What is a NullPointerException and how do I fix it

Question

What are Null Pointer Exceptions  java lang NullPointerException  and what causes them   What methods tools can be used to determine the cause so that you stop the exception from causing the program to terminate prematurely

User · Answer

In Java all the variables you declare are actually "references" to the objects (or primitives) and not the objects themselves.

When you attempt to execute one object method, the reference asks the living object to execute that method. But if the reference is referencing NULL (nothing, zero, void, nada) then there is no way the method gets executed. Then the runtime let you know this by throwing a NullPointerException.

Your reference is "pointing" to null, thus "Null -> Pointer".

The object lives in the VM memory space and the only way to access it is using this references. Take this example:

public class Some {
    private int id;
    public int getId(){
        return this.id;
    }
    public setId( int newId ) {
        this.id = newId;
    }
}

And on another place in your code:

Some reference = new Some();    // Point to a new object of type Some()
Some otherReference = null;     // Initiallly this points to NULL

reference.setId( 1 );           // Execute setId method, now private var id is 1

System.out.println( reference.getId() ); // Prints 1 to the console

otherReference = reference      // Now they both point to the only object.

reference = null;               // "reference" now point to null.

// But "otherReference" still point to the "real" object so this print 1 too...
System.out.println( otherReference.getId() );

// Guess what will happen
System.out.println( reference.getId() ); // :S Throws NullPointerException because "reference" is pointing to NULL remember...

This an important thing to know - when there are no more references to an object (in the example above when reference and otherReference both point to null) then the object is "unreachable". There is no way we can work with it, so this object is ready to be garbage collected, and at some point, the VM will free the memory used by this object and will allocate another.

User · Answer

Question  What causes a NullPointerException  NPE   As you should know  Java types are divided into primitive types  boolean  int  etc   and reference types  Reference types in Java allow you to use the special value null which is the Java way of saying  quot no object quot   A NullPointerException is thrown at runtime whenever your program attempts to use a null as if it was a real reference  For example  if you write this  public class Test       public static void main String   args            String foo   null          int length   foo length         HERE          the statement labeled  quot HERE quot  is going to attempt to run the length   method on a null reference  and this will throw a NullPointerException  There are many ways that you could use a null value that will result in a NullPointerException  In fact  the only things that you can do with a null without causing an NPE are   assign it to a reference variable or read it from a reference variable  assign it to an array element or read it from an array element  provided that array reference itself is non-null    pass it as a parameter or return it as a result  or test it using the    or    operators  or instanceof   Question  How do I read the NPE stacktrace  Suppose that I compile and run the program above    javac Test java    java Test Exception in thread  quot main quot  java lang NullPointerException     at Test main Test java 4     First observation  the compilation succeeds  The problem in the program is NOT a compilation error  It is a runtime error   Some IDEs may warn your program will always throw an exception     but the standard javac compiler doesn t   Second observation  when I run the program  it outputs two lines of  quot gobbledy-gook quot   WRONG   That s not gobbledy-gook  It is a stacktrace     and it provides vital information that will help you track down the error in your code if you take the time to read it carefully  So let s look at what it says  Exception in thread  quot main quot  java lang NullPointerException  The first line of the stack trace tells you a number of things   It tells you the name of the Java thread in which the exception was thrown   For a simple program with one thread  like this one   it will be  quot main quot   Let s move on     It tells you the full name of the exception that was thrown  i e  java lang NullPointerException  If the exception has an associated error message  that will be output after the exception name  NullPointerException is unusual in this respect  because it rarely has an error message   The second line is the most important one in diagnosing an NPE  at Test main Test java 4   This tells us a number of things    quot at Test main quot  says that we were in the main method of the Test class   quot Test java 4 quot  gives the source filename of the class  AND it tells us that the statement where this occurred is in line 4 of the file   If you count the lines in the file above  line 4 is the one that I labeled with the  quot HERE quot  comment  Note that in a more complicated example  there will be lots of lines in the NPE stack trace  But you can be sure that the second line  the first  quot at quot  line  will tell you where the NPE was thrown1  In short  the stack trace will tell us unambiguously which statement of the program has thrown the NPE  See also  What is a stack trace  and how can I use it to debug my application errors  1 - Not quite true  There are things called nested exceptions    Question  How do I track down the cause of the NPE exception in my code  This is the hard part  The short answer is to apply logical inference to the evidence provided by the stack trace  the source code  and the relevant API documentation  Let s illustrate with the simple example  above  first  We start by looking at the line that the stack trace has told us is where the NPE happened  int length   foo length       HERE  How can that throw an NPE  In fact  there is only one way  it can only happen if foo has the value null   We then try to run the length   method on null and    BANG  But  I hear you say  what if the NPE was thrown inside the length   method call  Well  if that happened  the stack trace would look different  The first  quot at quot  line would say that the exception was thrown in some line in the java lang String class and line 4 of Test java would be the second  quot at quot  line  So where did that null come from  In this case  it is obvious  and it is obvious what we need to do to fix it   Assign a non-null value to foo   OK  so let s try a slightly more tricky example  This will require some logical deduction  public class Test        private static String   foo   new String 2        private static int test String   bar  int pos            return bar pos  length               public static void main String   args            int length   test foo  1              javac Test java    java Test Exception in thread  quot main quot  java lang NullPointerException     at Test test Test java 6      at Test main Test java 10      So now we have two  quot at quot  lines  The first one is for this line  return args pos  length     and the second one is for this line  int length   test foo  1         Looking at the first line  how could that throw an NPE   There are two ways   If the value of bar is null then bar pos  will throw an NPE  If the value of bar pos  is null then calling length   on it will throw an NPE   Next  we need to figure out which of those scenarios explains what is actually happening  We will start by exploring the first one  Where does bar come from  It is a parameter to the test method call  and if we look at how test was called  we can see that it comes from the foo static variable  In addition  we can see clearly that we initialized foo to a non-null value   That is sufficient to tentatively dismiss this explanation   In theory  something else could change foo to null     but that is not happening here   So what about our second scenario  Well  we can see that pos is 1  so that means that foo 1  must be null  Is this possible  Indeed it is  And that is the problem  When we initialize like this  private static String   foo   new String 2    we allocate a String   with two elements that are initialized to null  After that  we have not changed the contents of foo     so foo 1  will still be null  What about on Android  On Android  tracking down the immediate cause of an NPE is a bit simpler   The exception message will typically tell you the  compile time  type of the null reference you are using and the method you were attempting to call when the NPE was thrown   This simplifies the process of pinpointing the immediate cause  But on the flipside  Android has some common platform-specific causes for NPEs   A very common is when getViewById unexpectedly returns a null   My advice would be to search for Q amp As about the cause of the unexpected null return value

