In context of
java "instance" means object.
When we create an object,we create it by using new keyword over its constructor.
Consider the below class:
Now if we are creating object of this class using the default constructor
and fetch the value for its members
then we get the default values
fruitName=Mango
fruitColor=Yellow
Now we go ahead and change these values using setters
After this the members become
fruitName=Apple
fruitColor=Red
After doing all these you serialize the object and then deserialize it.Now please go ahead and fetch the values you will get
fruitName=Apple
fruitColor=Red
implying that the object was persisted/stored and then the same object is reconstructed with the stored object info in the file which you specify during serialization.So the constructor is not called while doing all these.Hence its instance variables were not assigned the initial values.
Suggestion:Whenever you are reading anything try and implement it.Then the concept will become more clear.