Question

I’ve looked in the usual places (apache commons, google) and not been able to find one …

It should be opensource.

Pretty much looking for one based on a linked list. The use case is 10’000’s of maps, with not necessarily many values in. It does not need to scale up, as i can convert it when it gets too big.

Some numbers, sizes using some calculated jvm values (8bytes/java.lang.Object, 4bytes/ref) the HashMap is about 100+32n bytes, the theoretical best is 12+20*n. <– I want that, for small n.

Answer 1

Ok, implemented it myself in the end. I did a speed comparison, and found when compared to a HashMap that it was still slightly faster with 4 entries, but slower with 5 or more. I did the tests with a longish list of keys that I tried to give a similar makeup as a list of random english words.

import java.util.*;  // PUBLIC DOMAIN public class SmallMap extends AbstractMap {      private Entry entry = null;      public void clear() { entry = null; }     public boolean isEmpty() { return entry==null; }         public int size() {         int r = 0;         for(Entry e = entry; e!=null; e = e.next) r++;         return r;     }      public boolean containsKey(Object key) {         for(Entry e = entry; e!=null; e = e.next){             if(e.key.equals(key)){                 return true;             }         }         return false;     }      public boolean containsValue(Object value) {         for(Entry e = entry; e!=null; e = e.next){             if(e.value==null){                 if(value==null) return true;             }else if(e.value.equals(value)){                 return true;             }         }         return false;     }      public Object get(Object key) {         for(Entry e = entry; e!=null; e = e.next){             if(e.key.equals(key)){                 return e.value;             }         }         return null;     }      public Object put(Object key, Object value) {         for(Entry e = entry; e!=null; e = e.next){             if(e.key.equals(key)){                 Object r = e.value;                 e.value = value;                 return r;             }         }         entry = new Entry(key, value, entry);         return null;     }      public Object remove(Object key) {         if(entry!=null){             if(entry.key.equals(key)){                 Object r = entry.value;                 entry = entry.next;                 return r;             }             for(Entry e = entry; e.next!=null; e = e.next){                 if(key.equals(e.next.key)){                     Object r = e.next.value;                     e.next = e.next.next;                     return r;                 }             }         }         return null;     }      public Set entrySet() { return new EntrySet(); }      class EntrySet extends AbstractSet{         public Iterator iterator() {             return new Iterator(){                  Entry last = null;                 Entry e = entry;                 public boolean hasNext() { return e!=null; }                  public Object next() {                      last = e;                     e = e.next;                     return last;                 }                  public void remove() {                      if(last == null) throw new IllegalStateException();                     SmallMap.this.remove(last.key);                 }             };         }          public int size() { return SmallMap.this.size();}     }      static private class Entry implements java.util.Map.Entry {         final Object key;         Object value;         Entry next;          Entry(Object key, Object value, Entry next){             if(key==null) throw new NullPointerException();             this.key = key;             this.value = value;             this.next = next;         }         public Object getKey() { return key; }         public Object getValue() { return value; }         public Object setValue(Object value) {              Object r = this.value;             this.value = value;             return r;         }         public int hashCode() {             return (key   == null ? 0 :   key.hashCode()) ^                (value == null ? 0 : value.hashCode());         }     } }