User · Answer

When you declare a reference variable  i e   an object   you are really creating a pointer to an object  Consider the following code where you declare a variable of primitive type int  int x  x   10   In this example  the variable x is an int and Java will initialize it to 0 for you  When you assign the value of 10 on the second line  your value of 10 is written into the memory location referred to by x  But  when you try to declare a reference type  something different happens  Take the following code  Integer num  num   new Integer 10    The first line declares a variable named num  but it does not actually contain a primitive value yet  Instead  it contains a pointer  because the type is Integer which is a reference type   Since you have not yet said what to point to  Java sets it to null  which means  quot I am pointing to nothing quot   In the second line  the new keyword is used to instantiate  or create  an object of type Integer  and the pointer variable num is assigned to that Integer object  The NullPointerException  NPE  occurs when you declare a variable but did not create an object and assign it to the variable before trying to use the contents of the variable  called dereferencing   So you are pointing to something that does not actually exist  Dereferencing usually happens when using   to access a method or field  or using   to index an array  If you attempt to dereference num before creating the object you get a NullPointerException  In the most trivial cases  the compiler will catch the problem and let you know that  quot num may not have been initialized  quot  but sometimes you may write code that does not directly create the object  For instance  you may have a method as follows  public void doSomething SomeObject obj          Do something to obj  assumes obj is not null    obj myMethod       In which case  you are not creating the object obj  but rather assuming that it was created before the doSomething   method was called  Note  it is possible to call the method like this  doSomething null    In which case  obj is null  and the statement obj myMethod   will throw a NullPointerException  If the method is intended to do something to the passed-in object as the above method does  it is appropriate to throw the NullPointerException because it s a programmer error and the programmer will need that information for debugging purposes  In addition to NullPointerExceptions thrown as a result of the method s logic  you can also check the method arguments for null values and throw NPEs explicitly by adding something like the following near the beginning of a method     Throws an NPE with a custom error message if obj is null Objects requireNonNull obj   quot obj must not be null quot     Note that it s helpful to say in your error message clearly which object cannot be null  The advantage of validating this is that 1  you can return your own clearer error messages and 2  for the rest of the method you know that unless obj is reassigned  it is not null and can be dereferenced safely  Alternatively  there may be cases where the purpose of the method is not solely to operate on the passed in object  and therefore a null parameter may be acceptable  In this case  you would need to check for a null parameter and behave differently  You should also explain this in the documentation  For example  doSomething   could be written as           param obj An optional foo for       May be null  in which case      the result will be            public void doSomething SomeObject obj        if obj    null              Do something       else             Do something else          Finally  How to pinpoint the exception  amp  cause using Stack Trace  What methods tools can be used to determine the cause so that you stop the exception from causing the program to terminate prematurely   Sonar with find bugs can detect NPE  Can sonar catch null pointer exceptions caused by JVM Dynamically Now Java 14 has added a new language feature to show the root cause of NullPointerException  This language feature has been part of SAP commercial JVM since 2006  The following is 2 minutes read to understand this amazing language feature  https   jfeatures com blog NullPointerException In Java 14  the following is a sample NullPointerException Exception message   in thread  quot main quot  java lang NullPointerException  Cannot invoke  quot java util List size   quot  because  quot list quot  is null

User · Answer

In Java  everything  excluding primitive types  is in the form of a class   If you want to use any object then you have two phases    Declare Initialization   Example    Declaration  Object object  Initialization  object   new Object      Same for the array concept    Declaration  Item item     new Item 5   Initialization  item 0    new Item      If you are not giving the initialization section then the NullPointerException arise

User · Answer

In Java  everything  excluding primitive types  is in the form of a class   If you want to use any object then you have two phases    Declare Initialization   Example    Declaration  Object object  Initialization  object   new Object      Same for the array concept    Declaration  Item item     new Item 5   Initialization  item 0    new Item      If you are not giving the initialization section then the NullPointerException arise

User · Answer

It s like you are trying to access an object which is null  Consider below example   TypeA objA    At this time you have just declared this object but not initialized or instantiated  And whenever you try to access any property or method in it  it will throw  NullPointerException which makes sense   See this below example as well                                           String a   null  System out println a toString        NullPointerException will be thrown

User · Answer

A null pointer exception is an indicator that you are using an object without initializing it   For example  below is a student class which will use it in our code   public class Student        private int id       public int getId             return this id             public setId int newId            this id   newId            The below code gives you a null pointer exception   public class School        Student student       public School             try               student getId                      catch Exception e                System out println  Null pointer exception                        Because you are using student  but you forgot to initialize it like in the correct code shown below   public class School        Student student       public School             try               student   new Student                student setId 12               student getId                      catch Exception e                System out println  Null pointer exception

