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Lab 12: Inheritance

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• Introduction
  • This lab will reinforce the inheritance concept

  • Tasks in this lab:

    Define a subclass called TwoHighCard of the Card Add a constructor to the TwoHighCard class Add a beats method to the TwoHighCard class to compare the strength of 2 cards

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  • The two high card game

    In the Card class that was discussed in class, the rank of the cards are: Card Rank Card Rank --------+-------- --------+-------- A 14 7 7 K 13 6 6 Q 12 5 5 J 11 4 4 10 10 3 3 9 9 2 2 8 8 We will use the Card object in this lab to make the two high card object. In the two high card game, the deuce (or 2) has the highest rank of all the cards. So, you can imagine that 2 has the rank 15: Card Rank Card Rank --------+-------- --------+-------- A 14 7 7 K 13 6 6 Q 12 5 5 J 11 4 4 10 10 3 3 9 9 2 15 8 8 BUT: you must not make changes to the Card.java program (except for the change in Task 1). The rank of the 2 card will still be equal to 2 !!! In this lab, we will learn how to accomodate such change after we inherit from the base class.

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• Preparation
  • Copy-and-paste these commands to your terminal. They copy the files and cd to the lab directory:

    mkdir ~/cs170/lab12 cd ~cs170002/share/lab12 cp *.java ~/cs170/lab12 cd ~/cs170/lab12

    You will have these files:

    Card.java - the base class TwoHighCard.java - the derived class TestTwoHigh1.java - Test program used for task 1 TestTwoHigh2.java - Test program used for task 2 TestTwoHigh3.java - Test program used for task 3

    Execute the following command to list your lab12 directory and make sure you have copied all the files:

    ls ~/cs170/lab12

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• Task 1: make the TwoHighCard class into a subclass of Card
  • This is the program TestTwoHigh1:

    public class TestTwoHigh1 { public static void main( String[] args ) { TwoHighCard a = new TwoHighCard( ); // Make a TwoHightCard object System.out.println( a ); // Print it } }

    In this program, we create a TwoHighCard object using no parameters:

    new TwoHighCard( )

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  • Compile and run the TestTwoHigh1 program:

    javac TestTwoHigh1.java java TestTwoHigh1

    You will see this message:

    TwoHighCard@2a9931f5

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  • Task 1A:

    Make the TwoHighCard class into a subclass of the class Card

    After you have done so, compile and run the TestTwoHigh1 program again:

    javac TestTwoHigh1.java java TestTwoHigh1

    You will see this message:

    Can you figured out why you see this message ? Js

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    Explanation: (there are 2 special effects built into the Java compiler that made this happen)

    The TestTwoHigh1.java program defines a TwoHighCard onject using this statement: TwoHighCard a = new TwoHighCard( ); which will invoke the following constructor method inside the TestTwoHigh1 class with no parameters: public TowHighCard( ) .... Special effect 1 in the Java compiler: Every class definition in Java must contains at least one constructor method When a class does not contains any constructor method, then the Java compiler will insert the default constructor into the class definition. Since the TwoHighCard class does not contain any constructor method, the Java compiler will insert this default constructor into the class definition: public TwoHighCard( ) { } Special effect 2 in the Java compiler: Every constructor method in Java must invoke a constructor method in the super class as its first statement: super( ..... ); // ..... represents parameters in the invocation When a constructor method in class does not contains any super(....) statement, then the Java compiler will insert the default constructor inocation into the class definition. Since the TestTwoHigh1 constructor public TwoHighCard( ) { } does not contain any super( .... ) invocation, the Java compiler will insert this default constructor invocation into the constructor: public TwoHighCard( ) { super( ); } The invocation super( ) will invoke the constructor with no parameters inside the super class

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  • Now, go look inside the super class of TwoHighCard (Question: which class is the super class of TwoHighCard ???) to find out how this message is printed.

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  • Task 1B:

    Change the message to: I have figured out why I see this message ! Js

    After you have made this change, compile and run the TestTwoHigh1 program again:

    javac TestTwoHigh1.java java TestTwoHigh1

    You should see this message:

    I have figured out why I see this message ! Js

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  • For the lead TA:

    Give students 15 mins and then reveal the answer. Move on to task 2 simultaneously

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• Task 2: define a constructor for TwoHighCard
  • This is the TestTwoHigh2 program:

    public class TestTwoHigh2 { public static void main( String[] args ) { TwoHighCard a = new TwoHighCard( 4, 1 ); System.out.println( a ); } }

    In this program, we create a TwoHighCard object using these type of parameters:

    new TwoHighCard( int, int )

    The first integer is the code for the suit and the second integer is the code for the rank

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  • Compile and run the TestTwoHigh2 program:

    javac TestTwoHigh2.java

    You will see these kinds of errors:

    TestTwoHigh2.java:11: cannot find symbol symbol : constructor TwoHighCard(int,int) location: class TwoHighCard TwoHighCard a = new TwoHighCard( 4, 1 ); ^

    This is caused by the fact that there is no constructor method defined inside TwoHighCard.java that takes 2 integer parameters

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  • Task 2:

    Define the following constructor method: public TwoHighCard( int suit, int rank ) { .... // complete it } inside TwoHighCard.java that allow you to create a TwoHighCard with the suit and rank as given in the parameters suit and rank. Hint: You need to initial the inherited instance variables from the Card.java class. Take a look inside the Card.java program and you will see that the instance variables are private: private byte cardSuit; private byte cardRank; So you cannot access them from inside the TwoHighCard.java program !!! Solution: You need to use (= invoke) a constructor of the parent class !!!

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  • After you have complete this task, you can test whether you have been successful with:

    javac TestTwoHigh2.java java TestTwoHigh2 If successful, the output is: As

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  • For the lead TA:

    Give students 15 mins and then reveal the answer. Move on to task 3 simultaneously

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• Task 3: define a method to compare 2 TwoHighCard cards
  • In this task, we will define a method beats() to compare 2 TwoHighCard objects as follows:

    TwoHighCard a = new TwoHighCard( ..., ... ); TwoHighCard b = new TwoHighCard( ..., ... ); if ( a.beats(b) ) System.out.println( a + " beats " + b ); else System.out.println( a + " does not beat " + b );

    The call a.beats(b) will return:

    true if a beats b (Remember that the deuce (2) will beat every other card (except another deuce)). false otherwise. I.e., a and b have the same rank or rank of a is lower. (Remember that the deuce (2) ranks higher than any other card (except another deuce)).

    Task 3:

    Augment the TwoHighCard object with the above beats operation

    After you have done so, compile and run the TestTwoHigh1 program again:

    javac TestTwoHigh3.java java TestTwoHigh3

    You will see this output:

    The cards: 2s rank = 2 2h rank = 2 As rank = 14 Testing beats(): 2s beats 2h: false 2s beats As: true 2h beats 2s: false 2h beats As: true As beats 2s: false As beats 2h: false

    Notice that:

    The rank of the deuce (2) is 2 But a deuce (2) will beat an Ace (in this card game) !!!

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• Turnin
  • Use the following command to turn in your lab 12:

    cd ~/cs170/lab12 /home/cs170002/turnin-lab Card.java lab12 /home/cs170002/turnin-lab TwoHighCard.java lab12a