Generic Class Hierarchies in Java

Last Updated : 17 May, 2021

Generic means parameterized types introduced in java5. These help in creating classes, interfaces, methods, etc. A class or method which works on parameterized type known as "generic class " or "generic method". Generics is a combination of language properties of the definition and use of Generic types and methods. Collections were used before Generics which holds any type of objects i.e. non-generic. Using Generics, it has become possible to create a single class, interface, or method that automatically works with all types of data(Integer, String, Float, etc). It has expanded the ability to reuse the code safely and easily. Generics also provide type safety (ensuring that an operation is being performed on the right type of data before executing that operation).

Hierarchical classifications are allowed by Inheritance. Superclass is a class that is inherited. The subclass is a class that does inherit. It inherits all members defined by super-class and adds its own, unique elements. These uses extends as a keyword to do so.

Sometimes generic class acts like super-class or subclass. In Generic Hierarchy, All sub-classes move up any of the parameter types that are essential by super-class of generic in the hierarchy. This is the same as constructor parameters being moved up in a hierarchy.

Example 1: Generic super-class

Java

// Java Program to illustrate generic class hierarchies

// Importing all input output classes
import java.io.*;

// Helper class 1
// Class 1 - Parent class
class Generic1<T> {
    // Member variable of parent class
    T obj;

    // Constructor of parent class
    Generic1(T o1) { obj = o1; }

    // Member function of parent class
    // that returns an object
    T getobj1() { return obj; }
}

// Helper class 2
// Class 2 - Child class
class Generic2<T, V> extends Generic1<T> {
    // Member variable of child class
    V obj2;
    Generic2(T o1, V o2)
    {
        // Calling super class using super keyword
        super(o1);

        obj2 = o2;
    }

    // Member function of child class
    // that returns an object
    V getobj2() { return obj2; }
}

// Class 3 -  Main class
class GFG {
  
    // Main driver method
    public static void main(String[] args)
    {
        // Creating Generic2 (sub class) object
      // Custom inputs as parameters 
        Generic2<String, Integer> x
            = new Generic2<String, Integer>("value : ",
                                            100);

        // Calling method and printing 
        System.out.println(x.getobj1());
        System.out.println(x.getobj2());
    }
}

Output

value : 
100

Note: A subclass can freely add its own type parameters, if necessary.

Example 2: Non-generic sub-class of generic sub-class

Java

import java.io.*;
// non-generic super-class
class NonGen {
    int n;
    NonGen(int i) { n = i; }
    int getval() { return n; }
}
// generic class sub-class
class Gen<T> extends NonGen {
    T obj;
    Gen(T o1, int i)
    {
        super(i);
        obj = o1;
    }
    T getobj() { return obj; }
}

class GFG {
    public static void main(String[] args)
    {

        Gen<String> w = new Gen<String>("Hello", 2021);

        System.out.println(w.getobj() + " " + w.getval());
    }
}

Output

Hello 2021

Generic Class Hierarchies in Java

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Generic Class Hierarchies in Java

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