User · Answer

What is a NullPointerException   A good place to start is the JavaDocs  They have this covered      Thrown when an application attempts to use null in a case where an   object is required  These include          Calling the instance method of a null object    Accessing or modifying the field of a null object    Taking the length of null as if it were an array    Accessing or modifying the slots of null as if it were an array    Throwing null as if it were a Throwable value          Applications should throw instances of this class to indicate other   illegal uses of the null object    It is also the case that if you attempt to use a null reference with synchronized  that will also throw this exception  per the JLS    SynchronizedStatement      synchronized   Expression   Block          Otherwise  if the value of the Expression is null  a NullPointerException is thrown       How do I fix it   So you have a NullPointerException  How do you fix it  Let s take a simple example which throws a NullPointerException   public class Printer       private String name       public void setName String name            this name   name             public void print             printString name              private void printString String s            System out println s          s length                      public static void main String   args            Printer printer   new Printer            printer print              Identify the null values  The first step is identifying exactly which values are causing the exception  For this  we need to do some debugging  It s important to learn to read a stacktrace  This will show you where the exception was thrown   Exception in thread  main  java lang NullPointerException     at Printer printString Printer java 13      at Printer print Printer java 9      at Printer main Printer java 19    Here  we see that the exception is thrown on line 13  in the printString method   Look at the line and check which values are null by adding logging statements or using a debugger  We find out that s is null  and calling the length method on it throws the exception  We can see that the program stops throwing the exception when s length   is removed from the method   Trace where these values come from  Next check where this value comes from  By following the callers of the method  we see that s is passed in with printString name  in the print   method  and this name is null   Trace where these values should be set  Where is this name set  In the setName String  method  With some more debugging  we can see that this method isn t called at all  If the method was called  make sure to check the order that these methods are called  and the set method isn t called after the print method   This is enough to give us a solution  add a call to printer setName   before calling printer print     Other fixes  The variable can have a default value  and setName can prevent it being set to null    private String name         Either the print or printString method can check for null  for example   printString  name    null         name     Or you can design the class so that name always has a non-null value   public class Printer       private final String name       public Printer String name            this name   Objects requireNonNull name              public void print             printString name              private void printString String s            System out println s          s length                      public static void main String   args            Printer printer   new Printer  123            printer print              See also    Avoiding       null    statements in Java    I still can t find the problem  If you tried to debug the problem and still don t have a solution  you can post a question for more help  but make sure to include what you ve tried so far  At a minimum  include the stacktrace in the question  and mark the important line numbers in the code  Also  try simplifying the code first  see SSCCE

User · Answer

When you declare a reference variable  i e   an object   you are really creating a pointer to an object  Consider the following code where you declare a variable of primitive type int  int x  x   10   In this example  the variable x is an int and Java will initialize it to 0 for you  When you assign the value of 10 on the second line  your value of 10 is written into the memory location referred to by x  But  when you try to declare a reference type  something different happens  Take the following code  Integer num  num   new Integer 10    The first line declares a variable named num  but it does not actually contain a primitive value yet  Instead  it contains a pointer  because the type is Integer which is a reference type   Since you have not yet said what to point to  Java sets it to null  which means  quot I am pointing to nothing quot   In the second line  the new keyword is used to instantiate  or create  an object of type Integer  and the pointer variable num is assigned to that Integer object  The NullPointerException  NPE  occurs when you declare a variable but did not create an object and assign it to the variable before trying to use the contents of the variable  called dereferencing   So you are pointing to something that does not actually exist  Dereferencing usually happens when using   to access a method or field  or using   to index an array  If you attempt to dereference num before creating the object you get a NullPointerException  In the most trivial cases  the compiler will catch the problem and let you know that  quot num may not have been initialized  quot  but sometimes you may write code that does not directly create the object  For instance  you may have a method as follows  public void doSomething SomeObject obj          Do something to obj  assumes obj is not null    obj myMethod       In which case  you are not creating the object obj  but rather assuming that it was created before the doSomething   method was called  Note  it is possible to call the method like this  doSomething null    In which case  obj is null  and the statement obj myMethod   will throw a NullPointerException  If the method is intended to do something to the passed-in object as the above method does  it is appropriate to throw the NullPointerException because it s a programmer error and the programmer will need that information for debugging purposes  In addition to NullPointerExceptions thrown as a result of the method s logic  you can also check the method arguments for null values and throw NPEs explicitly by adding something like the following near the beginning of a method     Throws an NPE with a custom error message if obj is null Objects requireNonNull obj   quot obj must not be null quot     Note that it s helpful to say in your error message clearly which object cannot be null  The advantage of validating this is that 1  you can return your own clearer error messages and 2  for the rest of the method you know that unless obj is reassigned  it is not null and can be dereferenced safely  Alternatively  there may be cases where the purpose of the method is not solely to operate on the passed in object  and therefore a null parameter may be acceptable  In this case  you would need to check for a null parameter and behave differently  You should also explain this in the documentation  For example  doSomething   could be written as           param obj An optional foo for       May be null  in which case      the result will be            public void doSomething SomeObject obj        if obj    null              Do something       else             Do something else          Finally  How to pinpoint the exception  amp  cause using Stack Trace  What methods tools can be used to determine the cause so that you stop the exception from causing the program to terminate prematurely   Sonar with find bugs can detect NPE  Can sonar catch null pointer exceptions caused by JVM Dynamically Now Java 14 has added a new language feature to show the root cause of NullPointerException  This language feature has been part of SAP commercial JVM since 2006  The following is 2 minutes read to understand this amazing language feature  https   jfeatures com blog NullPointerException In Java 14  the following is a sample NullPointerException Exception message   in thread  quot main quot  java lang NullPointerException  Cannot invoke  quot java util List size   quot  because  quot list quot  is null

