Section 11

String recursion

Consider the following recursive method:

public static void mystery(String str, char a, char b) {
    if (str == null || str.equals("")) {
        System.out.println();
        return;
    }

    char first = str.charAt(0);
    if (first == a) {
        System.out.print(b);
    } else {
        System.out.print(first);
    }

    mystery(str.substring(1), a, b);
}

1. Trace the execution of mystery("snip", 'i', 'a') and determine the output. ¹
2. Briefly describe what this method does and how it works. ²

Classes as “blueprints”

As you’ve seen in lecture, a blueprint class is a class that defines a new type of object – it specifies the fields and methods that each object of that class will possess. An object built from a blueprint class is known as an instance of that class.

In this exercise, we will complete the implementation of a class for an object that represents a student at a university. Start with this code template.

Fields

Each Student object already has fields that capture the following state:

• the student’s first name
• the student’s last name
• the student’s year of graduation

Note that these fields are made private.

Methods

Now add the following methods to the Student class.

• A constructor that takes as parameters the student’s first name, last name, and year of graduation.

• Accessor methods for retrieving the first name, last name, and year of graduation for each student.

• An accessor method for the student’s full name (i.e., the first name, followed by a space, followed by the last name).

• A mutator method to change the student’s year of graduation, which should not allow the year of graduation to be moved earlier.

Write these methods so that they prevent “bad” values from getting into the fields. In particular, the constructor should not allow null values for the names, and the mutator should not allow the user to insert an earlier year of graduation. ³

Exercise

In this exercise, we will implement a class for an object that represents an airplane. We have started the Airplane.java file for you; your task is to fill out the rest of the class. The blueprint class should have the following functionality:

Fields

Each Airplane should have the following fields:

• the airplane’s manufacturer (a String)
• the airplane’s model number (an int)
• whether the plane is currently flying (a boolean)
• the number of miles the plane has flown (an int)

Methods

Each Airplane should have the following methods:

• A constructor that takes as parameters the plane’s manufacturer, model number, and the number of miles flown. This constructor should assume that the plane is not currently flying.

• An accessor method named getID that returns the plane’s full identification (i.e., the manufacturer, followed by a space, followed by the model number).

• An accessor method named isFlying that returns whether the plane is currently flying.

• An accessor method named getMilesFlown that returns the number of miles flown.

• A mutator method named changeFlightStatus that toggles the plane’s flying status. In other words, if this method is called when the plane is not flying, it should be set to flying. If this method is called when the plane is flying, it should be set to not flying.

• A mutator method named addMilesFlown that accepts a single int parameter and increases the number of miles flown by the given amount. However, you should not allow the number of miles flown to be decreased. If the user tries to call addMilesFlown with a negative number, an IllegalArgumentException should be thrown.

Once your Airplane class is implemented, download AirportSimulator.java and place it in the same folder as Airplane.java. Compile AirportSimulator.java and run it to verify that you correctly implemented Airplane.java. ⁴

Last updated on July 10, 2025.