Hashmap implementation

codereview/hashMap.md

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+## Purpose
+
+I've never implemented a `HashMap` and thought it would be a good data structure exercise. I tried looking at the Java
+source code as little as possible.
+
+## Discussion
+
+The `HashMap` was made up of an array of `Entry`s. Each `Entry` is points to another `Entry` (or `null` if it's the
+last `Entry` in the linked list).
+
+Since you'd want `Entry`s distributed as equally as possible across the different index values of the internal `Entry`
+array, each key's hashcode is hashed again to combat a bad hashcode (this code is from the Java 8 source code) and
+eventually an index value is calculated - this index value represents the index of the internal array that the
+key-value pair will exist under.
+
+Thus, the logic for getting a value for a particular key would be to translate the key into an index value, and then
+to get the first `Entry` for that index value from the internal array. And then to get the `Entry` in the linked list
+with the same key (or return `null` if the linked list has been exhausted).
+
+Another thing that was interesting to implement was array resizing. Every time `put` is called, it checks to see if array
+resizing needs to occur. I resize the array if the number of objects that have been added to the array is greater than
+75% of the allocated array capacity. I don't know if this is the best way to implement a resize check. When actually
+resizing the array, I needed to iterate through every `Entry` and reindex the `Entry`.
+
+Things I don't like:
+* My implementation seems really messy.
+  1. I don't like the `addEntry` method. I don't like it has a `boolean` return. However, I did this so that when `put`ting
+    I wouldn't update the `size` when I updated a key-value pair vs. added one.
+  2. I don't like how the `setEntry` method keeps the `next` value in memory before overwriting it (this was done so
+  that no `Entry` in the linked list did not get reindexed).
+* Is the way I'm resizing logical? I read [this article](http://coding-geek.com/how-does-a-hashmap-work-in-java/) and
+  that's where I got my ideas for how to resize the internal array.
+
+## Implementation
+
+<!-- language:lang-java --!>
+
+    public class HashMap<K, V> {
+        private int size = 0;
+        private int capacity = 16;
+        private Entry<K, V>[] entries = new Entry[capacity];
+        private double loadFactor = 0.75;
+
+        private static class Entry<K, V> {
+            private final K key;
+            private V value;
+            private Entry<K, V> next = null;
+
+            public Entry(K key, V value) {
+                this.key = key;
+                this.value = value;
+            }
+        }
+
+        public HashMap() {
+        }
+
+        public boolean isEmpty() {
+            return this.size == 0;
+        }
+
+        public int getSize() {
+            return this.size;
+        }
+
+        public boolean containsKey(K key) {
+            Entry<K, V> matchingEntry = getMatchingEntry(key);
+
+            return matchingEntry != null && matchingEntry.key == key;
+        }
+
+        public boolean containsValue(V value) {
+            for (Entry<K, V> entry : this.entries) {
+                while (entry != null && !matches(value, entry.value)) {
+                    entry = entry.next;
+                }
+
+                if (entry != null) {
+                    return true;
+                }
+            }
+
+            return false;
+        }
+
+        public V get(K key) {
+            Entry<K, V> matchingEntry = getMatchingEntry(key);
+
+            return matchingEntry == null ? null : matchingEntry.value;
+        }
+
+        public void put(K key, V value) {
+            if (this.shouldResize()) {
+                this.resize();
+            }
+