User · Answer

It s like you are trying to access an object which is null  Consider below example   TypeA objA    At this time you have just declared this object but not initialized or instantiated  And whenever you try to access any property or method in it  it will throw  NullPointerException which makes sense   See this below example as well                                           String a   null  System out println a toString        NullPointerException will be thrown

User · Answer

A lot of explanations are already present to explain how it happens and how to fix it  but you should also follow best practices to avoid NullPointerExceptions at all   See also  A good list of best practices  I would add  very important  make a good use of the final modifier  Using the  quot final quot  modifier whenever applicable in Java  Summary    Use the final modifier to enforce good initialization  Avoid returning null in methods  for example returning empty collections when applicable  Use annotations  NotNull and  Nullable Fail fast and use asserts to avoid propagation of null objects through the whole application when they shouldn t be null  Use equals with a known object first  if  knownObject  equals unknownObject  Prefer valueOf   over toString    Use null safe StringUtils methods StringUtils isEmpty null   Use Java 8 Optional as return value in methods  Optional class provide a solution for representing optional values instead of null references

User · Answer

A null pointer is one that points to nowhere   When you dereference a pointer p  you say  give me the data at the location stored in  p    When p is a null pointer  the location stored in p is nowhere  you re saying  give me the data at the location  nowhere     Obviously  it can t do this  so it throws a null pointer exception   In general  it s because something hasn t been initialized properly

User · Answer

A null pointer is one that points to nowhere   When you dereference a pointer p  you say  give me the data at the location stored in  p    When p is a null pointer  the location stored in p is nowhere  you re saying  give me the data at the location  nowhere     Obviously  it can t do this  so it throws a null pointer exception   In general  it s because something hasn t been initialized properly

User · Answer

A lot of explanations are already present to explain how it happens and how to fix it  but you should also follow best practices to avoid NullPointerExceptions at all   See also  A good list of best practices  I would add  very important  make a good use of the final modifier  Using the  quot final quot  modifier whenever applicable in Java  Summary    Use the final modifier to enforce good initialization  Avoid returning null in methods  for example returning empty collections when applicable  Use annotations  NotNull and  Nullable Fail fast and use asserts to avoid propagation of null objects through the whole application when they shouldn t be null  Use equals with a known object first  if  knownObject  equals unknownObject  Prefer valueOf   over toString    Use null safe StringUtils methods StringUtils isEmpty null   Use Java 8 Optional as return value in methods  Optional class provide a solution for representing optional values instead of null references

User · Answer

In Java all the variables you declare are actually "references" to the objects (or primitives) and not the objects themselves.

When you attempt to execute one object method, the reference asks the living object to execute that method. But if the reference is referencing NULL (nothing, zero, void, nada) then there is no way the method gets executed. Then the runtime let you know this by throwing a NullPointerException.

Your reference is "pointing" to null, thus "Null -> Pointer".

The object lives in the VM memory space and the only way to access it is using this references. Take this example:

public class Some {
    private int id;
    public int getId(){
        return this.id;
    }
    public setId( int newId ) {
        this.id = newId;
    }
}

And on another place in your code:

Some reference = new Some();    // Point to a new object of type Some()
Some otherReference = null;     // Initiallly this points to NULL

reference.setId( 1 );           // Execute setId method, now private var id is 1

System.out.println( reference.getId() ); // Prints 1 to the console

otherReference = reference      // Now they both point to the only object.

reference = null;               // "reference" now point to null.

// But "otherReference" still point to the "real" object so this print 1 too...
System.out.println( otherReference.getId() );

// Guess what will happen
System.out.println( reference.getId() ); // :S Throws NullPointerException because "reference" is pointing to NULL remember...

This an important thing to know - when there are no more references to an object (in the example above when reference and otherReference both point to null) then the object is "unreachable". There is no way we can work with it, so this object is ready to be garbage collected, and at some point, the VM will free the memory used by this object and will allocate another.

