Generics in Java

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.

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

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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);     } }

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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.