Classes/Object and Annotations

Question 1

Consider the following class:

public class IdentifyMyParts { public static int x = 7; public int y = 3; }

1. What are the class variables?

Answer 1

x

Question 2

Consider the following class:

public class IdentifyMyParts { public static int x = 7; public int y = 3; }

What are the instance variables?

Answer 2

y

Question 3

Consider the following class:

public class IdentifyMyParts { public static int x = 7; public int y = 3; }

//What is the output from the following code: IdentifyMyParts a = new IdentifyMyParts(); IdentifyMyParts b = new IdentifyMyParts(); a.y = 5; b.y = 6; a.x = 1; b.x = 2; System.out.println("a.y = " + a.y); System.out.println("b.y = " + b.y); System.out.println("a.x = " + a.x); System.out.println("b.x = " + b.x); System.out.println("IdentifyMyParts.x = " + IdentifyMyParts.x);

Answer 3

Here is the output:

a.y = 5 b.y = 6 a.x = 2 b.x = 2 IdentifyMyParts.x = 2

Because x is defined as a public static int in the class IdentifyMyParts, every reference to x will have the value that was last assigned because x is a static variable (and therefore a class variable) shared across all instances of the class. That is, there is only one x: when the value of x changes in any instance it affects the value of x for all instances of IdentifyMyParts.

This is covered in the Class Variables section of Understanding Instance and Class Members.

Question 4

Write a class whose instances represent a single playing card from a deck of cards. Playing cards have two distinguishing properties: rank and suit. Be sure to keep your solution as you will be asked to rewrite it in Enum Types.

Answer 4

Card.java

public class Card { private final int rank; private final int suit; // Kinds of suits public final static int DIAMONDS = 1; public final static int CLUBS = 2; public final static int HEARTS = 3; public final static int SPADES = 4; // Kinds of ranks public final static int ACE = 1; public final static int DEUCE = 2; public final static int THREE = 3; public final static int FOUR = 4; public final static int FIVE = 5; public final static int SIX = 6; public final static int SEVEN = 7; public final static int EIGHT = 8; public final static int NINE = 9; public final static int TEN = 10; public final static int JACK = 11; public final static int QUEEN = 12; public final static int KING = 13; public Card(int rank, int suit) { assert isValidRank(rank); assert isValidSuit(suit); this.rank = rank; this.suit = suit; } public int getSuit() { return suit; } public int getRank() { return rank; } public static boolean isValidRank(int rank) { return ACE \<= rank && rank \<= KING; } public static boolean isValidSuit(int suit) { return DIAMONDS \<= suit && suit \<= SPADES; } public static String rankToString(int rank) { switch (rank) { case ACE: return "Ace"; case DEUCE: return "Deuce"; case THREE: return "Three"; case FOUR: return "Four"; case FIVE: return "Five"; case SIX: return "Six"; case SEVEN: return "Seven"; case EIGHT: return "Eight"; case NINE: return "Nine"; case TEN: return "Ten"; case JACK: return "Jack"; case QUEEN: return "Queen"; case KING: return "King"; default: //Handle an illegal argument. There are generally two //ways to handle invalid arguments, throwing an exception //(see the section on Handling Exceptions) or return null return null; } } public static String suitToString(int suit) { switch (suit) { case DIAMONDS: return "Diamonds"; case CLUBS: return "Clubs"; case HEARTS: return "Hearts"; case SPADES: return "Spades"; default: return null; } } public static void main(String[] args) { // must run program with -ea flag (java -ea ..) to // use assert statements assert rankToString(ACE) == "Ace"; assert rankToString(DEUCE) == "Deuce"; assert rankToString(THREE) == "Three"; assert rankToString(FOUR) == "Four"; assert rankToString(FIVE) == "Five"; assert rankToString(SIX) == "Six"; assert rankToString(SEVEN) == "Seven"; assert rankToString(EIGHT) == "Eight"; assert rankToString(NINE) == "Nine"; assert rankToString(TEN) == "Ten"; assert rankToString(JACK) == "Jack"; assert rankToString(QUEEN) == "Queen"; assert rankToString(KING) == "King"; assert suitToString(DIAMONDS) == "Diamonds"; assert suitToString(CLUBS) == "Clubs"; assert suitToString(HEARTS) == "Hearts"; assert suitToString(SPADES) == "Spades"; } }

Question 5

Write a class whose instances represents a full deck of cards. You should also keep this solution.

You can use the assert statement to check your assignments. You write:

assert (boolean expression to test);

If the boolean expression is false, you will get an error message. For example,

assert toString(ACE) == "Ace";

should return true, so there will be no error message.

If you use the assert statement, you must run your program with the ea flag:

java -ea YourProgram.class

Answer 5

Deck.java.

import java.util.\*; public class Deck { public static int numSuits = 4; public static int numRanks = 13; public static int numCards = numSuits \* numRanks; private Card[][] cards; public Deck() { cards = new Card[numSuits][numRanks]; for (int suit = Card.DIAMONDS; suit \<= Card.SPADES; suit++) { for (int rank = Card.ACE; rank \<= Card.KING; rank++) { cards[suit-1][rank-1] = new Card(rank, suit); } } } public Card getCard(int suit, int rank) { return cards[suit-1][rank-1]; } }

Question 6

Write a small program to test your deck and card classes. The program can be as simple as creating a deck of cards and displaying its cards.

