Code Tip - 6/21/2010

If you want to create an immutable object but want to make sure that you don't have to create a different constructor for every combination of parameters, a consistent state, and readability, use the Builder pattern.

For an example, in my Stock application, I will have a Stock object that will not change throughout algorithm run.  It will always have the same name, description, dates, current price, and symbol.  The Stock object will always have a symbol but may not have some or all of the other parameters (or some combination of).  Here's the builder pattern implementation to create this object:

public class Stock
{
  private final int id;
  private final String stockName;
  private final String description;
  private final String symbol;
  private final float price;
  private final Date date;


  public static class StockBuilder
  {
    private final int id;
    private final String symbol;

    private String stockName;

    private String description;

    private float price;

    private Date date;

    public StockBuilder(int id, String symbol)
    {
      this.id = id;
      this.symbol = symbol;

    }

    public StockBuilder StockName(String name)
    {
      this.stockName = name;

      return this;

    }

    public StockBuilder Description(String description)

    {

      this.description= description;

      return this;

    }

    public StockBuilder Price(float price)

    {

      this.price= price;

      return this;

    }

    public StockBuilder StockName(String name)

    {

      this.stockName = name;

      return this;

    }

    public StockBuilder StockDate(Date date)

    {

      this.date= date;

      return this;

    }

    public StockBuilder Build()

    {

      return new Stock(this);

    }

  }


  private Stock(StockBuilder builder)
  {

    this.id = builder.id;

    this.symbol = builder.symbol;

    this.stockName = builder.stockName;

    this.description = builder.description;

    this.price = builder.price;

    this.date = builder.date;

  }


And now to initialize the Stock object:


Stock stock = new Stock.StockBuilder(5,"GOOG").StockName("Google").Price(500).Build();


Stock nextStock = new Stock.StockBuilder(5,"MSFT").Price(70.5).Description("Microsoft corporation description").Build();