Generics in Java

09 Feb, 2022
Generics means 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 a generic entity.
The Object is the superclass of all other classes, and Object reference can refer to any object. These features lack type safety. Generics add that type of safety feature. We will discuss that type of safety feature in later examples.
Generics in Java are 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 Method: Generic Java method takes a parameter and returns some value after performing a task. It is exactly like a normal function, however, a generic method has type parameters that are cited by actual type. This allows the generic method to be used in a more general way. The compiler takes care of the type of safety which enables programmers to code easily since they do not have to perform long, individual type castings.
Generic Classes: A generic class is implemented exactly like a non-generic class. The only difference is that it contains a type parameter section. There can be more than one type of parameter, separated by a comma. The classes, which accept one or more parameters, ​are known as parameterized classes or parameterized types.
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
// Java program to show working of user defined// Generic classes  // We use < > to specify Parameter typeclass 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 aboveclass 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());    }}
15
GeeksForGeeks
We can also pass multiple Type parameters in Generic classes. 
Java
// Java program to show multiple// type parameters in Java Generics  // We use < > to specify Parameter typeclass 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 aboveclass Main{    public static void main (String[] args)    {        Test <String, Integer> obj =            new Test<String, Integer>("GfG", 15);          obj.print();    }}
GfG
15
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
// 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);    }}
java.lang.Integer = 11
java.lang.String = GeeksForGeeks
java.lang.Double = 1.0
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.
Test<int> obj = new Test<int>(20); 
The above line results in a compile-time error that can be resolved using type wrappers to encapsulate a primitive type. 
But primitive type arrays can be passed to the type parameter because arrays are reference types.
ArrayList<int[]> a = new ArrayList<>();
Consider the following Java code.
Java
// Java program to show working// of user-defined Generic classes  // We use < > to specify Parameter typeclass 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 aboveclass 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.
The type parameters naming conventions are important to learn generics thoroughly. The common type parameters are as follows:
T – Type
E – Element
K – Key
N – Number
V – Value
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 the 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
// 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)
When defining ArrayList, we can specify that this list can take only String objects.
Java
// Using Java 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, 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: With the help of generics in Java, we can write code that will work with different types of data. For example,
public <T> void genericsMethod (T data) {...}
Here, we have created a generics method. This same method can be used to perform operations on integer data, string data, and so on.
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.
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 review-team@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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