+            if (addEntry(new Entry<>(key, value), this.entries)) {
+                this.size++;
+            }
+
+        }
+
+        public void remove(K key) {
+            int index = indexOf(key);
+            Entry<K, V> currentEntry = this.entries[index];
+
+            while (currentEntry != null && currentEntry.next != null && !matches(key, currentEntry.next.key)) {
+                currentEntry = currentEntry.next;
+            }
+
+            if (currentEntry != null) {
+                // this case can only occur if there is only one non-null entry at the index
+                if (matches(key, currentEntry.key)) {
+                    this.entries[index] = null;
+                // this case can only occur because the next entry's key matched
+                } else if (currentEntry.next != null) {
+                    currentEntry.next = currentEntry.next.next;
+                }
+
+                this.size--;
+            }
+        }
+
+        private boolean shouldResize() {
+            return this.size > Math.ceil((double) this.capacity * this.loadFactor);
+        }
+
+        private void resize() {
+            this.capacity = this.size * 2;
+
+            Entry<K, V>[] newEntries = new Entry[this.capacity];
+            for (Entry<K, V> entry : this.entries) {
+                if (entry != null) {
+                    this.setEntry(entry, newEntries);
+                }
+            }
+
+            this.entries = newEntries;
+        }
+
+        private void setEntry(Entry<K, V> entry, Entry<K, V>[] entries){
+            Entry<K, V> nextEntry = entry.next;
+            entry.next = null;
+
+            this.addEntry(entry, entries);
+
+            if (nextEntry != null) {
+                this.setEntry(nextEntry, entries);
+            }
+        }
+
+        private boolean addEntry(Entry<K, V> entry, Entry<K, V>[] entries) {
+            int index = indexOf(entry.key);
+            Entry<K, V> existingEntry = entries[index];
+
+            if (existingEntry == null) {
+                entries[index] = entry;
+                return true;
+            } else {
+                while (!this.matches(entry.key, existingEntry.key) && existingEntry.next != null) {
+                    existingEntry = existingEntry.next;
+                }
+
+                if (this.matches(entry.key, existingEntry.key)) {
+                    existingEntry.value = entry.value;
+                    return false;
+                }
+
+                existingEntry.next = entry;
+                return true;
+
+            }
+        }
+
+        private Entry<K, V> getMatchingEntry(K key) {
+            Entry<K, V> existingEntry = this.entries[indexOf(key)];
+
+            while (existingEntry != null && !matches(key, existingEntry.key)) {
+                existingEntry = existingEntry.next;
+            }
+
+            return existingEntry;
+        }
+
+        private int indexOf(K object) {
+            return object == null ? 0 : hash(object) & (this.capacity - 1);
+        }
+
+        private boolean matches(Object o1, Object o2) {
+            return (o1 == null && o2 == null) ||
+                    (o1 != null && o2 != null && o1.equals(o2));
+        }
+
+        /**
+         * Applies a supplemental hash function to a given hashCode, which
+         * defends against poor quality hash functions.  This is critical
+         * because HashMap uses power-of-two length hash tables, that
+         * otherwise encounter collisions for hashCodes that do not differ
+         * in lower bits. Note: Null keys always map to hash 0, thus index 0.
+         */
+        private static int hash(Object key) {
+            // This function ensures that hashCodes that differ only by
+            // constant multiples at each bit position have a bounded
+            // number of collisions (approximately 8 at default load factor).
+            int h;
+            return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
+        }
+    }

src/main/java/problems/impl/SimpleHashMap.java

@@ -0,0 +1,178 @@
+package problems.impl;
+
+import java.util.Objects;
+
+/**
+ * https://codereview.stackexchange.com/questions/171650/hashmap-implementation/171662#171662
+ * @param <K> Key Object
+ * @param <V> Value Object
+ */
+
+public class SimpleHashMap<K, V> {
+    private int size = 0;
+    private int capacity = 16;
+    private double loadFactor = 0.75;
+
+    @SuppressWarnings("unchecked")
+    private Entry<K, V>[] entries = new Entry[capacity];
+
+    private static class Entry<K, V> {
+        private final K key;
+        private V value;
+        private Entry<K, V> next;
+
+        Entry(K key, V value) {
+            this.key = key;
+            this.value = value;
+        }
+    }
+
+    public boolean isEmpty() {
+        return this.size == 0;
+    }
+
+    public int getSize() {
+        return this.size;
+    }
+
+    public boolean containsKey(K key) {
+        return getMatchingEntry(key) != null;
+    }
+
+    public boolean containsValue(V value) {
+        for (Entry<K, V> entry : this.entries) {
+            while (entry != null && !matches(value, entry.value)) {
+                entry = entry.next;
+            }
+
+            if (entry != null) {
+                return true;
+            }
+        }
+
+        return false;
+    }
+
+    public V get(K key) {
+        Entry<K, V> matchingEntry = getMatchingEntry(key);
+
+        return matchingEntry == null ? null : matchingEntry.value;
+    }
+
+    public void put(K key, V value) {
+        if (this.shouldResize()) {
+            this.resize();
+        }
+
+        if (addEntry(new Entry<>(key, value), this.entries)) {
+            this.size++;
+        }
+
+    }
+
+    public void remove(K key) {
+        int index = indexOf(key);
+        Entry<K, V> currentEntry = this.entries[index];
+
+        while (currentEntry != null && currentEntry.next != null && !matches(key, currentEntry.next.key)) {
+            currentEntry = currentEntry.next;
+        }
+
+        if (currentEntry != null) {
+            // this case can only occur if there is only one non-null entry at the index
+            if (matches(key, currentEntry.key)) {
+                this.entries[index] = null;
+            // this case can only occur because the next entry's key matched
+            } else if (currentEntry.next != null) {
+                currentEntry.next = currentEntry.next.next;
+            }
+
+            this.size--;
+        }
+    }
+
+    private boolean shouldResize() {
+        return this.size > Math.ceil((double) this.capacity * this.loadFactor);
+    }
+
+    private void resize() {
+        this.capacity = this.size * 2;
+
+        @SuppressWarnings("unchecked")
+        Entry<K, V>[] newEntries = new Entry[this.capacity];
+        for (Entry<K, V> entry : this.entries) {
+            if (entry != null) {
+                this.setEntry(entry, newEntries);
+            }
+        }
+
+        this.entries = newEntries;
+    }
+
+    private void setEntry(Entry<K, V> entry, Entry<K, V>[] entries){
+        Entry<K, V> nextEntry = entry.next;
+        entry.next = null;
+
+        this.addEntry(entry, entries);
+
+        if (nextEntry != null) {
+            this.setEntry(nextEntry, entries);
+        }
+    }
+
+    private boolean addEntry(Entry<K, V> entry, Entry<K, V>[] entries) {
+        int index = indexOf(entry.key);
+        Entry<K, V> existingEntry = entries[index];
+
+        if (existingEntry == null) {
+            entries[index] = entry;
+            return true;
+        } else {
+            while (!this.matches(entry.key, existingEntry.key) && existingEntry.next != null) {
+                existingEntry = existingEntry.next;
+            }
+
+            if (this.matches(entry.key, existingEntry.key)) {
+                existingEntry.value = entry.value;
+                return false;
+            }
+
+            existingEntry.next = entry;
+            return true;
+
+        }
+    }
+
+    private Entry<K, V> getMatchingEntry(K key) {
+        Entry<K, V> existingEntry = this.entries[indexOf(key)];
+
+        while (existingEntry != null && !matches(key, existingEntry.key)) {
+            existingEntry = existingEntry.next;
+        }
+
+        return existingEntry;
+    }
+
+    private int indexOf(K object) {
+        return object == null ? 0 : hash(object) & (this.capacity - 1);
+    }
+
+    private boolean matches(Object o1, Object o2) {
+        return Objects.equals(o1, o2);
+    }
+
+    /**
+     * Applies a supplemental hash function to a given hashCode, which
+     * defends against poor quality hash functions.  This is critical
+     * because HashMap uses power-of-two length hash tables, that
+     * otherwise encounter collisions for hashCodes that do not differ
+     * in lower bits. Note: Null keys always map to hash 0, thus index 0.
+     */
+    private static int hash(Object key) {
+        // This function ensures that hashCodes that differ only by
+        // constant multiples at each bit position have a bounded
+        // number of collisions (approximately 8 at default load factor).
+        int h;
+        return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
+    }
+}