How does Map work in Java?

HashMap in Java works on hashing principle. The Map is a data structure which allows us to store object with a key and retrieve same by the same key. Hash functions are used to link key and value in HashMap. Objects are stored by calling put(key, value) method of HashMap and retrieved by calling get(key) method.

 When we call put method,internally  hashcode() method of the key object is called so that hash function of the map can find a bucket location to store value object, which is actually an index of the internal LinkedList. If we dig down the structure we found HashMap internally stores mapping in the form of Map.Entry object which contains both key and value object. When you want to retrieve the object, you call the get() method with  a key object. get() method again generate  key object hash code and eventually ended up with same  bucket location. and returns the value.

Few important things to remember , If you want to use your custom object as a Key of Map

1. It has to be immutable .unless  some one can  change the state.of the object
2. Must override hashcode. As good hashing algorithm, distribution of objects in bucket will be as good
.3. Must override equals method so in  the time of get operation  map  can fetch the key which is identically equal.

public final class CustomKey{
private final String name = "Shamik";
public final String setName(String name)
{
this.name=name;
}
public final String getName(String name){
return name;
}
public int hascode()
{
return name.hascode()*1245/7;
}
public boolean equals(Object obj)
{
if(obj instanceof CustomKey)
{
return this.name.equals(obj.name);
}
return false;
}
}

Here  name is not final so following scenario can occur
CustomKey refKey= new CustomKey();
Map<CustomKey ,String> map = new HashMap<CustomKey,String>();
map.put(refKey,"Shamik");
refKey.setName("Bubun");
map.get(refKey);

It will return null as when equals method invokes it return false. So immutability is must required.

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