User · Answer

Question  What causes a NullPointerException  NPE   As you should know  Java types are divided into primitive types  boolean  int  etc   and reference types  Reference types in Java allow you to use the special value null which is the Java way of saying  quot no object quot   A NullPointerException is thrown at runtime whenever your program attempts to use a null as if it was a real reference  For example  if you write this  public class Test       public static void main String   args            String foo   null          int length   foo length         HERE          the statement labeled  quot HERE quot  is going to attempt to run the length   method on a null reference  and this will throw a NullPointerException  There are many ways that you could use a null value that will result in a NullPointerException  In fact  the only things that you can do with a null without causing an NPE are   assign it to a reference variable or read it from a reference variable  assign it to an array element or read it from an array element  provided that array reference itself is non-null    pass it as a parameter or return it as a result  or test it using the    or    operators  or instanceof   Question  How do I read the NPE stacktrace  Suppose that I compile and run the program above    javac Test java    java Test Exception in thread  quot main quot  java lang NullPointerException     at Test main Test java 4     First observation  the compilation succeeds  The problem in the program is NOT a compilation error  It is a runtime error   Some IDEs may warn your program will always throw an exception     but the standard javac compiler doesn t   Second observation  when I run the program  it outputs two lines of  quot gobbledy-gook quot   WRONG   That s not gobbledy-gook  It is a stacktrace     and it provides vital information that will help you track down the error in your code if you take the time to read it carefully  So let s look at what it says  Exception in thread  quot main quot  java lang NullPointerException  The first line of the stack trace tells you a number of things   It tells you the name of the Java thread in which the exception was thrown   For a simple program with one thread  like this one   it will be  quot main quot   Let s move on     It tells you the full name of the exception that was thrown  i e  java lang NullPointerException  If the exception has an associated error message  that will be output after the exception name  NullPointerException is unusual in this respect  because it rarely has an error message   The second line is the most important one in diagnosing an NPE  at Test main Test java 4   This tells us a number of things    quot at Test main quot  says that we were in the main method of the Test class   quot Test java 4 quot  gives the source filename of the class  AND it tells us that the statement where this occurred is in line 4 of the file   If you count the lines in the file above  line 4 is the one that I labeled with the  quot HERE quot  comment  Note that in a more complicated example  there will be lots of lines in the NPE stack trace  But you can be sure that the second line  the first  quot at quot  line  will tell you where the NPE was thrown1  In short  the stack trace will tell us unambiguously which statement of the program has thrown the NPE  See also  What is a stack trace  and how can I use it to debug my application errors  1 - Not quite true  There are things called nested exceptions    Question  How do I track down the cause of the NPE exception in my code  This is the hard part  The short answer is to apply logical inference to the evidence provided by the stack trace  the source code  and the relevant API documentation  Let s illustrate with the simple example  above  first  We start by looking at the line that the stack trace has told us is where the NPE happened  int length   foo length       HERE  How can that throw an NPE  In fact  there is only one way  it can only happen if foo has the value null   We then try to run the length   method on null and    BANG  But  I hear you say  what if the NPE was thrown inside the length   method call  Well  if that happened  the stack trace would look different  The first  quot at quot  line would say that the exception was thrown in some line in the java lang String class and line 4 of Test java would be the second  quot at quot  line  So where did that null come from  In this case  it is obvious  and it is obvious what we need to do to fix it   Assign a non-null value to foo   OK  so let s try a slightly more tricky example  This will require some logical deduction  public class Test        private static String   foo   new String 2        private static int test String   bar  int pos            return bar pos  length               public static void main String   args            int length   test foo  1              javac Test java    java Test Exception in thread  quot main quot  java lang NullPointerException     at Test test Test java 6      at Test main Test java 10      So now we have two  quot at quot  lines  The first one is for this line  return args pos  length     and the second one is for this line  int length   test foo  1         Looking at the first line  how could that throw an NPE   There are two ways   If the value of bar is null then bar pos  will throw an NPE  If the value of bar pos  is null then calling length   on it will throw an NPE   Next  we need to figure out which of those scenarios explains what is actually happening  We will start by exploring the first one  Where does bar come from  It is a parameter to the test method call  and if we look at how test was called  we can see that it comes from the foo static variable  In addition  we can see clearly that we initialized foo to a non-null value   That is sufficient to tentatively dismiss this explanation   In theory  something else could change foo to null     but that is not happening here   So what about our second scenario  Well  we can see that pos is 1  so that means that foo 1  must be null  Is this possible  Indeed it is  And that is the problem  When we initialize like this  private static String   foo   new String 2    we allocate a String   with two elements that are initialized to null  After that  we have not changed the contents of foo     so foo 1  will still be null  What about on Android  On Android  tracking down the immediate cause of an NPE is a bit simpler   The exception message will typically tell you the  compile time  type of the null reference you are using and the method you were attempting to call when the NPE was thrown   This simplifies the process of pinpointing the immediate cause  But on the flipside  Android has some common platform-specific causes for NPEs   A very common is when getViewById unexpectedly returns a null   My advice would be to search for Q amp As about the cause of the unexpected null return value

User · Answer

When you declare a reference variable  i e   an object   you are really creating a pointer to an object  Consider the following code where you declare a variable of primitive type int  int x  x   10   In this example  the variable x is an int and Java will initialize it to 0 for you  When you assign the value of 10 on the second line  your value of 10 is written into the memory location referred to by x  But  when you try to declare a reference type  something different happens  Take the following code  Integer num  num   new Integer 10    The first line declares a variable named num  but it does not actually contain a primitive value yet  Instead  it contains a pointer  because the type is Integer which is a reference type   Since you have not yet said what to point to  Java sets it to null  which means  quot I am pointing to nothing quot   In the second line  the new keyword is used to instantiate  or create  an object of type Integer  and the pointer variable num is assigned to that Integer object  The NullPointerException  NPE  occurs when you declare a variable but did not create an object and assign it to the variable before trying to use the contents of the variable  called dereferencing   So you are pointing to something that does not actually exist  Dereferencing usually happens when using   to access a method or field  or using   to index an array  If you attempt to dereference num before creating the object you get a NullPointerException  In the most trivial cases  the compiler will catch the problem and let you know that  quot num may not have been initialized  quot  but sometimes you may write code that does not directly create the object  For instance  you may have a method as follows  public void doSomething SomeObject obj          Do something to obj  assumes obj is not null    obj myMethod       In which case  you are not creating the object obj  but rather assuming that it was created before the doSomething   method was called  Note  it is possible to call the method like this  doSomething null    In which case  obj is null  and the statement obj myMethod   will throw a NullPointerException  If the method is intended to do something to the passed-in object as the above method does  it is appropriate to throw the NullPointerException because it s a programmer error and the programmer will need that information for debugging purposes  In addition to NullPointerExceptions thrown as a result of the method s logic  you can also check the method arguments for null values and throw NPEs explicitly by adding something like the following near the beginning of a method     Throws an NPE with a custom error message if obj is null Objects requireNonNull obj   quot obj must not be null quot     Note that it s helpful to say in your error message clearly which object cannot be null  The advantage of validating this is that 1  you can return your own clearer error messages and 2  for the rest of the method you know that unless obj is reassigned  it is not null and can be dereferenced safely  Alternatively  there may be cases where the purpose of the method is not solely to operate on the passed in object  and therefore a null parameter may be acceptable  In this case  you would need to check for a null parameter and behave differently  You should also explain this in the documentation  For example  doSomething   could be written as           param obj An optional foo for       May be null  in which case      the result will be            public void doSomething SomeObject obj        if obj    null              Do something       else             Do something else          Finally  How to pinpoint the exception  amp  cause using Stack Trace  What methods tools can be used to determine the cause so that you stop the exception from causing the program to terminate prematurely   Sonar with find bugs can detect NPE  Can sonar catch null pointer exceptions caused by JVM Dynamically Now Java 14 has added a new language feature to show the root cause of NullPointerException  This language feature has been part of SAP commercial JVM since 2006  The following is 2 minutes read to understand this amazing language feature  https   jfeatures com blog NullPointerException In Java 14  the following is a sample NullPointerException Exception message   in thread  quot main quot  java lang NullPointerException  Cannot invoke  quot java util List size   quot  because  quot list quot  is null

