learn-co-curriculum / java-creating-packages

Creating Packages

Learning Goals

• Use IntelliJ to create an application with multiple packages

Code-Along

Fork and clone this lesson. We will create an application with multiple packages.

The current project contains a class named Main that is in the default Java package:

Right-click on the "java" folder and select "New/Package" from the menu. Name the package "controller".

Your project structure should look like this:

Drag the Main class to move it out of the default package and into the "controller" package. Your project structure should look like this:

Notice the Main class now has package controller; as the first statement. The package statement is required when a class is not in the default package.

package controller;

public class Main {
    
}

Right-click again on the "java" folder (make sure you are not right-clicking the "controller" folder) and select "New/Package" from the menu. Name the second package "entity".

Your project structure should look like this:

Right-click on the "entity" package and select "New/Java Class". Name the class "Person".

Your project structure should look like the image below.

Notice the first statement in the Person class is package entity;.

package entity;

public class Person {
    
}

Edit the Person class as shown:

package entity;

public class Person {

    private String name;

    public Person(String name) {
        this.name = name;
    }

    @Override
    public String toString() {
        return "Person{" +
                "name='" + name + '\'' +
                '}';
    }
}

Now we will add a main() method to the Main class located in the controller package. The main() method should create an instance of Person as shown below. The code won't compile because we are missing the import statement for the Person class.

package controller;

public class Main {
    public static void main(String[] args) {
        Person person = new Person("Chai");
        System.out.println(person);
    }
}

IntelliJ's compiler indicates an error by changing the text color to red for each reference to the Person class. Hover over Person to have IntelliJ suggest a quick fix for the compiler error. Click the "Import class" suggestion.

IntelliJ adds the statement import entity.Person;. The import statement specifies the package of the Person class. The import statement is written after the package statement and before the class definition.

package controller;

import entity.Person;

public class Main {
    public static void main(String[] args) {
        Person person = new Person("Chai");
        System.out.println(person);
    }
}

We should be able to run the main() method now and see the output:

Person{name='Chai'}

The import statement can import a single class:

import entity.Person;

We can also use the character * to import all classes within a package, for example:

import entity.*;

Conclusion

A Java project may consist of multiple packages. Each class must specify the package as the first statement, unless the class is located in the default package. An import statement can import an entire package or a single class.