WordList

AP Computer Science A
2004 Free-Response Question

Directions: SHOW ALL YOUR WORK, REMEMBER THAT PROGRAM SEGMENTS ARE TO BE WRITTEN IN Java.

Notes:

• Assume that the classes listed in the Quick Reference found in the Appendix have been imported where appropriate.
• Unless otherwise noted in the question, assume that parameters in method calls are not null and that methods are called only when their preconditions are satisfied.
• In writing solutions for each question, you may use any of the accessible methods that are listed in classes defined in that question. Writing significant amounts of code that can be replaced by a call to one of these methods may not receive full credit.

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1. The following class WordList is designed to store and manipulate a list of words. The incomplete class declaration is shown below. You will be asked to implement two methods.

public class WordList
{
	private ArrayList myList;  // contains Strings made up of letters

	// postcondition: returns the number of words in the WordList that
	//                are exactly len letters long
	public int numWordsOfLength( int len )
	{  /* to be implemented in part (a) */ 	}

	// postcondition: all words that are exactly len letters long
	//                have been removed from this WordList, with the
	//                order of the remaining words unchanged
	public void removeWordsOfLength( int len )
	{  /* to be implemented in part (b) */	}


	// ... constructor and other methods not shown
}

(a) Write the WordList method numWordsOfLength. Method numWordsOfLength returns the number of words in the WordList that are exactly len letters long. For example, assume that the instance variable myList of the WordList animals contains the following.

["cat", "mouse", "frog", "dog", "dog"]

The table below shows several sample calls to numWordsOfLength.

Call	Result returned by call
animals.numWordsOfLength(4)	1
animals.numWordsOfLength(3)	3
animals.numWordsOfLength(2)	0

Complete method numWordsOfLength below.

// postcondition: returns the number of words in this WordList that
//                        are exactly len letters long
public int numWordsOfLength(int len)

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(b) Write the WordList method removeWordsOfLength. Method removeWordsOfLength removes all words from the WordList that are exactly len letters long, leaving the order of the remaining words unchanged. For example, assume that the instance variable myList of the WordList animals contains the following.

["cat", "mouse", "frog", "dog", "dog"]

The table below shows a sequence of calls to the removeWordsOfLength method.

Call	myList after the call
animals.removeWordsOfLength(4);	["cat", "mouse", "dog", "dog"]
animals.removeWordsOfLength(3);	["mouse"]
animals.removeWordsOfLength(2);	["mouse"]

Complete method removeWordsOfLength below.

// postcondition: all words that are exactly len letters long
//                        have been removed from this WordList, with the
//                        order of the remaining words unchanged
public void removeWordsOfLength(int len)