UML 2.0 Sequence Diagram depth

Question

I am trying to do my mid-term assignment which is creating a sequence diagram for a simple java program. However I encountered a problem where I could not decide which element to put in a diagram and which element to omit from. I am posting a JAVA code if it can help answering the question.

public class TestPOS {
public static void main(String[] args) {
    // POST 객체를 준비
    Store store = new Store();
    ProductCatalog catalog = new ProductCatalog();
    catalog.addSpec(1, new ProductSpec(1, "pencil", 1000));
    catalog.addSpec(2, new ProductSpec(2, "eraser", 500));
    catalog.addSpec(3, new ProductSpec(3, "fountain pen", 50000));
    POST post = new POST(store, catalog);

    // 첫 번째 판매
    post.enterItem(1, 12);
    post.enterItem(2, 4);
    post.enterItem(3, 1);

    post.makePayment();

    post.endSale();

    // 두 번째 판매
    post.enterItem(1, 2);
    post.enterItem(2, 1);

    post.makePayment();

    post.endSale();

    // 출력을 보여주어 이해를 돕기위한 코드
    for (Sale sale : store.completedSales) {
        System.out.println(sale.getDate());
        sale.printLineItems();
        System.out.println("total = " + sale.getTotal());
    }
  }
}

This is the main where it calls enterItem(), makePayment(), and endSale(). The assignment is to create sequence diagrams for three functions above. I will post each classes below.

-----------------------------------------------------------
import java.util.Date;
public class POST {
    private Store store;
    private ProductCatalog catalog;
    private Sale sale = null;

    public POST(Store store, ProductCatalog catalog) {
        this.store = store;
        this.catalog = catalog;
    }

    public void enterItem(int upc, int qty) {
        if (sale == null) {
            Date date = new Date(System.currentTimeMillis());
            sale = new Sale(date);
        }
        ProductSpec s = catalog.spec(upc);
        sale.makeLineItem(s, qty);
    }

    public void makePayment() {
        if (sale != null) sale.makePayment();
    }

    public void endSale() {
        store.addCompleteSale(sale);
        sale = null;
    }
}
-----------------------------------------------------------
import java.util.ArrayList;
public class Store {
    protected ArrayList<Sale> completedSales = null;

    public Store() {
        completedSales = new ArrayList<Sale>();
    }

    public void addCompleteSale(Sale sale) {
        completedSales.add(sale);
    }
}
-----------------------------------------------------------
import java.util.ArrayList;
import java.util.Date;
public class Sale {
    private Date date;
    private ArrayList<SalesLineItem> lineItem = null;
    private Payment payment = null;

    public Sale(Date date)
    {
        this.date = date;
        lineItem = new ArrayList<SalesLineItem>();
    }

    public void makeLineItem(ProductSpec s, int qty) {
        SalesLineItem item = new SalesLineItem(s, qty);
        lineItem.add(item);
    }

    public int getTotal() {
        int total = 0;  
        for (SalesLineItem item : lineItem) {
            total += item.getSubTotal();
        }
        return total;
    }

    public void makePayment() {
        int total = this.getTotal();
        payment = new Payment(total);   
    }

    // 출력을 보여주어 이해를 돕기위한 메소드, 클래스 다이어그램에 반영하지 않음
    public Date getDate() {
        return date;
    }

    // 출력을 보여주어 이해를 돕기위한 메소드, 클래스 다이어그램에 반영하지 않음
    public void printLineItems() {
        for (SalesLineItem item : lineItem) {
            System.out.println("upc : " + item.getItemUpc() +", name : " + item.getItemName() + ", price : "
                    + item.getItemPrice() + ", quantity : " + item.getQuantity());
        }
    }
}
-----------------------------------------------------------
public class Payment {
    private int amount;

    public Payment(int amount) {
        this.amount = amount;
    }

    // 출력을 보여주어 이해를 돕기위한 메소드, 클래스 다이어그램에 반영하지 않음
    public int getAmount() {
        return amount;
    }
}
-----------------------------------------------------------
public class SalesLineItem  {
    private int quantity;
    private ProductSpec spec;

    public SalesLineItem(ProductSpec spec, int quantity) {
        this.spec = spec;
        this.quantity = quantity;
    }

    public int getSubTotal() {
        int price = spec.getPrice();
        return price * quantity;
    }

    // 출력을 보여주어 이해를 돕기위한 메소드, 클래스 다이어그램에 반영하지 않음
    public int getItemUpc() {
        return spec.getUpc();
    }

    // 출력을 보여주어 이해를 돕기위한 메소드, 클래스 다이어그램에 반영하지 않음
    public String getItemName() {
        return spec.getName();
    }

    // 출력을 보여주어 이해를 돕기위한 메소드, 클래스 다이어그램에 반영하지 않음
    public int getItemPrice() {
        return spec.getPrice();
    }

    // 출력을 보여주어 이해를 돕기위한 메소드, 클래스 다이어그램에 반영하지 않음
    public int getQuantity() {
        return quantity;
    }
}
-----------------------------------------------------------
import java.util.HashMap;
public  class ProductCatalog  {
    private HashMap<Integer, ProductSpec> specTable = new HashMap<Integer, ProductSpec>();

    public void addSpec(int upc, ProductSpec spec) {
        specTable.put(upc, spec);
    }

    public ProductSpec spec(int upc) {
        return specTable.get(upc);
    }
}
-----------------------------------------------------------
public class ProductSpec  {
    private int upc;
    private String name;
    private int price;

    public ProductSpec(int upc, String name, int price) {
        this.upc = upc;
        this.name = name;
        this.price = price;
    }

    public int getPrice() {
        return price;
    }

    // 출력을 보여주어 이해를 돕기위한 메소드, 클래스 다이어그램에 반영하지 않음
    public int getUpc() {
        return upc;
    }

    // 출력을 보여주어 이해를 돕기위한 메소드, 클래스 다이어그램에 반영하지 않음
    public String getName() {
        return name;
    }
}

Never mind Korean in the code they are just for some description. Now a picture below is the diagram I drew wheter it is correct or not.

The major problem is that I don't have a clue whether I should draw every elements, interactions, and instances that are being used on the way to termination. I know i sound a bit unorganized but please someone help me with my problem.

Answer 1

I did not cross check with you code whether the drawing is correct. Just a note that the create ProductSpec returns directly to POST. That might be a legal abbreviation in your context, but it's likely not reflecting the code.

Anyhow, what you put in a SD should clarify a certain context and not obscure it. So it's never a good idea to put everything in a single SD. Rather create a rough overview to highlight the main flow. In a POS system this would probably two things in first place: Initialization of the whole stuff and entry of an article in the catalog. Means you would create two SDs here. You also should not be tempted to "program graphically" and use fragments en masse. Leave them for important path decisions and the like, not for each if or case you found in the code. In terms of SDs it's often better to use less than more.

Note: I don't know what your teacher intended here. It might contradict with my statement and he just wants you to create a (useless) wall paper.