What is delegation in oops concept?

Delegation allows the behavior of an object to be defined in terms of the behavior of another object.Delegation is alternative to class inheritance. Delegation is a way of making object composition as powerful as inheritance. In delegation, two objects are involved in handling a request.A receiving object delegates operations to its delegate. this is analogous to the child classes sending requests to the parent classes.

How to Iterate Over a Map in Java

1.Iterating over entries using For-Each loop.

Map<Integer, Integer> map = new HashMap<Integer, Integer>();
for (Map.Entry<Integer, Integer> entry : map.entrySet()) {
    System.out.println("Key = " + entry.getKey() + ", Value = " + entry.getValue());
}

 

2.Iterating over keys or values using For-Each loop.

Map<Integer, Integer> map = new HashMap<Integer, Integer>();
//iterating over keys only
for (Integer key : map.keySet()) {
    System.out.println("Key = " + key);
}
//iterating over values only
for (Integer value : map.values()) {
    System.out.println("Value = " + value);
}

 

 3. Iterating using Iterator.  

Using Generics

Map<Integer, Integer> map = new HashMap<Integer, Integer>();
Iterator<Map.Entry<Integer, Integer>> entries = map.entrySet().iterator();
while (entries.hasNext()) {
    Map.Entry<Integer, Integer> entry = entries.next();
    System.out.println("Key = " + entry.getKey() + ", Value = " + entry.getValue());
}

 Without Generics:

Map map = new HashMap();
Iterator entries = map.entrySet().iterator();
while (entries.hasNext()) {
    Map.Entry entry = (Map.Entry) entries.next();
    Integer key = (Integer)entry.getKey();
    Integer value = (Integer)entry.getValue();
    System.out.println("Key = " + key + ", Value = " + value);
} 

4. Iterating over keys and searching for values (inefficient).

Map<Integer, Integer> map = new HashMap<Integer, Integer>();
for (Integer key : map.keySet()) {
    Integer value = map.get(key);
    System.out.println("Key = " + key + ", Value = " + value);
}