Concept Behind Making A Class Static Final in Java

Question

I am new to Java , and at school level, i have one query can any one better explain me the concept behind it , what it's means and when to use and why .

public class ParentClass {
    private static final ParentClass ourInstance=new ParentClass();
    private ArrayList<Integer> arr=new ArrayList<>();
    public static ParentClass getinstance() {
        return ourInstance;
    }
    private ParentClass() {

    }
    public void setList(ArrayList<Integer> al) {
        arr.addAll(al);
    }

    public ArrayList<Integer>  getList(){
        return arr;
    }

}

I want to focus on this line private static final ParentClass ourInstance=new ParentClass(); what it's means and when we have to declare the syntax like this ? I know we can make the class Final then no subclass can extends the final class.In the Above code the way of declaration is something different ,that's waht confused me .

Answer 1

private static final ParentClass ourInstance=new ParentClass();

When you declare field of a class as final then you can't reassign its value after initialisation.

What it means is you cannot execute below code:

ParentClass otherInstance = new ParentClass();
ourInstance = otherInstance;   // Not allowed as ourInstance is final.

But you can perform below operations i.e. you can change fields of ourInstance by calling its setter methods: - e.g.

ourInstance.setSomeMethod(someValue);   // Allowed as you are not modifying object's reference but only its properties which are not final.