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.

Why Generics?

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. 

Types of Java Generics

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.

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’.

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We can also pass multiple Type parameters in Generic classes. 

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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.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.

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<>();

Generic Types Differ Based on Their Type Arguments: 

Consider the following Java code.

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.

Type Parameters in Java Generics

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

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 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.

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 do Generics Solve this Problem? 

When defining ArrayList, we can specify that this list can take only String objects.

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.

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,

Let’s say we want to Sort the array elements of various data types like int, char, String etc.

Basically we will be needing different functions for different data types.

For simplicity, we will be using Bubble sort.

But by using Generics, we can achieve the code reusability feature.

Sorted Integer array :  6, 22, 41, 50, 58, 100, 
Sorted Character array :  a, c, d, g, t, v, x, 
Sorted String array :  Abhinay, Bharat, Chandu, Kalam, Rohit, Sam, Virat, 

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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https://docs.oracle.com/javase/tutorial/java/generics/types.html

public class Box {
    private Object object;

    public void set(Object object) { this.object = object; }
    public Object get() { return object; }
}

class name<T1, T2, ..., Tn> { /* ... */ }

/**
 * Generic version of the Box class.
 * @param <T> the type of the value being boxed
 */
public class Box<T> {
    // T stands for "Type"
    private T t;

    public void set(T t) { this.t = t; }
    public T get() { return t; }
}

Box<Integer> integerBox;

Box<Integer> integerBox = new Box<Integer>();

Box<Integer> integerBox = new Box<>();

public interface Pair<K, V> {
    public K getKey();
    public V getValue();
}

public class OrderedPair<K, V> implements Pair<K, V> {

    private K key;
    private V value;

    public OrderedPair(K key, V value) {
	this.key = key;
	this.value = value;
    }

    public K getKey()	{ return key; }
    public V getValue() { return value; }
}

Pair<String, Integer> p1 = new OrderedPair<String, Integer>("Even", 8);
Pair<String, String>  p2 = new OrderedPair<String, String>("hello", "world");

OrderedPair<String, Integer> p1 = new OrderedPair<>("Even", 8);
OrderedPair<String, String>  p2 = new OrderedPair<>("hello", "world");

OrderedPair<String, Box<Integer>> p = new OrderedPair<>("primes", new Box<Integer>(...));