Learn

Previously we saw how generics can help make our code more manageable and flexible to future needs. We created the following generic class:

public class Box <T> {
  private T data;

  public Box(T data) {
    this.data = data; 
  }

  public T getData() {
    return this.data;
  }  
}

In the example above notice that:

The type parameter must be specified within the diamond operator (<>) after the class name.
The type parameter, T, is similar to a method parameter but instead receives a class or interface type as an argument as opposed to reference or primitive type.
The constructor has a T type parameter when wanting to alter data.
The getter returns the type parameter T when returning data.

Creating generic classes is great but we also need to be able to create references to them in our programs:

Box<String> myStringBox = new Box<>("Apple");

In the example above, we can see the reference myStringBox is created similarly to non-generic references but differ in:

Needing the diamond operator with the class or interface type argument, <String> in this example, after the class name.
Needing the empty diamond operator prior to calling the constructor, new Box<>("Apple").

When defining type parameters, although not being a hard requirement, it’s best practice to make them single, uppercase letters to easily distinguish them from the names of classes or variables. By convention common type parameters include E (Elements), K (Key), N (Number), T (Type), V (Value), and S (or U or V) for multiple type parameters.

One final thing to note is that prior to Java 7, generic references needed to be created like:

Box<String> myStringBox = new Box<String>("Apple");

In the example above, the <String> type argument also needed to be specified prior to calling the constructor. This is no longer necessary due to Java’s type inference where the compiler can infer the <String> type argument in the constructor from the context of the reference definition Box<String>.

Let’s practice creating a generic class and creating an instance of it.

Instructions

Let’s start by defining our generic class that will represent a real world container.

In Container.java, create a generic class Container with a type paramter T and create a type T field named data.

Note: It’s okay to see an error in the terminal window about a missing main() method.

Let’s complete our class so we can create references to it.

In the Container class, add a public constructor with a type T parameter named data. Set the data field to the value of the data parameter. Also, create a getter named getData() and a setter named setData() for the data field.

Note: It’s okay to see an error in the terminal window about a missing main() method.

Let’s create Container references and print it’s data field to the terminal.

We’ve provided a Book class for you to use with a single String argument constructor. In main() of Main.java, create the following references:

A String type Container named wordContainer. Use myWord to initialize it.
A Book type Container named bookContainer. Use myBook to initialize it.

Call System.out.println() to print wordContainers data field using it’s getter.

Call System.out.println() to print bookContainers data field using it’s getter.

Take this course for free

By signing up for Codecademy, you agree to Codecademy's Terms of Service & Privacy Policy.

Or sign up using:

Already have an account?

Log in from a computer to take this course

Instructions

Take this course for free

Or sign up using: