15.2. RandomStringChooser - Part A¶

The following is a free response question from 2016. It was question 1 on the exam. You can see all the free response questions from past exams at https://apstudents.collegeboard.org/courses/ap-computer-science-a/free-response-questions-by-year.

Question 1. This question involves the implementation and extension of a RandomStringChooser class.

Part a. A RandomStringChooser object is constructed from an array of non-null String values. When the object is first constructed, all of the strings are considered available. The RandomStringChooser class has a getNext method, which has the following behavior. A call to getNext returns a randomly chosen string from the available strings in the object. Once a particular string has been returned from a call to getNext, it is no longer available to be returned from subsequent calls to getNext. If no strings are available to be returned, getNext returns "NONE.

The following code segment shows an example of the behavior of RandomStringChooser.

String[] wordArray = {"wheels", "on", "the", "bus"};
RandomStringChooser sChooser = new RandomStringChooser(wordArray);
for (int k = 0; k < 6; k++)
{
   System.out.println(sChooser.getNext() + " ");
}

One possible output is shown below. Because sChooser has only four strings, the string "NONE is printed twice.

bus the wheels on NONE NONE

15.2.1. Try and Solve It - Again¶

Now that you have had practice with mixed up code, write the entire RandomStringChooser class again. Your implementation must include an appropriate constructor and any necessary methods. Any instance variables must be private. The code segment in the example above should have the indicated behavior (that is, it must compile and produce a result like the possible output shown. Neither the constructor or any of the methods should alter the parameter passed to the constructor, but your implementation may copy the contents of the array.

The code below has comments to help you get started. It also has a main method for testing. Finish writing the class and use the main method to test it.

import java.util.List;
import java.util.ArrayList;

// Declare a public RandomStringChooser class

{

/** Declare any fields (instance variables) **/

/** Declare any constructors */

/** Write the getNext method */

/** This is a main method for testing the class */
    public static void main(String[] args)
    {
        System.out.println("It should print the words in the array in a random order and then NONE twice");
        String[] wordArray = {"wheels", "on", "the", "bus"};
        RandomStringChooser sChooser = new RandomStringChooser(wordArray);
        for (int k = 0; k < 6; k++)
        {
           System.out.println(sChooser.getNext() + " ");
        }

} // end of main

} // end of class
====
import static org.junit.Assert.*;

import org.junit.*;

import java.io.*;

public class RunestoneTests extends CodeTestHelper
{
    public RunestoneTests()
    {
        super("RandomStringChooser");
        // CodeTestHelper.sort = true;
    }

@Test
    public void testMain1()
    {
        boolean passed = false;

String expect =
                "It should print the words in the array in a random order and then NONE twice\n"
                    + "on \n"
                    + "bus \n"
                    + "wheels \n"
                    + "the \n"
                    + "NONE \n"
                    + "NONE";

String output1 = getMethodOutput("main");

expect = expect.substring(expect.indexOf("\n") + 1);
        output1 = output1.substring(output1.indexOf("\n") + 1);

int num1 = countOccurences(output1, "wheels");
        int num2 = countOccurences(output1, "on");
        int num3 = countOccurences(output1, "the");
        int num4 = countOccurences(output1, "bus");
        int num5 = countOccurences(output1, "NONE");

passed = num1 == 1 && num2 == 1 && num3 == 1 && num4 == 1 && num5 == 2;

getResults(
                expect,
                output1,
                "Checking that each word is in output correct number of times",
                passed);
        assertTrue(passed);
    }

@Test
    public void testMain2()
    {
        boolean passed = false;

String output1 = getMethodOutput("main");
        String output2 = getMethodOutput("main");
        String output3 = getMethodOutput("main");

passed = !output1.equals(output2) || !output2.equals(output3) || !output1.equals(output3);

getResults(
                "Different results each time",
                "Same results each time",
                "Checking for random order",
                passed);
        assertTrue(passed);
    }
}

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