public class Person {
  private String name;

  public Person(String name) {
    this.name = name;
  }
}

Map<Person, String> testMap = new HashMap<>();
testMap.put(new Person("hello"), "world");
testMap.get(new Person("hello")); // ---> null

//: containers/SimpleHashMap.java
// A demonstration hashed Map.
import java.util.*;
import net.mindview.util.*;

public class SimpleHashMap<K,V> extends AbstractMap<K,V> {
 // Choose a prime number for the hash table size, to achieve a uniform distribution:
 static final int SIZE = 997;
 // You can't have a physical array of generics, but you can upcast to one:
 @SuppressWarnings("unchecked")
 LinkedList<MapEntry<K,V>>[] buckets =
  new LinkedList[SIZE];
 public V put(K key, V value) {
  V oldValue = null;
  int index = Math.abs(key.hashCode()) % SIZE;
  if(buckets[index] == null)
   buckets[index] = new LinkedList<MapEntry<K,V>>();
  LinkedList<MapEntry<K,V>> bucket = buckets[index];
  MapEntry<K,V> pair = new MapEntry<K,V>(key, value);
  boolean found = false;
  ListIterator<MapEntry<K,V>> it = bucket.listIterator();
  while(it.hasNext()) {
   MapEntry<K,V> iPair = it.next();
   if(iPair.getKey().equals(key)) {
    oldValue = iPair.getValue();
    it.set(pair); // Replace old with new
    found = true;
    break;
   }
  }
  if(!found)
   buckets[index].add(pair);
  return oldValue;
 }
 public V get(Object key) {
  int index = Math.abs(key.hashCode()) % SIZE;
  if(buckets[index] == null) return null;
  for(MapEntry<K,V> iPair : buckets[index])
   if(iPair.getKey().equals(key))
    return iPair.getValue();
  return null;
 }
 public Set<Map.Entry<K,V>> entrySet() {
  Set<Map.Entry<K,V>> set= new HashSet<Map.Entry<K,V>>();
  for(LinkedList<MapEntry<K,V>> bucket : buckets) {
   if(bucket == null) continue;
   for(MapEntry<K,V> mpair : bucket)
    set.add(mpair);
  }
  return set;
 }
 public static void main(String[] args) {
  SimpleHashMap<String,String> m =
   new SimpleHashMap<String,String>();
  m.putAll(Countries.capitals(25));
  System.out.println(m);
  System.out.println(m.get("ERITREA"));
  System.out.println(m.entrySet());
 }
}

/**
 * The table, resized as necessary. Length MUST Always be a power of two.
 */
transient Entry<K,V>[] table = (Entry<K,V>[]) EMPTY_TABLE;

static class Entry<K,V> implements Map.Entry<K,V> {
  final K key;
  V value;
  Entry<K,V> next;
  int hash;
  …
}

public V get(Object key) {
  if (key == null)
    return getForNullKey();
  Entry<K,V> entry = getEntry(key);

  return null == entry ? null : entry.getValue();
}

final Entry<K,V> getEntry(Object key) {
  if (size == 0) {
    return null;
  }

  int hash = (key == null) ? 0 : hash(key);
  for (Entry<K,V> e = table[indexFor(hash, table.length)];
     e != null;
     e = e.next) {
    Object k;
    if (e.hash == hash &&
      ((k = e.key) == key || (key != null && key.equals(k))))
      return e;
  }
  return null;
}