Answer 6

DisplayDeck.java.

import java.util.\*; public class DisplayDeck { public static void main(String[] args) { Deck deck = new Deck(); for (int suit = Card.DIAMONDS; suit \<= Card.SPADES; suit++) { for (int rank = Card.ACE; rank \<= Card.KING; rank++) { Card card = deck.getCard(suit, rank); System.out.format("%s of %s%n", card.rankToString(card.getRank()), card.suitToString(card.getSuit())); } } } }

Question 7

What’s wrong with the following program?

public class SomethingIsWrong { public static void main(String[] args) { Rectangle myRect; myRect.width = 40; myRect.height = 50; System.out.println("myRect's area is " + myRect.area()); } }

Answer 7

The code never creates a Rectangle object. With this simple program, the compiler generates an error. However, in a more realistic situation, myRect might be initialized to null in one place, say in a constructor, and used later. In that case, the program will compile just fine, but will generate a NullPointerException at runtime.

Question 8

The following code creates one array and one string object. How many references to those objects exist after the code executes? Is either object eligible for garbage collection?

... String[] students = new String[10]; String studentName = "Peter Smith"; students[0] = studentName; studentName = null; ...

Answer 8

There is one reference to the students array and that array has one reference to the string Peter Smith. Neither object is eligible for garbage collection. The array students is not eligible for garbage collection because it has one reference to the object studentName even though that object has been assigned the value null. The object studentName is not eligible either because students[0] still refers to it.

Question 9

How does a program destroy an object that it creates?

Answer 9

A program does not explicitly destroy objects. A program can set all references to an object to null so that it becomes eligible for garbage collection. But the program does not actually destroy objects.

Question 10

Fix the program called SomethingIsWrong shown in Question 1.

public class SomethingIsWrong { public static void main(String[] args) { Rectangle myRect; myRect.width = 40; myRect.height = 50; System.out.println("myRect's area is " + myRect.area()); } }

Answer 10

public class SomethingIsRight { public static void main(String[] args) { Rectangle myRect = new Rectangle(); myRect.width = 40; myRect.height = 50; System.out.println("myRect's area is " + myRect.area()); } }

Question 11

Given the following class, called NumberHolder, write some code that creates an instance of the class, initializes its two member variables, and then displays the value of each member variable.

public class NumberHolder { public int anInt; public float aFloat; }

Answer 11

public class NumberHolderDisplay { public static void main(String[] args) { NumberHolder aNumberHolder = new NumberHolder(); aNumberHolder.anInt = 1; aNumberHolder.aFloat = 2.3f; System.out.println(aNumberHolder.anInt); System.out.println(aNumberHolder.aFloat); } }

Question 12

The program Problem.java doesn’t compile. What do you need to do to make it compile? Why?

public class Problem {

String s;

static class Inner {

void testMethod() {

s = "Set from Inner";

}

}

}

Answer 12

Delete static in front of the declaration of the Inner class. An static inner class does not have access to the instance fields of the outer class. See ProblemSolved.java.

Question 13

Use the Java API documentation for the Box class (in the javax.swing package) to help you answer the following questions.

Question: What static nested class does Box define?

https://docs.oracle.com/javase/8/docs/api/javax/swing/Box.html

Answer 13

Box.Filler

Question 14

What inner class does Box define?
https://docs.oracle.com/javase/8/docs/api/javax/swing/Box.html

Answer 14

Box.AccessibleBox

Question 15

What is the superclass of Box's inner class?
https://docs.oracle.com/javase/8/docs/api/javax/swing/Box.html

Answer 15

[java.awt.]Container.AccessibleAWTContainer

Question 16

Which of Box’s nested classes can you use from any class?
https://docs.oracle.com/javase/8/docs/api/javax/swing/Box.html

Answer 16

Box.Filler

Question 17

How do you create an instance of Box’s Filler class?
https://docs.oracle.com/javase/8/docs/api/javax/swing/Box.html

Answer 17

new Box.Filler(minDimension, prefDimension, maxDimension)

Question 18

Get the file Class1.java. Compile and run Class1. What is the output?

public class Class1 { protected InnerClass1 ic; public Class1() { ic = new InnerClass1(); } public void displayStrings() { System.out.println(ic.getString() + "."); System.out.println(ic.getAnotherString() + "."); } static public void main(String[] args) { Class1 c1 = new Class1(); c1.displayStrings(); } protected class InnerClass1 { public String getString() { return "InnerClass1: getString invoked"; } public String getAnotherString() { return "InnerClass1: getAnotherString invoked"; } } }

Answer 18

InnerClass1: getString invoked.
InnerClass1: getAnotherString invoked.

Question 19

True or false: an Enum type can be a subclass of java.lang.String.

Answer 19

False. All enums implicitly extend java.lang.Enum. Because a class can only extend one parent, the Java language does not support multiple inheritance of state, and therefore an enum cannot extend anything else.

Question 20

Rewrite the class Card from the exercise in Questions and Exercises: Classes so that it represents the rank and suit of a card with enum types.

Answer 20

public class Card3 { private final Rank rank; private final Suit suit; public Card3(Rank rank, Suit suit) { this.rank = rank; this.suit = suit; } public Suit getSuit() { return suit; } public Rank getRank() { return rank; } public String toString() { return rank + " of " + suit; } }

public enum Suit { DIAMONDS, CLUBS, HEARTS, SPADES }

public enum Rank { DEUCE, THREE, FOUR, FIVE, SIX, SEVEN, EIGHT, NINE, TEN, JACK, QUEEN, KING, ACE }

Question 21

Rewrite the Deck class.

Answer 21

import java.util.\*; public class Deck3 { private static Card3[] cards = new Card3[52]; public Deck3() { int i = 0; for (Suit suit : Suit.values()) { for (Rank rank : Rank.values()) { cards[i++] = new Card3(rank, suit); } } } }