Best way to handle multiple constructors in Java

Solution 1:

A slightly simplified answer:

public class Book
{
    private final String title;

    public Book(String title)
    {
      this.title = title;
    }

    public Book()
    {
      this("Default Title");
    }

    ...
}

Solution 2:

Consider using the Builder pattern. It allows for you to set default values on your parameters and initialize in a clear and concise way. For example:

    Book b = new Book.Builder("Catcher in the Rye").Isbn("12345")
       .Weight("5 pounds").build();

Edit: It also removes the need for multiple constructors with different signatures and is way more readable.

Solution 3:

You need to specify what are the class invariants, i.e. properties which will always be true for an instance of the class (for example, the title of a book will never be null, or the size of a dog will always be > 0).

These invariants should be established during construction, and be preserved along the lifetime of the object, which means that methods shall not break the invariants. The constructors can set these invariants either by having compulsory arguments, or by setting default values:

class Book {
    private String title; // not nullable
    private String isbn;  // nullable

    // Here we provide a default value, but we could also skip the 
    // parameterless constructor entirely, to force users of the class to
    // provide a title
    public Book()
    {
        this("Untitled"); 
    }

    public Book(String title) throws IllegalArgumentException
    {
        if (title == null) 
            throw new IllegalArgumentException("Book title can't be null");
        this.title = title;
        // leave isbn without value
    }
    // Constructor with title and isbn
}

However, the choice of these invariants highly depends on the class you're writing, how you'll use it, etc., so there's no definitive answer to your question.

Solution 4:

You should always construct a valid and legitimate object; and if you can't using constructor parms, you should use a builder object to create one, only releasing the object from the builder when the object is complete.

On the question of constructor use: I always try to have one base constructor that all others defer to, chaining through with "omitted" parameters to the next logical constructor and ending at the base constructor. So:

class SomeClass
{
SomeClass() {
    this("DefaultA");
    }

SomeClass(String a) {
    this(a,"DefaultB");
    }

SomeClass(String a, String b) {
    myA=a;
    myB=b;
    }
...
}

If this is not possible, then I try to have an private init() method that all constructors defer to.

And keep the number of constructors and parameters small - a max of 5 of each as a guideline.