HashMap and its cousins

[Bill Denniston]

I have what I think should be some kind of collection of objects. I want to be able to quickly search the collection and retrieve a member object that has a given (string) value in a particular field. I thought that HashMap would be the right collection type. However, I have not been able to get a HashMap to accept a value that is other than a primitive; I can't get it to accept my object as the value with the key being the (string) value of a particular field in the object. (I have found lots of examples of this simple case, where the key is a string and the value is some other primitive, like an integer or another string, but not where the value is an object.) I also have not been able to create a HashMap that is typed to a generic of my object (e.g. HashMap<MyObject> hash = new HashMap<MyObject>(), which works great with an ArrayList. Any help, suggestions or references to examples would be appreciated. Thanks.

[Paul Clapham]

Apparently you are using Java 5, so it's okay (and even preferable) to use generics. It's quite simple:

Map<String, YourObject> map = new HashMap<String, YourObject)();
// Create a map whose keys are Strings and whose values are YourObjects
YourObject foobar = new YourObject("Foo", "bAR");
// Here's one of your objects
map.put(foobar.getName(), foobar);
// Assuming getName() returns a String, that's it.

I'm surprised you only found examples with primitives, since before Java 5 it was very hard to use Maps with primitives, and easy with Objects. You had to box the ints into Integers and so on.

[Keith Lynn]

I think you have some confusion on what a primitive type is. The primitives are boolean, byte, char, short, int, long, float, and double. All other types are objects. Here is an example of using a HashMap with objects as keys.

import java.awt.Point;
import java.util.*;
 
public class Test1 {
   private HashMap<Point,java.lang.Integer> map;
 
   public Test1() {
      map = new HashMap<Point,java.lang.Integer>();
 
      Point[] points = {new Point(1,2),
                        new Point(2,10),
                        new Point(3,5),
                        new Point(4,7)};
 
      for (Point point : points)
         map.put(point,new Random().nextInt());
   }
 
   public String toString() {
      String output = "";
 
      Iterator<Point> iterator = map.keySet().iterator();
 
      while (iterator.hasNext()) {
         Point point = iterator.next();
 
         output += point.x + " " + point.y + " " + map.get(point) + "\n";
      }
 
      return(output);
   }
 
   public static void main(String[] args) {
      Test1 test1 = new Test1();
 
      System.out.println(test1);
   }
}