1 public class opendressinghash<Key, Value> {
  2         private static final int INIT_CAPACITY = 4;
  3 
  4         private int n;          
  5         private int m;          
  6         private Key[] keys;      
  7         private Value[] vals;   
  8 
  9         public opendressinghash() {
 10             this(INIT_CAPACITY);
 11         }
 12 
 13         public opendressinghash(int capacity) {
 14             m = capacity;
 15             n = 0;
 16             keys = (Key[])   new Object[m];
 17             vals = (Value[]) new Object[m];
 18         }
 19 
 20        
 21         public int size() {
 22             return n;
 23         }
 24 
 25         
 26         public boolean isEmpty() {
 27             return size() == 0;
 28         }
 29 
 30        
 31         public boolean contains(Key key) {
 32             if (key == null) throw new NullPointerException("argument to contains() is null");
 33             return get(key) != null;
 34         }
 35 
 36        
 37         private int hash(Key key) {
 38             return (key.hashCode() & 0x7fffffff) % m;
 39         }
 40 
 41      
 42         private void resize(int capacity) {
 43             opendressinghash<Key, Value> temp = new opendressinghash<Key, Value>(capacity);
 44             for (int i = 0; i < m; i++) {
 45                 if (keys[i] != null) {
 46                     temp.put(keys[i], vals[i]);
 47                 }
 48             }
 49             keys = temp.keys;
 50             vals = temp.vals;
 51             m    = temp.m;
 52         }
 53 
 54         
 55         public void put(Key key, Value val) {
 56             if (key == null) throw new NullPointerException("first argument to put() is null");
 57 
 58             if (val == null) {
 59                 delete(key);
 60                 return;
 61             }
 62 
 63          
 64             if (n >= m/2) resize(2*m);
 65 
 66             int i;
 67             for (i = hash(key); keys[i] != null; i = (i + 1) % m) {
 68                 if (keys[i].equals(key)) {
 69                     vals[i] = val;
 70                     return;
 71                 }
 72             }
 73             keys[i] = key;
 74             vals[i] = val;
 75             n++;
 76         }
 77 
 78         public Value get(Key key) {
 79             if (key == null) throw new NullPointerException("argument to get() is null");
 80             for (int i = hash(key); keys[i] != null; i = (i + 1) % m)
 81                 if (keys[i].equals(key))
 82                     return vals[i];
 83             return null;
 84         }
 85 
 86       
 87         public void delete(Key key) {
 88             if (key == null) throw new NullPointerException("argument to delete() is null");
 89             if (!contains(key)) return;
 90 
 91             // find position i of key
 92             int i = hash(key);
 93             while (!key.equals(keys[i])) {
 94                 i = (i + 1) % m;
 95             }
 96 
 97             // delete key and associated value
 98             keys[i] = null;
 99             vals[i] = null;
100 
101             // rehash all keys in same cluster
102             i = (i + 1) % m;
103             while (keys[i] != null) {
104                 // delete keys[i] an vals[i] and reinsert
105                 Key   keyToRehash = keys[i];
106                 Value valToRehash = vals[i];
107                 keys[i] = null;
108                 vals[i] = null;
109                 n--;
110                 put(keyToRehash, valToRehash);
111                 i = (i + 1) % m;
112             }
113 
114             n--;
115 
116             // halves size of array if it‘s 12.5% full or less
117             if (n > 0 && n <= m/8) resize(m/2);
118 
119             assert check();
120         }
121 
122      
123        
124         private boolean check() {
125 
126             // check that hash table is at most 50% full
127             if (m < 2*n) {
128                 System.err.println("Hash table size m = " + m + "; array size n = " + n);
129                 return false;
130             }
131 
132             // check that each key in table can be found by get()
133             for (int i = 0; i < m; i++) {
134                 if (keys[i] == null) continue;
135                 else if (get(keys[i]) != vals[i]) {
136                     System.err.println("get[" + keys[i] + "] = " + get(keys[i]) + "; vals[i] = " + vals[i]);
137                     return false;
138                 }
139             }
140             return true;
141         }
142 
143 
144      
145     
146     }