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Ace the AP Computer Science Exam for High Schoolers

Constructors

Take a deeper look at class constructors in this lesson.

We'll cover the following...

Constructors are a special type of methods that are used to instantiate objects of a class. As we saw in the last couple of lessons, a class gives the blueprint of a non-primitive data type. To create objects from this class, we need constructors.

The code for a simple Introduction class is reproduced below. Let’s take a look at the anatomy of the constructor below.

Java
class Introduction 
{
    // A class can have its own variables
    String name;
    // This constructor requires the user to pass a String type variable or value
    Introduction(String enteredName)
    {
        // We can read/update the value of a class's variable in the constructor
        name = enteredName; 
    }
    
    // This class has the following method
    public void greet() 
    {
        // We can read/update the value of a class's variable in the class's methods
        System.out.format("Hello %s! This is AP CS A course.", name );  
    }
    public static void main (String args[])
    {
        // Introduction class has only one constructor that takes String as input
        Introduction user = new Introduction("Frodo Baggins");
        // user is now an object, we can call all the methods on it e.g. greet() method
        user.greet();
    }
}

Signature

The constructor’s signature defines what types of parameters it requires as input. For example, in our case, we only need a String type variable: enteredName. However, a constructor can have any number and type of parameters.

  • We can have a constructor with no parameters. In this case, we will leave the parenthesis empty.

    class Introduction()

  • We can also have any number of parameters. For example, the following signature takes three String type parameters.

    class Introduction(String enteredName, String secondName, String
...