Generics in Java - GeeksforGeeks

Generics in Java is similar to templates in C++. The idea is to allow type (Integer, String, … etc and user defined types) to be a parameter to methods, classes and interfaces. For example, classes like HashSet, ArrayList, HashMap, etc use generics very well. We can use them for any type.

Generic Class
Like C++, we use <> to specify parameter types in generic class creation. To create objects of generic class, we use 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'.

// 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.

// 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 generic method, the compiler handles each method.

// 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

Advantages of Generics:
Programs that uses Generics has got many benefits over non-generic code.

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

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 string, compiler allows it. But, when we retrieve this data from ArrayList, it causes problems at runtime.

// 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.

// 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); 
          ^

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 to typecast it every time.

// 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); 
    } 
} 

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.

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