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Another occurrence of a NullPointerException occurs when one declares an object array  then immediately tries to dereference elements inside of it   String   phrases   new String 10   String keyPhrase    Bird   for String phrase   phrases        System out println phrase equals keyPhrase        This particular NPE can be avoided if the comparison order is reversed  namely  use  equals on a guaranteed non-null object   All elements inside of an array are initialized to their common initial value  for any type of object array  that means that all elements are null   You must initialize the elements in the array before accessing or dereferencing them   String   phrases   new String     The bird    A bird    My bird    Bird    String keyPhrase    Bird   for String phrase   phrases        System out println phrase equals keyPhrase

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It s like you are trying to access an object which is null  Consider below example   TypeA objA    At this time you have just declared this object but not initialized or instantiated  And whenever you try to access any property or method in it  it will throw  NullPointerException which makes sense   See this below example as well                                           String a   null  System out println a toString        NullPointerException will be thrown

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When you declare a reference variable  i e   an object   you are really creating a pointer to an object  Consider the following code where you declare a variable of primitive type int  int x  x   10   In this example  the variable x is an int and Java will initialize it to 0 for you  When you assign the value of 10 on the second line  your value of 10 is written into the memory location referred to by x  But  when you try to declare a reference type  something different happens  Take the following code  Integer num  num   new Integer 10    The first line declares a variable named num  but it does not actually contain a primitive value yet  Instead  it contains a pointer  because the type is Integer which is a reference type   Since you have not yet said what to point to  Java sets it to null  which means  quot I am pointing to nothing quot   In the second line  the new keyword is used to instantiate  or create  an object of type Integer  and the pointer variable num is assigned to that Integer object  The NullPointerException  NPE  occurs when you declare a variable but did not create an object and assign it to the variable before trying to use the contents of the variable  called dereferencing   So you are pointing to something that does not actually exist  Dereferencing usually happens when using   to access a method or field  or using   to index an array  If you attempt to dereference num before creating the object you get a NullPointerException  In the most trivial cases  the compiler will catch the problem and let you know that  quot num may not have been initialized  quot  but sometimes you may write code that does not directly create the object  For instance  you may have a method as follows  public void doSomething SomeObject obj          Do something to obj  assumes obj is not null    obj myMethod       In which case  you are not creating the object obj  but rather assuming that it was created before the doSomething   method was called  Note  it is possible to call the method like this  doSomething null    In which case  obj is null  and the statement obj myMethod   will throw a NullPointerException  If the method is intended to do something to the passed-in object as the above method does  it is appropriate to throw the NullPointerException because it s a programmer error and the programmer will need that information for debugging purposes  In addition to NullPointerExceptions thrown as a result of the method s logic  you can also check the method arguments for null values and throw NPEs explicitly by adding something like the following near the beginning of a method     Throws an NPE with a custom error message if obj is null Objects requireNonNull obj   quot obj must not be null quot     Note that it s helpful to say in your error message clearly which object cannot be null  The advantage of validating this is that 1  you can return your own clearer error messages and 2  for the rest of the method you know that unless obj is reassigned  it is not null and can be dereferenced safely  Alternatively  there may be cases where the purpose of the method is not solely to operate on the passed in object  and therefore a null parameter may be acceptable  In this case  you would need to check for a null parameter and behave differently  You should also explain this in the documentation  For example  doSomething   could be written as           param obj An optional foo for       May be null  in which case      the result will be            public void doSomething SomeObject obj        if obj    null              Do something       else             Do something else          Finally  How to pinpoint the exception  amp  cause using Stack Trace  What methods tools can be used to determine the cause so that you stop the exception from causing the program to terminate prematurely   Sonar with find bugs can detect NPE  Can sonar catch null pointer exceptions caused by JVM Dynamically Now Java 14 has added a new language feature to show the root cause of NullPointerException  This language feature has been part of SAP commercial JVM since 2006  The following is 2 minutes read to understand this amazing language feature  https   jfeatures com blog NullPointerException In Java 14  the following is a sample NullPointerException Exception message   in thread  quot main quot  java lang NullPointerException  Cannot invoke  quot java util List size   quot  because  quot list quot  is null

