Java constructor with declaration in another class

[Alexander Visser]

I have read a lot of articles about Java constructors with declaration in the same class. Now I got a little confused how to make copy constructors when attributes are declared in another class?

public class Book {
  private String title = null;
  
  // Constructor
  public Book(String title) {
    this.title = title;
  }
}

public class BookCopy {
  private Book book = null;

  // Constructor for making a new copy
  public BookCopy(Book book) {
    this.book = book;
    System.out.println("New BookCopy with title: " + book.getTitle());
  }

  // Constructor for making a copy of a BookCopy
  public BookCopy(BookCopy bookCopy) {
    this.book = bookCopy.book; // Question: this rule doesn't work for making a copy of a BookCopy, how to do?
    System.out.println("Copy of BookCopy with title: " + bookcopy.getTitle());
  }

}

[Piet Souris]

What are you trying to copy? A book or a BookCopy?

[Alexander Visser]

I am trying to make a copy of a BookCopy.
I tried "return new BookCopy()" too, but doesn't work.

[Piet Souris]

Can you show the code that you used to test the BookCopy class?

[Alexander Visser]

Book b1 = new Book("Java Development");
BookCopy b2 = new BookCopy(b1); // Refers to b1 (alias)
BookCopy b3 = new BookCopy(b2); // Must make a copy of b2 (no alias, new object)

[Piet Souris]

Well, that looks okay. Did you get any errors? And do you know why 'return new BookCopy()' doesn't work?

[Alexander Visser]

Didn't get any errors. But b2 and b3 both refer to the same object, but they must be different objects. For example: I have a book, named "Java Development" in the library and the library has 10 BookCopies of "Java Development". So I need to change public BookCopy(BookCopy bookcopy), so it will make a new object without referring to the same object.

[Campbell Ritchie]

I don't quote understand what you are trying to do, but I don't think that is how you make a copy constructor. The whole idea is to make two different objects with the same state, but whose state may now change independently of each other.
Don't assign fields to null at declaration time, and make the class final.

public final class Book {
  private String title;
   
  // Constructor
  public Book(String title) {
    this.title = title;
  }
  /* getXXX(), hashCode(), equals(), toString() and other methods.
   * No mutators */
}

Declare that field final, and you now have an immutable class with all its advantages, including the fact that you never need to take copies of it.

public final class Book {
  private final String title;
   
  // Constructor
  public Book(String title) {
    this.title = title;
  }
  /* getXXX(), hashCode(), equals(), toString() and other methods.
   * No mutators */
}

Now, let's imagine your book class is no longer immutable. Let's add a field which will have to change from time to time.

public final class Book {
  private final String title;
  private BigDecimal price;
   
  // Constructor
  public Book(String title, BigDecimal price) {
    this.title = title;
    this.price = price;
  }

  /**
   * ... for price reductions use negative argument ...
   */
  public void increasePrice(BigDecimal priceChange) {
     price = price.add(priceChange);
  }
  /* getXXX(), hashCode(), equals(), toString() and other methods.
   * No other mutators */
}

Now it is worth creating a copy constructor so you can have an object preserving the old price:-[

public final class Book {
  private final String title;
  private BigDecimal price;
   
  // Constructor
  public Book(String title, BigDecimal price) {
    this.title = title;
    this.price = price;
  }

  public Book(Book b) {
    price = b.price;
    title = b.title;
  }

  /**
   * ... for price reductions use negative argument ...
   */
  public void increasePrice(BigDecimal priceChange) {
     price = price.add(priceChange);
  }
  /* getXXX(), hashCode(), equals(), toString() and other methods.
   * No other mutators */
}

[Alexander Visser]

Thanks for additional suggestions.
Hope I can make it more clear.
With class Book I can create a new book, for example "Java Development".
Within class BookCopy there must be two constructors.
First constructor is to create a new BookCopy with a book as parameter.
Second constructor is to create a copy of a BookCopy with a bookcopy as parameter.
If it is not clear, please let me know how I can make it more clear.

[Carey Brown]

Both your Book and BookCopy classes need copy constructors. The BookCopy copy constructor needs to invoke the Book copy constructor.

[Alexander Visser]

See code below I've tried, but b2 and b3 still refer to the same object. What did I do wrong?

public class Book {
	private String title;
	
	// Constructor
	public Book(String title) {
		this.title = title;
		System.out.println("Book added: " + title);
	}
	
	// Copy constructor
	public Book(Book book) {
		this.title = book.title;
	}
	
	// Getter
	public String getTitle() {
		return title;
	}
}

public class BookCopy {
	private Book book;
	
	// Constructor for making a new BookCopy
	public BookCopy(Book book) {
		this.book = book;
		System.out.println("New BookCopy with title: " + book.getTitle());
	}
	
	// Constructor for making a copy of a BookCopy
	public BookCopy(BookCopy bookcopy) {
		this.book = bookcopy.book; // Question: this rule doesn't work for making a copy of a BookCopy, how to do?
		System.out.println("Copy of BookCopy with title: " + bookcopy.getTitle());
	}

	// Getter
	private String getTitle() {
		return book.getTitle();
	}
}

[Carey Brown]

    // Constructor for making a copy of a BookCopy
    public BookCopy(BookCopy bookcopy) {
        this.book = new Book( bookcopy.book ); // TRY THIS
        System.out.println("Copy of BookCopy with title: " + bookcopy.getTitle());
    }
 

[Carey Brown]

Out of curiosity why do you need a BookCopy class? Doesn't the copy constructor in Book give you a Book copy? Now you've got two means by which a Book can be copied: directly via Book copy constructor, and indirectly via BookCopy copy constructor. It would seem that Book copy constructor should be all you'd need. Like this:

public void someMethod( Book thatBook ) {
    Book copy = new Book( thatBook );
    // do something with copy
}