What is null in Java

Question

What is null    Is null an instance of anything    What set does null belong to   How is it represented in the memory

User · Answer

An interesting way to see null in java in my opinion is to see it as something that DOES NOT denote an absence of information but simply as a literal value that can be assigned to a reference of any type  If you think about it if it denoted absence of information then for a1  a2 to be true doesn t make sense  in case they were both assigned a value of null  as they could really could be pointing to ANY object  we simply don t know what objects they should be pointing to     By the way null    null returns true in java  If java e g  would be like SQL 1999 then null  null would return unknown  a boolean value in SQL 1999 can take three values   true false and unknown but in practise unknown is implemented as null in real systems     http   en wikipedia org wiki SQL

User · Answer

There are two major categories of types in Java  primitive and reference  Variables declared of a primitive type store values  variables declared of a reference type store references    String x   null    In this case  the initialization statement declares a variables    x        x    stores String reference  It is null here  First of all  null is not a valid object instance  so there is no memory allocated for it  It is simply a value that indicates that the object reference is not currently referring to an object

User · Answer

Is null an instance of anything     No   That is why null instanceof X will return false for all classes X    Don t be fooled by the fact that you can assign null to a variable whose type is an object type   Strictly speaking  the assignment involves an implicit type conversion  see below       What set does  null  belong to    It is the one and only member of the null type  where the null type is defined as follows    There is also a special null type  the type of the expression null  which has no name  Because the null type has no name  it is impossible to declare a variable of the null type or to cast to the null type  The null reference is the only possible value of an expression of null type  The null reference can always be cast to any reference type  In practice  the programmer can ignore the null type and just pretend that null  is merely a special literal that can be of any reference type    JLS 4 1     What is null    See above   In some contexts  null is used to denote  no object  or  unknown  or  unavailable   but these meanings are application specific      How is it represented in the memory    That is implementation specific  and you won t be able to see the representation of null in a pure Java program    But null is represented as a zero machine address   pointer in most if not all Java implementations

User · Answer

What is null   It is nothing   Is null an instance of anything   No as it is nothing It can t be instance of any thing   What set does null belong to   No any set  How is it represented in the memory   If some reference points to it like   Object o new Object      In heap memory some space assigned to new created object  And o will point to that assigned space in memory   Now o null   This means now o will not point to that memory space of object

User · Answer

Is null an instance of anything     No  there is no type which null is an instanceof   15 20 2 Type Comparison Operator instanceof   RelationalExpression      RelationalExpression instanceof ReferenceType       At run time  the result of the instanceof operator is true if the value of the RelationalExpression is not null and the reference could be cast to the ReferenceType without raising a ClassCastException  Otherwise the result is false    This means that for any type E and R  for any E o  where o    null  o instanceof R is always false        What set does  null  belong to    JLS 4 1 The Kinds of Types and Values     There is also a special null type  the type of the expression null  which has no name  Because the null type has no name  it is impossible to declare a variable of the null type or to cast to the null type  The null reference is the only possible value of an expression of null type  The null reference can always be cast to any reference type  In practice  the programmer can ignore the null type and just pretend that null is merely a special literal that can be of any reference type         What is null    As the JLS quote above says  in practice you can simply pretend that it s  merely a special literal that can be of any reference type    In Java  null    null  this isn t always the case in other languages   Note also that by contract  it also has this special property  from java lang Object       public boolean equals Object obj       For any non-null reference value x  x equals null  should return false     It is also the default value  for variables that have them  for all reference types   JLS 4 12 5 Initial Values of Variables        Each class variable  instance variable  or array component is initialized with a default value when it is created           For all reference types  the default value is null          How this is used varies  You can use it to enable what is called lazy initialization of fields  where a field would have its initial value of null until it s actually used  where it s replaced by the  real  value  which may be expensive to compute    There are also other uses  Let s take a real example from java lang System      public static Console console        Returns  The system console  if any  otherwise null    This is a very common use pattern  null is used to denote non-existence of an object   Here s another usage example  this time from java io BufferedReader      public String readLine   throws IOException      Returns  A String containing the contents of the line  not including any line-termination characters  or null if the end of the stream has been reached    So here  readLine   would return instanceof String for each line  until it finally returns a null to signify the end  This allows you to process each line as follows   String line  while   line   reader readLine       null       process line       One can design the API so that the termination condition doesn t depend on readLine   returning null  but one can see that this design has the benefit of making things concise  Note that there is no problem with empty lines  because an empty line       null   Let s take another example  this time from java util Map lt K V gt       V get Object key       Returns the value to which the specified key is mapped  or null if this map contains no mapping for the key       If this map permits null values  then a return value of null does not necessarily indicate that the map contains no mapping for the key  it s also possible that the map explicitly maps the key to null  The containsKey operation may be used to distinguish these two cases     Here we start to see how using null can complicate things  The first statement says that if the key isn t mapped  null is returned  The second statement says that even if the key is mapped  null can also be returned   In contrast  java util Hashtable keeps things simpler by not permitting null keys and values  its V get Object key   if returns null  unambiguously means that the key isn t mapped   You can read through the rest of the APIs and find where and how null is used  Do keep in mind that they aren t always the best practice examples   Generally speaking  null are used as a special value to signify    Uninitialized state Termination condition Non-existing object An unknown value        How is it represented in the memory    In Java  None of your concern  And it s best kept that way     Is null a good thing   This is now borderline subjective  Some people say that null causes many programmer errors that could ve been avoided  Some say that in a language that catches NullPointerException like Java  it s good to use it because you will fail-fast on programmer errors  Some people avoid null by using Null object pattern  etc   This is a huge topic on its own  so it s best discussed as answer to another question   I will end this with a quote from the inventor of null himself  C A R Hoare  of quicksort fame       I call it my billion-dollar mistake  It was the invention of the null reference in 1965  At that time  I was designing the first comprehensive type system for references in an object oriented language  ALGOL W   My goal was to ensure that all use of references should be absolutely safe  with checking performed automatically by the compiler  But I couldn t resist the temptation to put in a null reference  simply because it was so easy to implement  This has led to innumerable errors  vulnerabilities  and system crashes  which have probably caused a billion dollars of pain and damage in the last forty years    The video of this presentation goes deeper  it s a recommended watch

