When should null values of Boolean be used

Question

Java boolean allows values of true and false while Boolean allows true  false  and null  I have started to convert my booleans to Booleans  This can cause crashes in tests such as  Boolean set   null      if  set        while the test  if  set    null  amp  amp  set        seems contrived and error-prone   When  if ever  is it useful to use Booleans with null values  If never  then what are the main advantages of the wrapped object   UPDATE  There has been such a lot of valuable answers that I have summarised some of it in my own answer  I am at best an intermediate in Java so I have tried to show the things that I find useful  Note that the question is  incorrectly phrased   Boolean cannot  have a null value   but I have left it in case others have the same misconception

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ANSWER TO OWN QUESTION  I thought it would be useful to answer my own question as I have learnt a lot from the answers  This answer is intended to help those - like me - who do not have a complete understanding of the issues  If I use incorrect language please correct me    The null  value  is not a value and is fundamentally different from true and false  It is the absence of a pointer to objects  Therefore to think that Boolean is 3-valued is fundamentally wrong The syntax for Boolean is abbreviated and conceals the fact that the reference points to Objects   Boolean a   true    conceals the fact that true is an object  Other equivalent assignments might be   Boolean a   Boolean TRUE    or  Boolean a   new Boolean true      The abbreviated syntax   if  a        is different from most other assignments and conceals the fact that a might be an object reference or a primitive  If an object it is necessary to test for null to avoid NPE  For me it is psychologically easier to remember this if there is an equality test   if  a    true       where we might be prompted to test for null  So the shortened form is only safe when a is a primitive   For myself I now have the recommendations    Never use null for a 3-valued logic  Only use true and false  NEVER return Boolean from a method as it could be null  Only return boolean  Only use Boolean for wrapping elements in containers  or arguments to methods where objects are required

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Main purpose for Boolean is null value  Null value says  that property is undefined  for example take database nullable column   If you really need to convert everyting from primitive boolean to wrapper Boolean  then you could use following to support old code   Boolean set   Boolean FALSE    set to default value primitive value  false      if  set

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I almost never use Boolean because its semantics are vague and obscure  Basically you have 3-state logic  true  false or unknown  Sometimes it is useful to use it when e g  you gave user a choice between two values and the user didn t answer at all and you really want to know that information  think  NULLable database column    I see no reason to convert from boolean to Boolean as it introduces extra memory overhead  NPE possibility and less typing  Typically I use awkward BooleanUtils isTrue   to make my life a little bit easier with Boolean   The only reason for the existence of Boolean is the ability to have collections of Boolean type  generics do not allow boolean  as well as all other primitives

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There are many uses for the   null   value in the Boolean wrapper       For example  you may have in a form a field named  newsletter  that indicate if the user want or doesn t want a newsletter from your site  If the user doesn t select a value in this field  you may want to implement a default behaviour to that situation  send  don t send   question again   etc    Clearly  not set  or not selected or   null     is not the same that true or false   But  if  not set  doesn t apply to your model  don t change the boolean primitive

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There are three quick reasons    to represent Database boolean values  which may be true  false or null to represent XML Schema s xsd boolean values declared with xsd nillable  true  to be able to use generic types  List lt Boolean gt  - you can t use List lt boolean gt

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Boolean wrapper is useful when you want to whether value was assigned or not apart from true and false  It has the following three states    True False Not defined which is null   Whereas boolean has only two states    True False   The above difference will make it helpful in Lists of Boolean values  which can have True  False or Null

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The best way would be to avoid booleans completely  since every boolean implies that you have a conditional statement anywhere else in your code  see http   www antiifcampaign com  and this question  Can you write any algorithm without an if statement     However  pragmatically you have to use booleans from time to time  but  as you have already found out by yourself  dealing with Booleans is more error prone and more cumbersome  So I would suggest using booleans wherever possible  Exceptions from this might be a legacy database with nullable boolean-columns  although I would try to hide that in my mapping as well

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I suppose in some case  you should have a mechanism to distinguish a Boolean field which already set value or not

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In a strict definition of a boolean element  there are only two values   In a perfect world  that would be true   In the real world  the element may be missing or unknown   Typically  this involves user input   In a screen based system  it could be forced by an edit   In a batch world using either a database or XML input  the element could easily be missing   So  in the non-perfect world we live in  the Boolean object is great in that it can represent the missing or unknown state as null   After all  computers just model the real world an should account for all possible states and handle them with throwing exceptions  mostly since there are use cases where throwing the exception would be the correct response    In my case  the Boolean object was the perfect answer since the input XML sometimes had the element missing and I could still get a value  assign it to a Boolean and then check for a null before trying to use a true or false test with it   Just my 2 cents

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Wow  what on earth  Is it just me or are all these answers wrong or at least misleading   The Boolean class is a wrapper around the boolean primitive type  The use of this wrapper is to be able to pass a boolean in a method that accepts an object or generic  Ie vector   A Boolean object can NEVER have a value of null  If your reference to a Boolean is null  it simply means that your Boolean was never created    You might find this useful  http   grepcode com file repository grepcode com java root jdk openjdk 6-b14 java lang Boolean java  A null Boolean reference should only be used to trigger similar logic to which you have any other null reference  Using it for three state logic is clumsy   EDIT  notice  that Boolean a   true  is a misleading statement  This really equals something closer to Boolean a   new Boolean true   Please see autoboxing here  http   en wikipedia org wiki Boxing  28computer science 29 Autoboxing  Perhaps this is where much of the confusion comes from   EDIT2  Please read comments below  If anyone has an idea of how to restructure my answer to incorporate this  please do so

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Wrapper classes for primitives can be used where objects are required  collections are a good sample   Imagine you need for some reason store a sequence of boolean in an ArrayList  this can be done by boxing boolean in Boolean   There is a few words about this here  From documentation      As any Java programmer knows  you can   t put an int  or other primitive   value  into a collection  Collections can only hold object references    so you have to box primitive values into the appropriate wrapper class    which is Integer in the case of int   When you take the object out of   the collection  you get the Integer that you put in  if you need an   int  you must unbox the Integer using the intValue method  All of this   boxing and unboxing is a pain  and clutters up your code  The   autoboxing and unboxing feature automates the process  eliminating the   pain and the clutter    http   docs oracle com javase 1 5 0 docs guide language autoboxing html

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Boolean can be very helpful when you need three state  Like in software  testing if Test is passed send  true    if failed send false and if test case interrupted send null which will denote test case not executed

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Use boolean rather than Boolean every time you can  This will avoid many NullPointerExceptions and make your code more robust   Boolean is useful  for example   to store booleans in a collection  List  Map  etc   to represent a nullable boolean  coming from a nullable boolean column in a database  for example   The null value might mean  we don t know if it s true or false  in this context  each time a method needs an Object as argument  and you need to pass a boolean value  For example  when using reflection or methods like MessageFormat format

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For all the good answers above  I m just going to give a concrete example in Java servlet HttpSession class  Hope this example helps to clarify some question you may still have   If you need to store and retrieve values for a session  you use setAttribute String  Object   and getAttribute String  Object  method  So for a boolean value  you are forced to use the Boolean class if you want to store it in an http session    HttpSession sess   request getSession false   Boolean isAdmin    Boolean  sess getAttribute  admin    if    isAdmin        The last line will cause a NullPointerException if the attribute values is not set   which is the reason led me to this post   So the 3 logic state is here to stay  whether you prefer to use it or not

[java] When should null values of Boolean be used?

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