I was trying sample code and some got result which I don't understand:
Map<Integer,Integer> map = new HashMap<>();
map.put(1, 2);
System.out.println(map.get(1));
Integer k = map.get(1);
k++;
System.out.println(map.get(1));
The result:
2
2
But as Integer is an object, the change should get reflected in the map value as well? So why is the value not changing?
Integer is immutable, and k++ doesn't change the value of the Integer stored in the Map. It create a new Integer instance.
You should put the new value in the Map in order for the Map to be modified:
Map<Integer,Integer> map = new HashMap<>();
map.put(1, 2);
System.out.println(map.get(1));
Integer k = map.get(1);
k++;
map.put(1, k);
System.out.println(map.get(1));
If Integer was a mutable class, and you would have called a method that mutates its state, you wouldn't have needed to put the value again in the Map.
h++ unboxes the Integer to an int, increments that int and assigns the incremented value back to the h variable (which requires boxing the int to Integer).h++ only assigns a new value (new Object) to h. It doesn't change the value that was already put in the Map.Misconception on your end: Integer is immutable!
That means that k++ creates a new Integer object. It is impossible to change the value of an existing Integer object!
Integer(and all other boxed types) is immutable.k++returns a new Integer with the value 3, the original Integer stays 2.