Writing your own classes in Java

Demonstrating various methods of Person class:

public class TestPerson {
	public static void main(String[] args) {
		// create a new Person
		Person bob = new Person("Robert", "Smith", 1985);
		// create another person
		Person mary = new Person("Mary", "Green", 1987);

		System.out.println("Testing names:");

		// expect to print: Robert
		System.out.println(bob.getFirstName());
		// expect to print: Green
		System.out.println(mary.getLastName());
		// expect to print: Mary Green
		System.out.println(mary.getFullName());

		System.out.println("Changing name:");
		bob.changeName("Bob", "");
		// expect: Bob Smith
		System.out.println(bob.getFullName());

		System.out.println("Checking the age:");
		int year = 2016;
		// expected: 31
		System.out.println("In the year " + year + " " + bob.getFullName()
				+ " turned or will turn " + bob.age(year));
		year = 2017;
		// expected: 30
		System.out.println("In the year " + year + " " + mary.getFullName()
				+ " turned or will turn " + mary.age(year));

		// write and test a method "isOlder"

		// write toString method so that a Person object
		// can be printed directly with System.out.println
	}
}

The Person class:

/**
 * This simple class describes a person: their name (first, last) and date of
 * birth.
 **/
public class Person {
	// instance variables (a.k.a. fields):
	// different for each instance of a person
	// private variables are invisible outside of the
	// Person class: no other part of the program can change them
	// or even see them
	private String firstName;
	private String lastName;
	private int yearBirth; // for simplicity only have year

	// Constructor method
	// note: no return type, not even void
	/**
	 * creates a person with the given first and last names and a given year of
	 * birth A year that's < 0 or > 2017 causes the program to exit
	 **/
	public Person(String first, String last, int year) {
		firstName = first;
		lastName = last;
		// check if the year is valid, exit the program if
		// it's not. We will learn a better approach to error
		// handling later, but this will do for now
		if (year < 0 || year > 2017) {
			System.out.println("invalid year");
			System.exit(0);
		}
		yearBirth = year;
	}

	/**
	 * returns the person's first name
	 **/
	public String getFirstName() {
		return firstName;
	}

	/**
	 * returns the person's last name
	 **/
	public String getLastName() {
		return lastName;
	}

	/**
	 * returns the person's full name: concatenation of their first and last
	 * name separated by a space, e.g. Robert Smith
	 **/
	public String getFullName() {
		return firstName + " " + lastName;
	}

	/**
	 * the method changes the first and the last name of the person to the given
	 * strings. If the new first name or the the new last name is an empty
	 * string "" then the corresponding name is not changed
	 **/
	public void changeName(String newFirst, String newLast) {
		if (!newFirst.equals("")) {
			firstName = newFirst;
		}
		if (!newLast.equals("")) {
			lastName = newLast;
		}
	}

	/**
	 * The method takes a year and returns the age the person turns in that year
	 **/
	public int age(int year) {
		return year - yearBirth;
	}
}

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