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Create a class diagram and Java code for the following system and scenario, taking into account the possibility of future extensions.

The system is a command line utility that prints a short 'quote of the day' on the user's terminal when run. To begin with the quote is selected randomly from a set of hard-coded strings within the program itself, but that might change later on -- the quotes might be based on the user's history, the time of day, the date, etc.

Scenario:

  1. User types "java QuoteOfTheDay" on the command line.
  2. System prints out a quote of the day, with an attribution.

I have created an interface QuoteProvider and implemented it with the class HardCodedQuoteProvider. I am storing the times when a quote was previously used in a file 'history.txt'. A quote is provided only if it has not been used before.

Is there any way to make this code better? Also is there any way to ensure that this class is instantiated only once? Since otherwise the different objects will overwrite each other's history in the history file.

QuoteProvider.java

package beg_assignment4;

import java.io.IOException;
import java.util.List;

public interface QuoteProvider {
    public String getQuote() throws IOException;
    public List<String> getHistory() throws IOException;
    public void clearHistory() throws IOException;
}

HardCodedQuoteProvider.java

package beg_assignment4;

import java.io.IOException;
import java.nio.charset.Charset;
import java.nio.file.FileAlreadyExistsException;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.text.SimpleDateFormat;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Date;
import java.util.List;
import java.util.concurrent.ThreadLocalRandom;

public class HardCodedQuoteProvider implements QuoteProvider {
    // quotes to be used if the time is A.M.
    private final String  AM_QUOTES[] = {
        "Be yourself; everyone else is already taken.― Oscar Wilde",
        "A room without books is like a body without a soul. ― Marcus Tullius Cicero",
        "Be the change that you wish to see in the world. ― Mahatma Gandhi",
        "If you tell the truth, you don't have to remember anything. ― Mark Twain",
        "If you want to know what a man's like, take a good look at how he treats his inferiors, not his equals.― J.K. Rowling",
        "To live is the rarest thing in the world. Most people exist, that is all.― Oscar Wilde"
        };
    // quotes to be used if the time is P.M.
    private final String PM_QUOTES[] = {
        "Without music, life would be a mistake. ― Friedrich Nietzsche",
        "Always forgive your enemies, nothing annoys them so much. ― Oscar Wilde",
        "Life isn't about getting and having, it's about giving and being. –Kevin Kruse",
        "Whatever the mind of man can conceive and believe, it can achieve. –Napoleon Hill",
        "Strive not to be a success, but rather to be of value. –Albert Einstein"                          
        }; 

    // the time when a quote was last used is stored in the file
    // the value is 0 if the quote was previously not used
    private Path historyPath = Paths.get("history.txt");

    private long[] lastUsedAM = new long[AM_QUOTES.length];
    private long[] lastUsedPM = new long[PM_QUOTES.length];

    public HardCodedQuoteProvider() throws IOException {
        // history has to be read from the history file to intialize the
        // lastUsed arrays
        readHistory();
    }

    public String getQuote() throws IOException {
        String[] QUOTES;
        long[] lastUsed;
        if (!isPM()) {
            QUOTES = AM_QUOTES;
            lastUsed = lastUsedAM;
        } else {
            QUOTES = PM_QUOTES;
            lastUsed = lastUsedPM;
        }

        String quote;
        while (true) {
            int index =
                ThreadLocalRandom.current().nextInt(0, QUOTES.length - 1);
            if (lastUsed[index] == 0) {
                lastUsed[index] = System.currentTimeMillis();
                quote = QUOTES[index];
                break;
            }
        } 
        writeHistory();
        return new String(quote);
    }

    private void readHistory() throws IOException {
        if (Files.exists(historyPath)){
            List<String> lines =
                Files.readAllLines(historyPath, Charset.defaultCharset());
            int i = 0;
            for (String line: lines) {
                Long time = Long.parseLong(line);
                if (i < lastUsedAM.length) {
                    lastUsedAM[i] = time;
                } else {
                    lastUsedPM[i - lastUsedAM.length] = time;
                }
                i++;
            }
        }
    }

    private void writeHistory() throws IOException {
        try {
            Files.createFile(historyPath);
        } catch (FileAlreadyExistsException e) {
        }

        List<String> lines = new ArrayList<>();
        for (long time : lastUsedAM) {
            lines.add(Long.toString(time));
        }

        for (long time : lastUsedPM) {
            lines.add(Long.toString(time));
        }

        Files.write(historyPath, lines, Charset.defaultCharset());
    }

    public void clearHistory() throws IOException {
        Arrays.fill(lastUsedAM, 0);
        Arrays.fill(lastUsedPM, 0);
        writeHistory();
    }

    public List<String> getHistory() {
        List<String> history = new ArrayList<>();
        history.addAll(getHistoryFromArrays(AM_QUOTES, lastUsedAM));
        history.addAll(getHistoryFromArrays(PM_QUOTES, lastUsedPM));
        return history;
    }

    private List<String> getHistoryFromArrays(String[] QUOTES,
            long[] lastUsed) {
        List<String> history = new ArrayList<>();
        for (int i = 0; i < QUOTES.length; i++) {
            if (lastUsed[i] != 0) {
                String timestamp =
                    new SimpleDateFormat("yyyy.MM.dd.HH.mm.ss")
                    .format(new Date(lastUsed[i]));
                history.add(QUOTES[i] + " " + timestamp);
            }
        }
        return history;
    }

    private boolean isPM() {
        String currentTimeStamp = getCurrentTimeStamp();
        return currentTimeStamp.substring(currentTimeStamp.length() - 2)
            .equals("PM");
    }

    private String getCurrentTimeStamp() {
        return new SimpleDateFormat("h:mm a").format(new Date());
    }
}

QuoteOfTheDay.java

package beg_assignment4;

import java.io.IOException;
import java.util.List;

public class QuoteOfTheDay {
    public static void main(String[] args) throws IOException {
        QuoteProvider qp = new HardCodedQuoteProvider();
        if (args.length == 0) {
            System.out.println(qp.getQuote());
        } else if (args[0].equals("show-history")) {
            List<String> list = qp.getHistory();
            for (String string: list) {
                System.out.println(string);
            }
        } else if (args[0].equals("clear-history")) {
            qp.clearHistory();
        } else {
            System.out.println("Invalid options.");
        }
    }
}
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  • 1
    \$\begingroup\$ To know how to create a class that instantiates only ones, read up on Singleton design pattern. \$\endgroup\$ – PM 77-1 Mar 26 '16 at 16:34
  • 1
    \$\begingroup\$ Instead of aiming to ensure that the class is only instantiated once, why not try to add support for having multiple histories? \$\endgroup\$ – Simon Forsberg Mar 30 '16 at 5:35
4
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Single responsibility principle

HardCodedQuoteProvider has too many responsibilities:

  • Provide quotes
  • Provide quotes depending on the time of the day
  • Manage history

Composition can be very useful here. For example, instead of HardCodedQuoteProvider managing the quotes for AM and PM, you could have:

  • A simple quote provider with no notion of AM and PM
  • A composite quote provider that has two simple quote providers, one for AM and one for PM, and delegate to them appropriately depending on the time of day. Note that this composite quote provider wouldn't manage the history directly, the contained simple quote providers would do that

And for managing the history, it would be good to create an interface, so that you can have different implementations, for example file-based, in-memory, database-backed.

Your question

Also is there any way to ensure that this class is instantiated only once? Since otherwise the different objects will overwrite each other's history in the history file.

You can use the Singleton pattern. (See for example Item 3 in Effective Java by Joshua Bloch.)

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