Working with HashMaps

java

A HashMap basically designates unique keys to corresponding values that can be retrieved at any given point. Some core features of the HashMap are-

a)      The values can be stored in a map by forming a key-value pair. The value can be retrieved using the key by passing it to the correct method.

b)      If no element exists in the Map, it will throw a ‘NoSuchElementException’.

c)       HashMap stores only object references. That’s why, it’s impossible to use primitive data types like double or int. Use wrapper class (like Integer or Double) instead.

java1

Using Maps in Java Programs:

Following are the two ways to declare a HashMap:

HashMap<String, Object> map = new HashMap<String, Object>();
Map<String, Object> map = new HashMap<String, Object>();

The most important methods of HashMap are-

  • get(Object KEY) – This will return the value associated with specified key in this hash map.
  • put(Object KEY, String VALUE) – This method stores the specified value and associates it with the specified key in this map.

Following is a sample implementation of HashMap:

  import java.util.HashMap;
  import java.util.Map;

public class Sample_TestMaps{

    public static void main(String[] args){

      Map<String, String> objMap = new HashMap<String, String>();
      objMap.put("Name", "Suzuki");
      objMap.put("Power", "220");
      objMap.put("Type", "2-wheeler");
      objMap.put("Price", "85000");

      System.out.println("Elements of the Map:");
      System.out.println(objMap);

    }
}


Output of the above code:

Elements of the Map:
{Name=Suzuki, Type=2-wheeler, Power=220, Price=85000}

Lets us ask a few queries to the HashMap itself to know it better

Q: So Mr.HashMap, how can I find if a particular key has been assigned to you?

A: Cool, you can use the containsKey(Object KEY) method with me, it will return a Boolean value if I have a value for the given key.

Q: How do I find all the available keys that are present in the Map?

A: I have a method called as keyset() that will return all the keys in the map. In the above example, if you write a line as –
System.out.println(objMap.keySet());

It will return an output as-
[Name, Type, Power, Price]

Similarly, if you need all the values only, I have a method as values().
System.out.println(objMap.values());

It will return an output as-
[Suzuki, 2-wheeler, 220, 85000]

Q: Suppose, I need to remove only a particular key from the Map, do I need to delete the entire Map?

A: No buddy!! I have a method as remove(Object KEY) that will remove only that particular key-value pair.

Q: How can we check if you actually contain some key-value pairs?

A: Just check if I am empty or not!! In short, use isEmpty() method against me ;)

On December 21, 2012   /   Java   /   Comments Off

Comments are closed.