Generics in Java

The Java Generics programming is introduced in J2SE 5 to deal with type-safe objects. It makes the code stable by detecting the bugs at compile time.

Before generics, we can store any type of objects in the collection, i.e., non-generic. Now generics force the java programmer to store a specific type of objects.

Advantage of Java Generics

There are mainly 3 advantages of generics. They are as follows:

1) Type-safety: We can hold only a single type of objects in generics. It doesn?t allow to store other objects.

Without Generics, we can store any type of objects.

2) Type casting is not required: There is no need to typecast the object.

Before Generics, we need to type cast.

3) Compile-Time Checking: It is checked at compile time so problem will not occur at runtime. The good programming strategy says it is far better to handle the problem at compile time than runtime.

Syntax to use generic collection

Example to use Generics in java

Full Example of Generics in Java

Here, we are using the ArrayList class, but you can use any collection class such as ArrayList, LinkedList, HashSet, TreeSet, HashMap, Comparator etc.

Output:

element is: jai
rahul
jai 

Example of Java Generics using Map

Now we are going to use map elements using generics. Here, we need to pass key and value. Let us understand it by a simple example:

Output

1 vijay
2 ankit 
4 umesh

Generic class

A class that can refer to any type is known as a generic class. Here, we are using the T type parameter to create the generic class of specific type.

Let's see a simple example to create and use the generic class.

Creating a generic class:

The T type indicates that it can refer to any type (like String, Integer, and Employee). The type you specify for the class will be used to store and retrieve the data.

Using generic class:

Let's see the code to use the generic class.

Output

2

Type Parameters

The type parameters naming conventions are important to learn generics thoroughly. The common type parameters are as follows:

1. T - Type
2. E - Element
3. K - Key
4. N - Number
5. V - Value

Generic Method

Like the generic class, we can create a generic method that can accept any type of arguments. Here, the scope of arguments is limited to the method where it is declared. It allows static as well as non-static methods.

Let's see a simple example of java generic method to print array elements. We are using here E to denote the element.

Output

Printing Integer Array
10
20
30
40
50
Printing Character Array
J
A
V
A
T
P
O
I
N
T

Wildcard in Java Generics

The ? (question mark) symbol represents the wildcard element. It means any type. If we write <? extends Number>, it means any child class of Number, e.g., Integer, Float, and double. Now we can call the method of Number class through any child class object.

We can use a wildcard as a type of a parameter, field, return type, or local variable. However, it is not allowed to use a wildcard as a type argument for a generic method invocation, a generic class instance creation, or a supertype.

Let's understand it by the example given below:

Output

drawing rectangle
drawing circle
drawing circle

Upper Bounded Wildcards

The purpose of upper bounded wildcards is to decrease the restrictions on a variable. It restricts the unknown type to be a specific type or a subtype of that type. It is used by declaring wildcard character ("?") followed by the extends (in case of, class) or implements (in case of, interface) keyword, followed by its upper bound.

Syntax

Here,

? is a wildcard character.

extends, is a keyword.

Number, is a class present in java.lang package

Suppose, we want to write the method for the list of Number and its subtypes (like Integer, Double). Using List<? extends Number> is suitable for a list of type Number or any of its subclasses whereas List<Number> works with the list of type Number only. So, List<? extends Number> is less restrictive than List<Number>.

Example of Upper Bound Wildcard

In this example, we are using the upper bound wildcards to write the method for List<Integer> and List<Double>.

Output

displaying the sum= 30.0
displaying the sum= 70.0

Unbounded Wildcards

The unbounded wildcard type represents the list of an unknown type such as List<?>. This approach can be useful in the following scenarios: -

• When the given method is implemented by using the functionality provided in the Object class.
• When the generic class contains the methods that don't depend on the type parameter.

Example of Unbounded Wildcards

Output

displaying the Integer values
1
2
3
displaying the String values
One
Two
Three

Lower Bounded Wildcards

The purpose of lower bounded wildcards is to restrict the unknown type to be a specific type or a supertype of that type. It is used by declaring wildcard character ("?") followed by the super keyword, followed by its lower bound.

Syntax

Here,

? is a wildcard character.

super, is a keyword.

Integer, is a wrapper class.

Suppose, we want to write the method for the list of Integer and its supertype (like Number, Object). Using List<? super Integer> is suitable for a list of type Integer or any of its superclasses whereas List<Integer> works with the list of type Integer only. So, List<? super Integer> is less restrictive than List<Integer>.

Example of Lower Bound Wildcard

In this example, we are using the lower bound wildcards to write the method for List<Integer> and List<Number>.

Output

displaying the Integer values
1
2
3
displaying the Number values
1.0
2.0
3.0