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A null pointer exception is an indicator that you are using an object without initializing it   For example  below is a student class which will use it in our code   public class Student        private int id       public int getId             return this id             public setId int newId            this id   newId            The below code gives you a null pointer exception   public class School        Student student       public School             try               student getId                      catch Exception e                System out println  Null pointer exception                        Because you are using student  but you forgot to initialize it like in the correct code shown below   public class School        Student student       public School             try               student   new Student                student setId 12               student getId                      catch Exception e                System out println  Null pointer exception

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NullPointerExceptions are exceptions that occur when you try to use a reference that points to no location in memory  null  as though it were referencing an object   Calling a method on a null reference or trying to access a field of a null reference will trigger a NullPointerException   These are the most common  but other ways are listed on the NullPointerException javadoc page   Probably the quickest example code I could come up with to illustrate a NullPointerException would be   public class Example        public static void main String   args            Object obj   null          obj hashCode               On the first line inside main  I m explicitly setting the Object reference obj equal to null   This means I have a reference  but it isn t pointing to any object   After that  I try to treat the reference as though it points to an object by calling a method on it   This results in a NullPointerException because there is no code to execute in the location that the reference is pointing    This is a technicality  but I think it bears mentioning  A reference that points to null isn t the same as a C pointer that points to an invalid memory location   A null pointer is literally not pointing anywhere  which is subtly different than pointing to a location that happens to be invalid

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A null pointer is one that points to nowhere   When you dereference a pointer p  you say  give me the data at the location stored in  p    When p is a null pointer  the location stored in p is nowhere  you re saying  give me the data at the location  nowhere     Obviously  it can t do this  so it throws a null pointer exception   In general  it s because something hasn t been initialized properly

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NullPointerExceptions are exceptions that occur when you try to use a reference that points to no location in memory  null  as though it were referencing an object   Calling a method on a null reference or trying to access a field of a null reference will trigger a NullPointerException   These are the most common  but other ways are listed on the NullPointerException javadoc page   Probably the quickest example code I could come up with to illustrate a NullPointerException would be   public class Example        public static void main String   args            Object obj   null          obj hashCode               On the first line inside main  I m explicitly setting the Object reference obj equal to null   This means I have a reference  but it isn t pointing to any object   After that  I try to treat the reference as though it points to an object by calling a method on it   This results in a NullPointerException because there is no code to execute in the location that the reference is pointing    This is a technicality  but I think it bears mentioning  A reference that points to null isn t the same as a C pointer that points to an invalid memory location   A null pointer is literally not pointing anywhere  which is subtly different than pointing to a location that happens to be invalid

User · Answer

A null pointer is one that points to nowhere   When you dereference a pointer p  you say  give me the data at the location stored in  p    When p is a null pointer  the location stored in p is nowhere  you re saying  give me the data at the location  nowhere     Obviously  it can t do this  so it throws a null pointer exception   In general  it s because something hasn t been initialized properly

User · Answer

In Java all the variables you declare are actually "references" to the objects (or primitives) and not the objects themselves.

When you attempt to execute one object method, the reference asks the living object to execute that method. But if the reference is referencing NULL (nothing, zero, void, nada) then there is no way the method gets executed. Then the runtime let you know this by throwing a NullPointerException.

Your reference is "pointing" to null, thus "Null -> Pointer".

The object lives in the VM memory space and the only way to access it is using this references. Take this example:

public class Some {
    private int id;
    public int getId(){
        return this.id;
    }
    public setId( int newId ) {
        this.id = newId;
    }
}

And on another place in your code:

Some reference = new Some();    // Point to a new object of type Some()
Some otherReference = null;     // Initiallly this points to NULL

reference.setId( 1 );           // Execute setId method, now private var id is 1

System.out.println( reference.getId() ); // Prints 1 to the console

otherReference = reference      // Now they both point to the only object.

reference = null;               // "reference" now point to null.

// But "otherReference" still point to the "real" object so this print 1 too...
System.out.println( otherReference.getId() );

// Guess what will happen
System.out.println( reference.getId() ); // :S Throws NullPointerException because "reference" is pointing to NULL remember...

This an important thing to know - when there are no more references to an object (in the example above when reference and otherReference both point to null) then the object is "unreachable". There is no way we can work with it, so this object is ready to be garbage collected, and at some point, the VM will free the memory used by this object and will allocate another.

User · Answer

NullPointerExceptions are exceptions that occur when you try to use a reference that points to no location in memory  null  as though it were referencing an object   Calling a method on a null reference or trying to access a field of a null reference will trigger a NullPointerException   These are the most common  but other ways are listed on the NullPointerException javadoc page   Probably the quickest example code I could come up with to illustrate a NullPointerException would be   public class Example        public static void main String   args            Object obj   null          obj hashCode               On the first line inside main  I m explicitly setting the Object reference obj equal to null   This means I have a reference  but it isn t pointing to any object   After that  I try to treat the reference as though it points to an object by calling a method on it   This results in a NullPointerException because there is no code to execute in the location that the reference is pointing    This is a technicality  but I think it bears mentioning  A reference that points to null isn t the same as a C pointer that points to an invalid memory location   A null pointer is literally not pointing anywhere  which is subtly different than pointing to a location that happens to be invalid

User · Answer

A null pointer exception is thrown when an application attempts to use null in a case where an object is required  These include    Calling the instance method of a null object  Accessing or modifying the field of a null object  Taking the length of null as if it were an array  Accessing or modifying the slots of null as if it were an array  Throwing null as if it were a Throwable value     Applications should throw instances of this class to indicate other illegal uses of the null object    Reference  http   docs oracle com javase 8 docs api java lang NullPointerException html

User · Answer

It s like you are trying to access an object which is null  Consider below example   TypeA objA    At this time you have just declared this object but not initialized or instantiated  And whenever you try to access any property or method in it  it will throw  NullPointerException which makes sense   See this below example as well                                           String a   null  System out println a toString        NullPointerException will be thrown

User · Answer

In Java all the variables you declare are actually "references" to the objects (or primitives) and not the objects themselves.

