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MusicDownloadsTester.java

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


public class MusicDownloadsTester {

	public static void main(String[] args) {
		MusicDownloads webMusicA = new MusicDownloads();
		webMusicA.add(new DownloadInfo("Hey Jude",5));
		webMusicA.add(new DownloadInfo("Soul Sister",3));
		webMusicA.add(new DownloadInfo("Aqualung",10));
		System.out.println("Part A tests:");
		System.out.println("webMusicA has:\n"+webMusicA);

		System.out.println( getDownloadInfoTest(webMusicA, "Aqualung") +" [expected found at 2]");
		System.out.println( getDownloadInfoTest(webMusicA, "Happy Birthday") +" [expected null]");

		String found = getDownloadInfoTest(webMusicA, "Aqualung");
		found+=getDownloadInfoTest(webMusicA, "Happy Birthday");
		String expected="'Aqualung' found at 2'Happy Birthday' returned a null";
		if (found.equals(expected))
			System.out.print("No ");
		System.out.println("problems found in part (a)\n");

		System.out.println("Part B tests:");
		MusicDownloads webMusicB = new MusicDownloads();
		webMusicB.add(new DownloadInfo("Hey Jude",5));
		webMusicB.add(new DownloadInfo("Soul Sister",3));
		webMusicB.add(new DownloadInfo("Aqualung",10));
		System.out.println("Before the call, webMusicB has:\n"+webMusicB);

		List<String> songTitles = new ArrayList<String>();
		songTitles.add("Lights");
		songTitles.add("Aqualung");
		songTitles.add("Soul Sister");
		songTitles.add("Go Now");
		songTitles.add("Lights");
		songTitles.add("Soul Sister");	

		webMusicB.updateDownloads(songTitles);
		System.out.println("After the call, webMusicB has:\n"+webMusicB);
		found = webMusicB.toString();
		expected = "0) 'Hey Jude' 5 downloads\n1) 'Soul Sister' 5 downloads\n2) 'Aqualung' 11 downloads\n3) 'Lights' 2 downloads\n4) 'Go Now' 1 downloads\n";
		if (found.equals(expected))
			System.out.print("No ");
		System.out.println("problems found in part (b)");

	}
	/**
	 * this tester method has spoilers,
	 * so don't look too carefully if you're still 
	 * working on the part (a) solution
	 */
	public static String getDownloadInfoTest(MusicDownloads md, String title){
		DownloadInfo di = md.getDownloadInfo(title);
		if (di == null)
			return "'"+title+"'"+" returned a null";
		List<DownloadInfo> list = md.getList();
		int i=0;
		boolean found=false;
		while (i<list.size() && !found){
			String s = list.get(i).getTitle();
			if (s.equals(title))
				found=true;
			else
				i++;
		}
		if (!found)
			return "'"+title+"'"+" not found but NOT null";
		return "'"+title+"'"+" found at "+i;
	}
}

DownloadInfo.java

public class DownloadInfo
{
	private String title;
	private int count;
	/** Creates a new instance with the given unique title and sets the
	 * number of times downloaded to 1.
	 * @param title the unique title of the downloaded song
	 */
	public DownloadInfo(String title)
	{
		//this implementation was not shown
		this.title=title;
		count=1;
	}
	//this implementation was not shown
	public DownloadInfo(String title, int count) {
		this(title);
		this.count=count;
	}
	/** @return the title */
	public String getTitle()
	{ 
		return title;
	}
	/** Increment the number times downloaded by 1 
	 * 
	 */ public void incrementTimesDownloaded() 
	 { 
		 count++;
	 }
	// There may be instance variables, constructors, and methods that are not shown. 
	public String toString(){
		return "'"+getTitle()+"' "+count+" downloads";
	}
}

MusicDownloads.java

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


public class MusicDownloads 
{
	/** The list of downloaded information.
	* Guaranteed not to be null and not to contain duplicate titles. 
	*/
	private List<DownloadInfo> downloadList;

	/** Creates the list of downloaded information. */
	public MusicDownloads()
	{
		downloadList = new ArrayList<DownloadInfo>();
	}
	/** Returns a reference to the DownloadInfo object with the requested title if it exists.
	 * @param title the requested title
	 * @return a reference to the DownloadInfo object with the
	 * title that matches the parameter title if it exists in the list;
	 * null otherwise.
	 * 
	 * Postcondition 
	 *  - no changes were made to downloadList
	 */
	public DownloadInfo getDownloadInfo(String title)
	{
		// part (a)
                 return null; // replace this line with your code

	}

	/** Updates downloadList with information from titles.
	 *  @param titles a list of song titles
	 *  Postcondition:
	 *  - there are no duplicate titles in downloadList.
	 *  - no entries were removed from downloadList.
	 *  - all songs in titles are represented in downloadList. 
	 *  - for each existing entry in downloadList, the download count is increased by
	 *    the number of times its title appeared in titles.
	 *  - the order of the existing entries in downloadList is not changed.
	 *  - the first time an object with a title from titles is added to downloadList, it
	 *    is added to the end of the list.
	 *  - new entries in downloadList appear in the same order
	 *    in which they first appear in titles.
	 *  - for each new entry in downloadList, the download count is equal to 
	 *    the number of times its title appeared in titles
	 */
	public void updateDownloads(List<String> titles)
	{
		// part (b)

	}
	// other methods to get the examples to work
	public void add(DownloadInfo d){
		downloadList.add(d);
	}
    public List<DownloadInfo> getList(){
    		return downloadList;
    }
    public String toString(){
    		String result ="";
    		for (int i=0; i< downloadList.size(); i++)
    			result+= i+") "+downloadList.get(i).toString()+"\n";
    		return result;
    }
}