User · Answer

Bytecode representation  Java s null has direct JVM support  three instructions are used to implement it    aconst null  e g  to set a variable to null as in Object o   null  ifnull and ifnonnull  e g  to compare an object to null as in if  o    null    Chapter 6  The Java Virtual Machine Instruction Set   then mentions the effects of null on other instructions  it throws a NullPointerException for many of them   2 4   Reference Types and Values  also mentions null in generic terms      A reference value may also be the special null reference  a reference to no object  which will be denoted here by null  The null reference initially has no run-time type  but may be cast to any type  The default value of a reference type is null

User · Answer

null is special value  it is not instance of anything  For obviously reason it cannot be instanceof anything

User · Answer

null in Java is like similar to nullptr in C     Program in C     class Point       private         int x         int y      public         Point int ix  int iy                      x   ix             y   iy                  void print     std  cout  lt  lt       lt  lt  x  lt  lt       lt  lt  y  lt  lt            int main         Point  p   new Point 3 5       if  p    nullptr               p- gt print           p   nullptr            else               std  cout  lt  lt   p is null   lt  lt  std  endl            return 0      Same program in Java   public class Point       private int x      private int y      public Point int ix  int iy            x   ix          y   iy            public void print     System out print       x         y             class Program       public static void main String   args            Point p   new Point 3 5           if  p    null                        p print                p   null                    else                       System out println  p is null                        Now do you understand from the codes above what is null in Java  If no then I recommend you to learn pointers in C C   and then you will understand   Note that in C  unlike C    nullptr is undefined  but NULL is used instead  which can also be used in C   too  but in C   nullptr is more preferable than just NULL  because the NULL in C is always related to pointers and that s it  so in C   the suffix  ptr  was appended to end of the word  and also all letters are now lowercase  but this is less important   In Java every variable of type class non-primitive is always reference to object of that type or inherited and null is null class object reference  but not null pointer  because in Java there is no such a thing  pointer   but references to class objects are used instead  and null in Java is related to class object references  so you can also called it as  nullref  or  nullrefobj   but this is long  so just call it  null    In C   you can use pointers and the nullptr value for optional members variables  i e  member variable that has no value and if it has no value then it equals to nullptr  so how null in Java can be used for example

User · Answer

Null in Java tm   In C and C     NULL  is a constant defined in a header file  with a value like       0   or       0L   or         void  0    depending on the compiler and memory model options  NULL is not  strictly speaking  part of C C   itself   In Java tm    null  is not a keyword  but a special literal of the null type  It can be cast to any reference type  but not to any primitive type such as int or boolean  The null literal doesn t necessarily have value zero  And it is impossible to cast to the null type or declare a variable of this type

User · Answer

Null is not an instance of any class   However  you can assign null to variables of any  object or array  type       this is false     boolean nope    null instanceof String        but you can still use it as a String  String x   null    abc  startsWith null

User · Answer

Short and precise answer which answers all your questions formally from JLS           3 10 7  The Null Literal         The null type has one value  the null reference  represented by the   null literal null  which is formed from ASCII characters        A null literal is always of the null type    Only a reference of type which is assigned to null is allocated  You don t assign any value  object  to the reference   Such allocation is specific to JVM how much reference will take and in which memory area it will be allocated

User · Answer

The null keyword is a literal that represents a null reference  one that does not refer to any object  null is the default value of reference-type variables   Also maybe have a look at   null   Java Glossary

User · Answer

null is a special value that is not an instance of any class  This is illustrated by the following program   public class X      void f Object o              System out println o instanceof String        Output is  false          public static void main String   args          new X   f null

User · Answer

No it s not the instance of anything  instanceof will always be false

[java] What is null in Java?

Examples related to java

Examples related to null

Examples related to terminology