CODEWITHSUNDEEP
java
jijo
jijo

Reputation: 815

Java: Get instance of super class from subclass

Consider I have a class like below.

public class SuperClass
{
  private String a;
  private String b;
  //getter/setters
}

And a subclass

public class ChildClass extends SuperClass
    {
      private String c;
      private String d;
      //getter/setters
    }

In another class I have an object of the ChildClass. This object has both the parent and child class member variables populated with data.
Using this object, I need to get an instance of parent class, i.e, an object which only has the variables in the parent class (i.e., a and b). How can I achieve it ?

Upvotes: 2

Views: 4503

Answers (2)

Kartik
Kartik

Reputation: 7917

Let's take a step back and understand what happens when you create an instance of the ChildClass. A memory location is made to store the fields of ChildClass and SuperClass. Now you cannot make that memory location "forget" the ChildClass fields and retain only the SuperClass fields. The best you can do is make the ChildClass fields null.

So to do what you want, you'll have a create a new object. You can do so by using a method like this:

public class ChildClass extends SuperClass {
    private String c;
    private String d;

    public SuperClass getSuperClassInstance() {
        SuperClass sc = new SuperClass();
        sc.a = this.a; //or use getters/setters
        sc.b = this.b; //you might need to deep clone objects
        return sc;
    }
}

As @ruakh said, merely casting to SuperClass is not enough as the data can be retrieved by casting it back to ChildClass as long as it is pointing to the same memory location.

Update based on comments

So this is an XY problem!

You can annotate the fields in your child class by @JsonIgnore or do it at class level using @JsonIgnoreProperties(value = { "c", "d" }).

Also, the answer in this duplicate question should help.

If you need to serialize those fields in some other case, you can use JSON Views.

Upvotes: 3

Fernandoms
Fernandoms

Reputation: 444

You can achieve this by casting the class, if you have an ChildClass instance, you can get the SuperClass with the following command:

SuperClass sc = (SuperClass) childClass;

or 

SuperClass sc = childClass;

As the comments bellow say, you don't really need the cast in this case, just making a SuperClass variable receiving your ChildClass is enough.

This happens because the Upcast (from child to super) is always possible, so it's safe to do it implicit. Only needing to explicit in cases of Downcast.

Here you can learn more about java casting.

Upvotes: 2

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