CS 199 EMP

Even More Practice

Attendance Link:



  • Reference equality: If two references refer to the same object, they are equal
    • "==", ".equals()"
  • Object equality
    • Class-defined equality, usually based on field values
    • ".equals()"

Pass by value vs reference

  • by value: a copy of the variable will be pased into the method
  • by reference: the method will be working with the original variable


Abstract Classes

  • you can inherit from an abstract class
  • you cannot create an instane of it
  • Abstract classes provide methods for the class to use
  • If any of your methods are marked abstract, you must make the class abstract


  • an Interface is a shared boundary across which two or more separate components of a computer system exchange information
  • Interfaces have to be implemented by a specific class, they can't function on their own, using the keyword implements.
  • You also have to implement all the methods the interface declares.

Interface vs. Inheritance

  • Only one inheritance per class

    • different classes could have different functionalities
    • Java can't decide which implementation to use
  • Multiple interfaces is allowed

    • Interface just says what the class is doing, not how it's doing it
    • We define the class functionality!

Problem 1: Lets implement an interface!

allow me to start it off for you, but from here, you should create a class that implements the following interface I made (I tried to get creative, you can too!)

interface Monster {
  // interface methods (do not have bodies)

  public void didTheMash();    // it was a graveyard smash
  public void playedTheMash(); // it caught on in a flash

Problem 2: remember me?

Create a public class named Minimum with a single class method named min. min should take in an array of Objects that implement Comparable and return the smallest value in the array. Don't forget your null checks!

Problem 3: practice with arrays

Write a function that given an integer, creates a 2D char array that, when called with printGrid, prints this pattern:

for x=1,2,3,4,5x = 1, 2, 3, 4, 5

x   xx    x o x     x o o x     x o o o x
    xx    o x o     o x x o     o x o x o
          x o x     o x x o     o o x o o
                    x o o x     o x o x o
                                x o x o x

Weekly recap

  • Remaining time is for you to ask questions about literally anything the class has covered so far!
  • Don't hesitate :)

Additional Sources

Want more coding challenges?


(no peeking, it's for your own good!)

(no seriously, attempt the problems with your groupmates first!)

Problem 1

public class Mon implements Monster {
  public String name;
  public Mon(String setName) {
    name = setName;
  public void didTheMash() {
    System.out.println(name + " did the monster mash");
  public void playedTheMash() {
    System.out.println(name + " was caught in on a flash!");

Problem 2

public class Minimum {
  public static Comparable min(Comparable[] arr) {
    if (arr == null || arr.length == 0) {
      return null;
    Comparable temp = arr[0];
    for (int i = 0; i < arr.length; i++) {
      if (arr[i].compareTo(temp) < 0) {
        temp = arr[i];
    return temp;

Problem 3

public static String[][] xtremeVDay(int n) {
  String[][] returnArr = new String[n][n];
  for (int i = 0; i < n; i++) {
    for (int j = 0; j < n; j++) {
      if (i == j || (i + j) == n - 1) {
        returnArr[i][j] = "x";
      } else {
        returnArr[i][j] = "o";
  return returnArr;