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Generics in Java
  • Difficulty Level : Easy
  • Last Updated : 17 May, 2021

Generics mean parameterized types. The idea is to allow type (Integer, String, … etc, and user-defined types) to be a parameter to methods, classes, and interfaces. Using Generics, it is possible to create classes that work with different data types. 
An entity such as class, interface, or method that operates on a parameterized type is called a generic entity.

Why Generics?

The Object is the superclass of all other classes and Object reference can refer to any type object. These features lack type safety. Generics add that type safety feature. We will discuss that type of safety feature in later examples.
Generics in Java is similar to templates in C++. For example, classes like HashSet, ArrayList, HashMap, etc use generics very well. There are some fundamental differences between the two approaches to generic types. 
 
Generic Class 
Like C++, we use <> to specify parameter types in generic class creation. To create objects of a generic class, we use the following syntax. 
 

// To create an instance of generic class 
BaseType <Type> obj = new BaseType <Type>()

Note: In Parameter type we can not use primitives like 
      'int','char' or 'double'.

Java




// A Simple Java program to show working of user defined
// Generic classes
   
// We use < > to specify Parameter type
class Test<T>
{
    // An object of type T is declared
    T obj;
    Test(T obj) {  this.obj = obj;  }  // constructor
    public T getObject()  { return this.obj; }
}
   
// Driver class to test above
class Main
{
    public static void main (String[] args)
    {
        // instance of Integer type
        Test <Integer> iObj = new Test<Integer>(15);
        System.out.println(iObj.getObject());
   
        // instance of String type
        Test <String> sObj =
                          new Test<String>("GeeksForGeeks");
        System.out.println(sObj.getObject());
    }
}

Output: 
 

15
GeeksForGeeks

We can also pass multiple Type parameters in Generic classes.
 



Java




// A Simple Java program to show multiple
// type parameters in Java Generics
  
// We use < > to specify Parameter type
class Test<T, U>
{
    T obj1;  // An object of type T
    U obj2;  // An object of type U
  
    // constructor
    Test(T obj1, U obj2)
    {
        this.obj1 = obj1;
        this.obj2 = obj2;
    }
  
    // To print objects of T and U
    public void print()
    {
        System.out.println(obj1);
        System.out.println(obj2);
    }
}
  
// Driver class to test above
class Main
{
    public static void main (String[] args)
    {
        Test <String, Integer> obj =
            new Test<String, Integer>("GfG", 15);
  
        obj.print();
    }
}

Output: 
 

GfG
15

Generic Functions: 
We can also write generic functions that can be called with different types of arguments based on the type of arguments passed to the generic method, the compiler handles each method.
 

Java




// A Simple Java program to show working of user defined
// Generic functions
   
class Test
{
    // A Generic method example
    static <T> void genericDisplay (T element)
    {
        System.out.println(element.getClass().getName() +
                           " = " + element);
    }
   
    // Driver method
    public static void main(String[] args)
    {
         // Calling generic method with Integer argument
        genericDisplay(11);
   
        // Calling generic method with String argument
        genericDisplay("GeeksForGeeks");
   
        // Calling generic method with double argument
        genericDisplay(1.0);
    }
}

Output : 
 

java.lang.Integer = 11
java.lang.String = GeeksForGeeks
java.lang.Double = 1.0

Generics work only with Reference Types: 
When we declare an instance of a generic type, the type argument passed to the type parameter must be a reference type. We cannot use primitive data types like int, char.
 

Java




Test<int> obj = new Test<int>(20); 

The above line results in a compile-time error, that can be resolved by using type wrappers to encapsulate a primitive type. 

But primitive type array can be passed to the type parameter because arrays are reference type.

Java






ArrayList<int[]> a = new ArrayList<>();

Generic Types Differ Based on Their Type Arguments: 

Consider the following Java code. 

Java




// A Simple Java program to show working
// of user-defined Generic classes
   
// We use < > to specify Parameter type
class Test<T>
{
    // An object of type T is declared
    T obj;
    Test(T obj) {  this.obj = obj;  }  // constructor
    public T getObject()  { return this.obj; }
}
   
// Driver class to test above
class Main
{
    public static void main (String[] args)
    {
        // instance of Integer type
        Test <Integer> iObj = new Test<Integer>(15);  
        System.out.println(iObj.getObject());
   
        // instance of String type
        Test <String> sObj =
                          new Test<String>("GeeksForGeeks");
        System.out.println(sObj.getObject());
        iObj = sObj; //This results an error  
    }
}

Output: 
 

 
error:
 incompatible types:
 Test cannot be converted to Test 

Even though iObj and sObj are of type Test, they are the references to different types because their type parameters differ. Generics add type safety through this and prevent errors.
Advantages of Generics: 
Programs that use Generics has got many benefits over non-generic code. 
 

1. Code Reuse: We can write a method/class/interface once and use it for any type we want.

2. Type Safety: Generics make errors to appear compile time than at run time (It’s always better to know problems in your code at compile time rather than making your code fail at run time). Suppose you want to create an ArrayList that store name of students and if by mistake programmer adds an integer object instead of a string, the compiler allows it. But, when we retrieve this data from ArrayList, it causes problems at runtime.

Java




// A Simple Java program to demonstrate that NOT using
// generics can cause run time exceptions
import java.util.*;
  
class Test
{
    public static void main(String[] args)
    {
        // Creatinga an ArrayList without any type specified
        ArrayList al = new ArrayList();
  
        al.add("Sachin");
        al.add("Rahul");
        al.add(10); // Compiler allows this
  
        String s1 = (String)al.get(0);
        String s2 = (String)al.get(1);
  
        // Causes Runtime Exception
        String s3 = (String)al.get(2);
    }
}

Output :

Exception in thread "main" java.lang.ClassCastException: 
   java.lang.Integer cannot be cast to java.lang.String
    at Test.main(Test.java:19)

How generics solve this problem? 
At the time of defining ArrayList, we can specify that this list can take only String objects.
 

Java




// Using generics converts run time exceptions into 
// compile time exception.
import java.util.*;
  
class Test
{
    public static void main(String[] args)
    {
        // Creating a an ArrayList with String specified
        ArrayList <String> al = new ArrayList<String> ();
  
        al.add("Sachin");
        al.add("Rahul");
  
        // Now Compiler doesn't allow this
        al.add(10); 
  
        String s1 = (String)al.get(0);
        String s2 = (String)al.get(1);
        String s3 = (String)al.get(2);
    }
}


Output:
15: error: no suitable method found for add(int)
        al.add(10); 
          ^

3. Individual Type Casting is not needed: If we do not use generics, then, in the above example every time we retrieve data from ArrayList, we have to typecast it. Typecasting at every retrieval operation is a big headache. If we already know that our list only holds string data then we need not typecast it every time.

Java




// We don't need to typecast individual members of ArrayList
import java.util.*;
  
class Test
{
    public static void main(String[] args)
    {
        // Creating a an ArrayList with String specified
        ArrayList <String> al = new ArrayList<String> ();
  
        al.add("Sachin");
        al.add("Rahul");
  
        // Typecasting is not needed 
        String s1 = al.get(0);
        String s2 = al.get(1);
    }
}

4. Generics promotes code reusability.
5. Implementing generic algorithms: By using generics, we can implement algorithms that work on different types of objects and at the same, they are type safe too.

References: 
https://docs.oracle.com/javase/tutorial/java/generics/why.html
This article is contributed by Dharmesh Singh. If you like GeeksforGeeks and would like to contribute, you can also write an article and mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.
Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above
 

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