first commit

src/main/java/core/basesyntax/MyHashMap.java

@@ -1,19 +1,100 @@
 package core.basesyntax;
 
+import java.util.Objects;
+
 public class MyHashMap<K, V> implements MyMap<K, V> {
 
+    private static class Node<K, V> {
+        private final K key;
+        private V value;
+        private Node<K, V> next;
+
+        Node(K key, V value) {
+            this.key = key;
+            this.value = value;
+        }
+    }
+
+    private static final int DEFAULT_CAPACITY = 16;
+    private static final float DEFAULT_LOAD_FACTOR = 0.75f;
+
+    private Node<K, V>[] table;
+    private int size;
+    private int capacity;
+    private final float loadFactor;
+    private int threshold;
+
+    @SuppressWarnings("unchecked")
+    public MyHashMap() {
+        this.capacity = DEFAULT_CAPACITY;
+        this.loadFactor = DEFAULT_LOAD_FACTOR;
+        this.threshold = (int) (capacity * loadFactor);
+        this.table = (Node<K, V>[]) new Node[capacity];
+        this.size = 0;
+    }
+
+    private int hash(K key) {
+        return (key == null) ? 0 : Math.abs(key.hashCode()) % capacity;
+    }
+
     @Override
     public void put(K key, V value) {
+        int index = hash(key);
+        Node<K, V> head = table[index];
+
+        // check existing key
+        for (Node<K, V> node = head; node != null; node = node.next) {
+            if (Objects.equals(node.key, key)) {
+                node.value = value; // update
+                return;
+            }
+        }
+
+        // insert new node
+        Node<K, V> newNode = new Node<>(key, value);
+        newNode.next = head;
+        table[index] = newNode;
+        size++;
 
+        // check resize
+        if (size > threshold) {
+            resize();
+        }
     }
 
     @Override
     public V getValue(K key) {
+        int index = hash(key);
+        for (Node<K, V> node = table[index]; node != null; node = node.next) {
+            if (Objects.equals(node.key, key)) {
+                return node.value;
+            }
+        }
         return null;
     }
 
     @Override
     public int getSize() {
-        return 0;
+        return size;
+    }
+
+    @SuppressWarnings("unchecked")
+    private void resize() {
+        int oldCapacity;
+        oldCapacity = capacity;
+        capacity = capacity * 2;
+        threshold = (int) (capacity * loadFactor);
+
+        Node<K, V>[] oldTable = table;
+        table = (Node<K, V>[]) new Node[capacity];
+        size = 0;
+
+        for (int i = 0; i < oldCapacity; i++) {
+            Node<K, V> node = oldTable[i];
+            while (node != null) {
+                put(node.key, node.value); // rehash into new table
+                node = node.next;
+            }
+        }
     }
 }