Question

I have a HashMap with millions of entries.

Need to retrieve all entries whose keys match a specific set of criteria (in this case, each key is an object with two integer properties; I need to retrieve all keys where each of these integers fall within a specified range).

What is the fastest, most efficient way to iterate through all such keys?

UPDATE: In this particular case, though I didn’t specify it up front, the first integer in the key has a natural precedence over the second integer.

Answer 1

Here’s a solution using TreeMap:

public static void main(String[] args) {     Comparator<Foo> fooComparator = new Comparator<Foo>() {         @Override         public int compare(Foo o1, Foo o2) {             return o1.compareTo(o2);         }     };      TreeMap<Foo, String> map = new TreeMap<Foo, String>(fooComparator);      map.put(new Foo(1, 4), '');     map.put(new Foo(1, 3), '');     map.put(new Foo(2, 4), '');     map.put(new Foo(3, 4), '');     map.put(new Foo(8, 10), '');     map.put(new Foo(8, 17), '');     map.put(new Foo(10, 10), '');      int a = 2;     int b = 5;      for (Foo f : getKeysInRange(map, a, b)) {         System.out.println(f);     } }  public static List<Foo> getKeysInRange(TreeMap<Foo, String> map, int low, int high) {     Foo key1 = new Foo(low, low);     Foo key2 = new Foo(high, high);      Foo fromKey = map.ceilingKey(key1);     Foo toKey = map.floorKey(key2);      if (fromKey != null && toKey != null && fromKey.compareTo(toKey) < 0)         return new ArrayList<Foo>(map.subMap(fromKey, true, toKey, true).keySet());     return new ArrayList<Foo>(); }  public static class Foo implements Comparable<Foo> {     private int i;     private int j;      private Foo(int i, int j) {         super();         this.i = i;         this.j = j;     }      public int min() {         if (i < j)             return i;         else             return j;     }      public int max() {         if (i > j)             return i;         else             return j;     }      @Override     public String toString() {         return 'I=' + i + 'J=' + j;     }      @Override     public int compareTo(Foo o) {         if (this.min() > o.min()) {             return 1;         } else if (this.min() < o.min())             return -1;         else {             if (this.max() > o.max())                 return 1;             else if (this.max() < o.max())                 return -1;             else                 return 0;         }     } }