• Develop code to designate access and visibility constraints to classes, data, constructors, and methods

Assignment

With this section, you’ll start designing and writing your own classes. Classes are made up of three main parts: instance variables, constructors, and methods.

Methods you’ve seen before, and instance variables at least in passing. Classes typically represent objects, so methods are the actions the objects can take (or have done to them) and instance variables are their properties.

In the class below, a Person object would have three properties/instance variables with their name, email and phone number. They also have one action, where their details will be printed out to the console.

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public class Person {
    
    // instance variables
    private String name;
    private String email;
    private String phoneNumber;

    // constructor: construct a Person copying in the data into the instance
    // variables
    public Person(String initName, String initEmail, String initPhone) {
        name = initName;
        email = initEmail;
        phoneNumber = initPhone;
    }

    // Print all the data for a person
    public void print() {
        System.out.println("Name: " + name);
        System.out.println("Email: " + email);
        System.out.println("Phone Number: " + phoneNumber);
    }

}

The new part is the constructor. This is more of a topic for 3.4, but these are special methods responsible for creating the object when new is called during object creation.

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Person p1 = new Person("Bob", "bob@bob.bob", "(262) 555-2626");

The constructor calls for three arguments, so we need to provide them when the object is created.

Missing from the class is main method. These are not required, but can always be included. They are points of entry for a program, so not every class needs one.

Public vs. Private

Up until this point, you’ve seen public as the access modifier for nearly everything, but another option is private. These can be applied to classes, methods, instance variables, and your constructors, and they control what part of a program has access to that data or method.

public allows for any part of the program to access it, while private restricts access to inside the class itself. When to use each depends on the design, but some quick rules are that your main methods always needs to be public or else they won’t run, and instance variables should be private. You can always write public methods to grant access to them if needed, which we’ll cover in 3.5.