Best way to handle multiple constructors in Java

Question

I ve been wondering what the best  i e  cleanest safest most efficient  way of handling multiple constructors in Java is  Especially when in one or more constructors not all fields are specified   public class Book        private String title      private String isbn       public Book                 nothing specified             public Book String title                only title                           What should I do when fields are not specified  I ve so far been using default values in the class so that a field is never null  but is that a  good  way of doing things

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A slightly simplified answer   public class Book       private final String title       public Book String title              this title   title             public Book               this  Default Title

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Some general constructor tips    Try to focus all initialization in a single constructor and call it from the other constructors   This works well if multiple constructors exist to simulate default parameters  Never call a non-final method from a constructor   Private methods are final by definition Polymorphism can kill you here  you can end up calling a subclass implementation before the subclass has been initialized If you need  helper  methods  be sure to make them private or final  Be explicit in your calls to super     You would be surprised at how many Java programmers don t realize that super   is called even if you don t explicitly write it  assuming you don t have a call to this         Know the order of initialization rules for constructors  It s basically    this      if present  just move to another constructor  call super       if not explicit  call super   implicitly   construct superclass using these rules recursively  initialize fields via their declarations run body of current constructor return to previous constructors  if you had encountered this      calls     The overall flow ends up being    move all the way up the superclass hierarchy to Object while not done   init fields run constructor bodies drop down to subclass    For a nice example of evil  try figuring out what the following will print  then run it  package com javadude sample       THIS IS REALLY EVIL CODE  BEWARE       class A       private int x   10      public A             init              protected void init             x   20            public int getX             return x           class B extends A       private int y   42      protected void init             y   getX              public int getY             return y           public class Test       public static void main String   args            B b   new B            System out println  x     b getX             System out println  y     b getY               I ll add comments describing why the above works as it does    Some of it may be obvious  some is not

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It might be worth considering the use of a static factory method instead of constructor   I m saying instead  but obviously you can t replace the constructor  What you can do  though  is hide the constructor behind a static factory method  This way  we publish the static factory method as a part of the class API but at the same time we hide the constructor making it private or package private   It s a reasonably simple solution  especially in comparison with the Builder pattern  as seen in Joshua Bloch s Effective Java 2nd Edition     beware  Gang of Four s Design Patterns define a completely different design pattern with the same name  so that might be slightly confusing  that implies creating a nested class  a builder object  etc   This approach adds an extra layer of abstraction between you and your client  strengthening encapsulation and making changes down the road easier  It also gives you instance-control     since the objects are instantiated inside the class  you and not the client decide when and how these objects are created   Finally  it makes testing easier     providing a dumb constructor  that just assigns the values to the fields  without performing any logic or validation  it allows you to introduce invalid state into your system to test how it behaves and reacts to that  You won t be able to do that if you re validating data in the constructor   You can read much more about that in  already mentioned  Joshua Bloch s Effective Java 2nd Edition     it s an important tool in all developer s toolboxes and no wonder it s the subject of the 1st chapter of the book   -   Following your example   public class Book        private static final String DEFAULT TITLE    The Importance of Being Ernest        private final String title      private final String isbn       private Book String title  String isbn            this title   title          this isbn   isbn             public static Book createBook String title  String isbn            return new Book title  isbn              public static Book createBookWithDefaultTitle String isbn            return new Book DEFAULT TITLE  isbn                       Whichever way you choose  it s a good practice to have one main constructor  that just blindly assigns all the values  even if it s just used by another constructors

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Another consideration  if a field is required or has a limited range  perform the check in the constructor   public Book String title        if  title  null          throw new IllegalArgumentException  title can t be null        this title   title

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I would do the following    public class Book       private final String title      private final String isbn       public Book final String t  final String i                if t    null                        throw new IllegalArgumentException  t cannot be null                       if i    null                        throw new IllegalArgumentException  i cannot be null                       title   t          isbn    i            I am making the assumption here that   1  the title will never change  hence title is final  2  the isbn will never change  hence isbn is final  3  that it is not valid to have a book without both a title and an isbn    Consider a Student class    public class Student       private final StudentID id      private String firstName      private String lastName       public Student final StudentID i                     final String    first                     final String    last                if i    null                        throw new IllegalArgumentException  i cannot be null                        if first    null                        throw new IllegalArgumentException  first cannot be null                        if last    null                        throw new IllegalArgumentException  last cannot be null                        id          i          firstName   first          lastName    last            There a Student must be created with an id  a first name  and a last name   The student ID can never change  but a persons last and first name can change  get married  changes name due to losing a bet  etc       When deciding what constrructors to have you really need to think about what makes sense to have   All to often people add set get methods because they are taught to - but very often it is a bad idea    Immutable classes are much better to have  that is classes with final variables  over mutable ones   This book  http   books google com books id ZZOiqZQIbRMC amp pg PA97 amp sig JgnunNhNb8MYDcx60Kq4IyHUC58 PPP1 M1  Effective Java  has a good discussion on immutability   Look at items 12 and 13

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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

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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

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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

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Several people have recommended adding a null check   Sometimes that s the right thing to do  but not always   Check out this excellent article showing why you d skip it   http   misko hevery com 2009 02 09 to-assert-or-not-to-assert

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