Help with Hashmaps in Java

It's important to realize that maps are directional. That is, if you have a Map<Person, Date> storing birthdays, then it's easy to look up any person's birthday, and impossible to look up a birthday's person (there could easily be zero, or more than one).

Now, do you want to be able to look up the email address of a person, or the person of an email address? Your code is mixing these things around:

• You declare the map to be Map<Person,String>, suggesting that you'll be using Persons as the key and Strings as the value -- that is, looking up a person's email address.
• You add data with staff.put(p, p.getEmail()), which also suggests that you'll use the Persons as the key.
• But you try to define a findByEmail method, which would necessarily use the email address as the key -- that's backwards from how you've got your map set up.

So, a map can only go in one direction. You get to decide which direction it is, but you have to be consistent. If you need to be able to do lookups in both directions, consider using two maps!

Here's a snippet that shows most of the Map functionalities:

import java.util.*; public class MapExample { public static void main(String[] args) { Map<String,Integer> map = new HashMap<String,Integer>(); map.put("One", 1); map.put("Two", 2); map.put("Three", 3); System.out.println(map.size()); // prints "3" System.out.println(map); // prints "{Three=3, One=1, Two=2}" // HashMap allows null keys and values // Map also allows several keys to map to the same values map.put(null, 1); map.put("?", null); System.out.println(map.size()); // prints "5" System.out.println(map); // prints "{null=1, Three=3, ?=null, One=1, Two=2}" // get values mapped by key System.out.println(map.get("One")); // prints "1" System.out.println(map.get("Two")); // prints "2" System.out.println(map.get("Three")); // prints "3" // get returns null if // (i) there's no such key, or // (ii) there's such key, and it's mapped to null System.out.println(map.get("Four") == null); // prints "true" System.out.println(map.get("?") == null); // prints "true" // use containsKey to check if map contains key System.out.println(map.containsKey("Four")); // prints "false" System.out.println(map.containsKey("?")); // prints "true" // use keySet() to get view of keys Set<String> keys = map.keySet(); System.out.println(keys); // prints "[null, Three, ?, One, Two]" // the view supports removal keys.remove("Three"); System.out.println(map); // prints "{null=1, ?=null, One=1, Two=2}" // use values() to get view of values Collection<Integer> values = map.values(); System.out.println(values); // prints "[1, null, 1, 2]" // the view supports removal values.remove(null); System.out.println(map); // prints "{null=1, One=1, Two=2}" values.remove(1); // removes at most one mapping System.out.println(map); // prints "{One=1, Two=2}" // iterating all entries using for-each for (Map.Entry<String,Integer> entry : map.entrySet()) { System.out.println(entry.getKey() + "->" + entry.getValue()); } // prints "One->1", "Two->2" map.clear(); System.out.println(map.isEmpty()); // prints "true" } }