Learning HashMap

I'm just learning myself how to use hashMaps, can someone check this piece of code I have written and tell me if it's correct? The idea is to have a list of staff working i开发者_如何学编程n a company, I want to add and remove staff from a hashMap.

public class Staff
{
    private HashMap<String, String>id;

    public Staff(String name, String number)
    {
        id = new HashMap<>(name,number);
    }

    public addStaff()
    {
       id.add("Joe","1234A2);   
    }

    public removeStaff()
    {
       id.remove("Joe","1234A2);
    }
}

In your class, you need to change / add things to look like:

private HashMap<String, String> id;

public Staff(String name, String number)
{
    id = new HashMap<String, String>();
}

public addStaff()
{
   id.put("Joe","1234A2");   
}

public removeStaff()
{
   id.remove("Joe");
}

to use a HashMap properly.

Note that addStaff() and removeStaff() are not likely to be very useful for most purposes because they only add and remove the one staff member "Joe". A more useful way of doing things would be

public void addStaff(StaffMember member) {
   id.put(member.getName(), member);
}

public StaffMember get(String name) {
   // this will return null if the member's name isn't a key in the map.
   return id.get(name);
}

What makes a Map different from other data structures is that it has a "key" which allows you to possibly retrieve one item. You cannot use a Map in the ways that give you a boost in performance if you don't have the item's key beforehand.

Note that HashMaps require keys to follow the rules for equality correctly in addition to equal items returning the same hashCode. Basically if on object a and another object b are considered equal, then

// Reflexive property
a.equals(a) must return true

// Symmetric property
if (a.equals(b)) then b.equals(a) must return true

// Transitive property
if (a.equals(b) and b.equals(c)) then a.equals(c) must return true

// Additional requirements to make hash related algorithms work properly
a.hashCode() == b.hashCode() // must be true

The hashCode() part is above and beyond simple equality, and is required for the HashMap to work correctly.

Looks good to me, however I would have created a Employee class and use a unique id for each staff member.

class Employee {
  Integer id;
  String name;
}

Map<Integer, Employee> staff = new HashMap<Integer, Employee>();

This way you can extend the staff info at will without redefining your staff map. Also never assume that human names are unique.

First of all please read this http://download.oracle.com/javase/6/docs/api/java/util/Map.html and a lot of things will be more clear for you. Then take a look here for some samples:

• http://www.java-samples.com/showtutorial.php?tutorialid=369
• http://www4.ncsu.edu/~kaltofen/courses/Languages/JavaExamples/jgl3.1.0/doc/api/com.objectspace.jgl.examples.HashMapExamples.html