Generics in Java


  • Many algorithms work in a similar way irrespective of the data types on which they are applied on
    • All Stacks work in a similar way irrespective of the type of object they hold
  • Generics help to write algorithms independent of a data type
    • These algorithms can be applied to a variety of data types
    • The data type is parameterized
    • A class, interface or a method that works on a parameterized type is called a generic
    public class List<T>{
    	protected T [] list;
    	public List(T [] list){
    		this.list = list;
    	public void put(T element, int position){
    		list[position] = element;
    	public T get(int position){
    		return list[position];

In above code, T represents a data type. While declaring an object of List, the programmer can decide the actual type for T.

List<String> list = new List<String>(new String[5]);
list.put("Hello", 0);
list.put("Welcome", 1);
list.put("Ok", 2);


Just like String objects are stored and retrieved in the above example, the List class can be used for any kind of object

    • A generic can restrict the type of object that can be used as the parameter
      • A generic List class that performs arithmetic operations on the elements may want to restrict the parameters to Integer, Float, Double etc
      • Number is the super class of all the numeric wrapper classes
    • Only Number or its sub classes can be used as a parameter to List
    • Wildcards can be used with the Generic and Wildcards also can be bounded
public double getSum(List<? extends Double> list){
		//Code goes here


    • A Generic Class can be extended to create another Generic class
public class LinkedList<T> extends List<T>{
		//Code goes here
    • Generic interfaces can be created and used as follows
	interface List<T>{
		public void put(T element, int position);
		public T get(int position);

	class LinkedList<T> implements List<T>{
		//Implementation goes here

Generic Restrictions

  • A type parameter cannot be instantiated
  • T t1 = new T(); //Error, this will not work
  • T [] t2 = new T[5]; //Error, this will not work
  • A static method of a generic class cannot use the parameterized type
  • A generic class cannot extend Throwable
    • We cannot have generic exception classes


One thought on “Generics in Java

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