What are Generics in Java?

Java Generics is a set of related methods or a set of similar types. Generics allow types Integer, String, or even user-defined types to be passed as a parameter to classes, methods, or interfaces. Generics are mostly used by classes like HashSet or HashMap.

Advantages of using generics

Generics ensure compile-time safety which allows the programmer to catch the invalid types while compiling the code.
Java Generics helps the programmer to reuse the code for whatever type he/she wishes. For instance, a programmer writes a generic method for sorting an array of objects. Generics allow the programmer to use the same method for Integer arrays, Double arrays, and even String arrays.
Another advantage of using generics is that Individual typecasting isn’t required. The programmer defines the initial type and then lets the code do its job.
It allows us to implement non-generic algorithms.

Constraints

Java Generics does not support sub-typing.
You cannot create generic arrays in Java Generics.

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 which 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 parametrized classes or parameterized types.

class GenericMethodTest {
   // generic method printArray
   public static < E > void printArray( E[] inputArray ) {
      // Display array elements
      for(E element : inputArray) {
         System.out.printf("%s ", element);
      }
      System.out.println();
   }
   public static void main(String args[]) {
      // Create arrays of Integer, Double and Character
      Integer[] integerArray = { 5, 4, 3, 2, 1 };
      Double[] doubleArray = { 1.21, 22.12, 13.32 };
      Character[] characterArray = { 'Y', 'o', 'u', ' ', 'a', 'r', 'e', ' ', 'a','w','e','s','o','m','e' };
      System.out.println("integerArray contains:");
      printArray(integerArray);   // pass an Integer array
      System.out.println("\ndoubleArray contains:");
      printArray(doubleArray);   // pass a Double array
      System.out.println("\ncharacterArray contains:");
      printArray(characterArray);   // pass a Character array
   }
}

class container<T> {
   private T obj1;
   public void add(T obj1) {
      this.obj1 = obj1;
   }
   public T get() {
      return obj1;
   }
   public static void main(String[] args) {
      container<Integer> integerContainer= new container<Integer>();
      container<String> stringContainer = new container<String>();
    
      integerContainer.add(new Integer(7));
      stringContainer.add(new String("You are awesome"));
      System.out.printf("Integer Value :%d\n\n", integerContainer.get());
      System.out.printf("String Value :%s\n", stringContainer.get());
   }
}

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What are Generics in Java?

Advantages of using generics

Constraints

Types of Java Generics

Examples

1. Single generic method

2. Generic class