Java Generics

Last Updated on March 22, 2023 by Prepbytes

Java Generics were introduced in Java 5. Java generics are primarily used in programs to enforce type safety. Type safety occurs when the compiler validates the datatype of constants, variables, and methods to ensure it is correctly assigned. We cannot, for example, initialize a variable as an integer and then assign it a string value. To avoid such mismatches, we can limit the use of a specific type of object, such as Integer, Float, and so on. Let’s look at some code examples to see why we need generics.

Here we will discuss and explore java generics, how it works, and different types of java generics.

What are Generics in Java?

Java generic is a feature that allows you to parameterize types in classes, interfaces, and methods. Generics in Java are a way to write code that can work with different data types. They provide a way to define a class, interface, or method that can operate on objects of different types while maintaining type safety. Generics are similar to templates in C++, but they are easier to use.

Java Generic methods and generic classes allow programmers to specify a set of related methods or a set of related types with a single method declaration or a single class declaration, respectively.

Using the Java Generic concept, we could create a generic method for sorting an array of objects, then use Integer arrays, Double arrays, String arrays, and so on to sort the array elements.

Syntax with an example:
The syntax for defining java generics uses angle brackets and a type parameter. The type parameter is a placeholder for the actual type that will be used when an instance of the generic type is created. For example, we define a generic class

public class MyGenericClass {
  private T myVariable;

  public void setVariable(T variable) {
    this.myVariable = variable;
  }

  public T getVariable() {
    return myVariable;
  }
}

In this example, T is the type parameter that can be replaced with any data type when an instance of the MyGenericClass is created. The setVariable method takes a parameter of type T, and the getVariable method returns an object of type T. When we create an instance of MyGenericClass, we can specify the actual type to be used for T, like this:

MyGenericClass stringClass = new MyGenericClass<>();
stringClass.setVariable("Hello, world!");
String myString = stringClass.getVariable();

In this example, we have created an instance of MyGenericClass that will use String as the type parameter. We then set a string value using the setVariable method and retrieve it using the getVariable method. The type safety is ensured at compile time, as the compiler will not allow us to use any other data type than String with this instance of MyGenericClass.

Advantages of Java Generics

Generics provide several benefits or advantages to Java developers, including:

• Type safety: Generics ensure that the type of data used with a generic class, interface, or method is consistent, which helps prevent runtime errors.
• Code reusability: Generics allow us to write code that can work with different data types, which makes the code more reusable.
• Efficiency: Generics allow us to write code that is more efficient, as it can avoid unnecessary type conversions and reduce the amount of boilerplate code.
• Cleaner code: Generics can make code easier to read and understand, as it can remove the need for casting and other type-related operations.

Types of Java Generics

There are two types of java generics:

1) Generic Class

A generic class is implemented in the same way that a non-generic class is. The only difference is that it includes a section for type parameters. There can be multiple types of parameters, each separated by a comma. Parameterized classes or parameterized types are classes that accept one or more parameters.

Below we have an example with code implementation and explanation

/* package whatever; // don't place package name! */

import java.util.*;
import java.lang.*;
import java.io.*;




class GenericClass<T> {
    private T value;
 
    public void setValue(T value) {
        this.value = value;
    }
 
    public T getValue() {
        return value;
    }
}

/* Name of the class has to be "Main" only if the class is public. */
class prepbytes
{
    public static void main (String[] args) throws java.lang.Exception
    {
        // your code goes here
        
        GenericClass<String> stringObj = new GenericClass<>();
        stringObj.setValue("Hello, prepbytes!");
        System.out.println(stringObj.getValue());
        
        
        GenericClass<Integer> intObj = new GenericClass<>();
        intObj.setValue(17);
        System.out.println(intObj.getValue());
        
    }
}

Output

Hello, prepbytes!
17

Explanation
In the above java program, we have defined a Generic class called GenericClass that takes a type parameter T. This type parameter is then used as the type of the value field. The class contains two methods – setValue and getValue. The setValue method takes an argument of type T and sets it as the value of the value field. The getValue method returns the value of the value field. In this example, we have created an instance of the GenericClass class that takes a String type parameter. We have set the value of the value field to "Hello, prepbytes!" using the setValue method.

Now, we have another instance of the GenericClass class that takes an Integer type parameter and test it out. we have created an instance of the GenericClass class that takes an Integer type parameter. We have set the value of the value field to 17 using the setValue method. We then print out the value of the value field using the getValue method.

2) Generic Method

A java generics method is a method that is defined with one or more type parameters, allowing the method to work with different types. The type parameters are specified in angle brackets < >, and can be used to declare the return type, parameter types, and local variable types within the method. It works exactly like a normal function, except that a generic method has type parameters that are referred to by the actual type. This enables the generic method to be used more broadly. The compiler handles type safety, allowing programmers to code more easily because they do not have to perform lengthy, individual type castings.

Below we have an example with code implementation and explanation

class Main {
  public static void main(String[] args) {

    // initialize the class with Integer data
    DemoClass demo = new DemoClass();

    // generics method working with String
    demo.<String>genericsMethod("Java Programming");

    // generics method working with integer
    demo.<Integer>genericsMethod(25);
  }
}

class DemoClass {

  // creae a generics method
  public <T> void genericsMethod(T data) {
    System.out.println("Generics Method:");
    System.out.println("Data Passed: " + data);
  }
}

Output:

Generics Method:
Data Passed: Java Programming
Generics Method:
Data Passed: 25

Explanation
In the above example, we have created a generic method named genericsMethod.

public  void genericMethod(T data) {...}

Here, the type parameter < T > is inserted after the modifier public and before the return type void. We can call the generics method by placing the actual type < String > and < Integer > inside the bracket before the method name.

demo.genericMethod("Java Programming");
demo.genericMethod(25);

Note: We can call the generics method without including the type parameter. For example,

demo.genericsMethod("Java Programming");

In this case, the compiler can match the type parameter based on the value passed to the method.

Conclusion
In conclusion, Java Generics allow types to be parameterized so that they can work with different data types. Generics were introduced in Java 5 and are implemented using angle brackets (< >) to specify the type parameter. The type parameter can be any non-primitive data type, including classes, interfaces, and other generics. Java Generics provides compile-time type safety by ensuring that the correct type is used at compile time and can prevent runtime errors caused by incorrect type casting. Java Generics provides several benefits, including increased type safety, code reusability, and improved code clarity.

Frequently Asked Questions(FAQs)

Here are the FAQs on java generics

1) How do Generics work in Java?
Ans: A generic type is a class or interface that is parameterized over types, which means that a type can be assigned by invoking the generic type, which replaces the generic type with the assigned concrete type.

2) Can you use primitives with Java Generics?
Ans: No, primitives cannot be used with Java Generics. Instead, you must use their corresponding wrapper classes, such as Integer, Float, and Double.

3) What is a wildcard in Java Generics?
Ans: A wildcard is a type argument that represents an unknown type. It can be used to specify a class or method that can work with any type of object or to specify a specific subset of types.

4) What is type erasure in Java Generics?
Ans: Type erasure is a process that removes the type parameter information from a generic class or method when the code is compiled. This allows the generic code to be used with any type of object that meets the specified requirements.

5) What are some best practices for using Java Generics?
Ans: Some best practices for using Java Generics include using descriptive names for type parameters, using wildcards sparingly, and avoiding unchecked warnings by specifying the type parameters correctly.