private int capacity;
+    private int size;
+    private TreeMap<Integer, DLL> freq;
+    private HashMap<Integer, Node> map;
+
+    /**
+     * instantiates LFUCache with fixed capacity
+     * 
+     * @param capacity The capacity of the cache. Once the cache reaches capacity,
+     *                 new elements will replace old elements in LFU manner
+     */
+    public LFUCache(int capacity) {
+        this.capacity = capacity;
+        size = 0;
+        freq = new TreeMap<Integer, DLL>();
+        map = new HashMap<Integer, Node>();
+        System.out.println("LFUCache initialised with capacity: " + capacity);
+    }
+
+    /**
+     * To get the cached value for given key
+     * 
+     * @param key The key (int) of the expected value
+     * @return corresponding value for input key
+     * @throws NoSuchElementException if key is absent
+     */
+    public T get(int key) {
+        // Cache hit condition
+        if (map.containsKey(key)) {
+            Node node = map.get(key);
+            System.out.println("Returning value from cache:" + node.toString());
+            DLL dll = freq.get(node.freq);
+            dll.deleteNode(node);
+            if (dll.len == 0)
+                freq.remove(node.freq);
+            node.freq += 1;
+            dll = freq.computeIfAbsent(node.freq, k -> new DLL());
+            dll.addToHead(node);
+            return node.value;
+        }
+        // Cache miss condition
+        throw new NoSuchElementException("No element for key: " + key);
+    }
+
+    /**
+     * To put a value in LFU cache with corresponding key
+     * 
+     * @param key   The key (int)
+     * @param value The value to be cached
+     */
+    public void put(int key, T value) {
+        if (capacity == 0) {
+            System.out.println("Cache set to 0 capacity. No element will be cached");
+            return;
+        }
+        if (map.containsKey(key)) {
+            System.out.println("Key " + key + " already present in cache.Value will be replaced");
+            Node node = map.get(key);
+            node.value = value;
+            DLL dll = freq.get(node.freq);
+            dll.deleteNode(node);
+            if (dll.len == 0)
+                freq.remove(node.freq);
+            node.freq += 1;
+            dll = freq.computeIfAbsent(node.freq, k -> new DLL());
+            dll.addToHead(node);
+        } else {
+            System.out.println("Adding new key " + key + " to cache");
+            Node node = new Node(key, value, 1);
+            map.put(key, node);
+
+            if (size < capacity) {
+                size++;
+                DLL dll = freq.computeIfAbsent(1, k -> new DLL());
+                dll.addToHead(node);
+            } else {
+                System.out.println("Cache at peak capacity.Old values will be removed in LFU fashion");
+                Integer lowest = freq.firstKey();
+                DLL dll = freq.get(lowest);
+                map.remove(dll.tail.pre.key);
+                System.out.println("Value removed:" + dll.tail.pre.value.toString());
+                dll.deleteNode(dll.tail.pre);
+                if (dll.len == 0 && lowest != 1)
+                    freq.remove(lowest);
+                DLL freq_one = freq.computeIfAbsent(1, k -> new DLL());
+                freq_one.addToHead(node);
+            }
+        }
+
+    }
+
+}
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src/test/java/com/caching/LFUCacheTest.java

+package com.caching;
+
+import java.util.NoSuchElementException;
+
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+
+public class LFUCacheTest {
+
+    @Test
+    public void testLFUCache() {
+        LFUCache<Integer> cache = new LFUCache<Integer>(2 /* capacity */ );
+
+        cache.put(1, 1);
+        cache.put(2, 2);
+        Assertions.assertEquals(1, cache.get(1)); // returns 1
+        cache.put(3, 3); // evicts key 2
+        Assertions.assertThrows(NoSuchElementException.class, () -> cache.get(2));// throws exception
+        Assertions.assertEquals(3, cache.get(3)); // returns 3.
+        cache.put(4, 4); // evicts key 1.
+        Assertions.assertThrows(NoSuchElementException.class, () -> cache.get(1));// throws exception
+        Assertions.assertEquals(3, cache.get(3)); // returns 3
+        Assertions.assertEquals(4, cache.get(4)); // returns 4
+    }
+}
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