When you attempt to execute one object method, the reference asks the living object to execute that method. But if the reference is referencing NULL (nothing, zero, void, nada) then there is no way the method gets executed. Then the runtime let you know this by throwing a NullPointerException.

Your reference is "pointing" to null, thus "Null -> Pointer".

The object lives in the VM memory space and the only way to access it is using this references. Take this example:

public class Some {
    private int id;
    public int getId(){
        return this.id;
    }
    public setId( int newId ) {
        this.id = newId;
    }
}

And on another place in your code:

Some reference = new Some();    // Point to a new object of type Some()
Some otherReference = null;     // Initiallly this points to NULL

reference.setId( 1 );           // Execute setId method, now private var id is 1

System.out.println( reference.getId() ); // Prints 1 to the console

otherReference = reference      // Now they both point to the only object.

reference = null;               // "reference" now point to null.

// But "otherReference" still point to the "real" object so this print 1 too...
System.out.println( otherReference.getId() );

// Guess what will happen
System.out.println( reference.getId() ); // :S Throws NullPointerException because "reference" is pointing to NULL remember...

This an important thing to know - when there are no more references to an object (in the example above when reference and otherReference both point to null) then the object is "unreachable". There is no way we can work with it, so this object is ready to be garbage collected, and at some point, the VM will free the memory used by this object and will allocate another.

User · Answer

What is a NullPointerException   A good place to start is the JavaDocs  They have this covered      Thrown when an application attempts to use null in a case where an   object is required  These include          Calling the instance method of a null object    Accessing or modifying the field of a null object    Taking the length of null as if it were an array    Accessing or modifying the slots of null as if it were an array    Throwing null as if it were a Throwable value          Applications should throw instances of this class to indicate other   illegal uses of the null object    It is also the case that if you attempt to use a null reference with synchronized  that will also throw this exception  per the JLS    SynchronizedStatement      synchronized   Expression   Block          Otherwise  if the value of the Expression is null  a NullPointerException is thrown       How do I fix it   So you have a NullPointerException  How do you fix it  Let s take a simple example which throws a NullPointerException   public class Printer       private String name       public void setName String name            this name   name             public void print             printString name              private void printString String s            System out println s          s length                      public static void main String   args            Printer printer   new Printer            printer print              Identify the null values  The first step is identifying exactly which values are causing the exception  For this  we need to do some debugging  It s important to learn to read a stacktrace  This will show you where the exception was thrown   Exception in thread  main  java lang NullPointerException     at Printer printString Printer java 13      at Printer print Printer java 9      at Printer main Printer java 19    Here  we see that the exception is thrown on line 13  in the printString method   Look at the line and check which values are null by adding logging statements or using a debugger  We find out that s is null  and calling the length method on it throws the exception  We can see that the program stops throwing the exception when s length   is removed from the method   Trace where these values come from  Next check where this value comes from  By following the callers of the method  we see that s is passed in with printString name  in the print   method  and this name is null   Trace where these values should be set  Where is this name set  In the setName String  method  With some more debugging  we can see that this method isn t called at all  If the method was called  make sure to check the order that these methods are called  and the set method isn t called after the print method   This is enough to give us a solution  add a call to printer setName   before calling printer print     Other fixes  The variable can have a default value  and setName can prevent it being set to null    private String name         Either the print or printString method can check for null  for example   printString  name    null         name     Or you can design the class so that name always has a non-null value   public class Printer       private final String name       public Printer String name            this name   Objects requireNonNull name              public void print             printString name              private void printString String s            System out println s          s length                      public static void main String   args            Printer printer   new Printer  123            printer print              See also    Avoiding       null    statements in Java    I still can t find the problem  If you tried to debug the problem and still don t have a solution  you can post a question for more help  but make sure to include what you ve tried so far  At a minimum  include the stacktrace in the question  and mark the important line numbers in the code  Also  try simplifying the code first  see SSCCE

User · Answer

NullPointerExceptions are exceptions that occur when you try to use a reference that points to no location in memory  null  as though it were referencing an object   Calling a method on a null reference or trying to access a field of a null reference will trigger a NullPointerException   These are the most common  but other ways are listed on the NullPointerException javadoc page   Probably the quickest example code I could come up with to illustrate a NullPointerException would be   public class Example        public static void main String   args            Object obj   null          obj hashCode               On the first line inside main  I m explicitly setting the Object reference obj equal to null   This means I have a reference  but it isn t pointing to any object   After that  I try to treat the reference as though it points to an object by calling a method on it   This results in a NullPointerException because there is no code to execute in the location that the reference is pointing    This is a technicality  but I think it bears mentioning  A reference that points to null isn t the same as a C pointer that points to an invalid memory location   A null pointer is literally not pointing anywhere  which is subtly different than pointing to a location that happens to be invalid

User · Answer

A null pointer exception is thrown when an application attempts to use null in a case where an object is required  These include    Calling the instance method of a null object  Accessing or modifying the field of a null object  Taking the length of null as if it were an array  Accessing or modifying the slots of null as if it were an array  Throwing null as if it were a Throwable value     Applications should throw instances of this class to indicate other illegal uses of the null object    Reference  http   docs oracle com javase 8 docs api java lang NullPointerException html

User · Answer

Another occurrence of a NullPointerException occurs when one declares an object array  then immediately tries to dereference elements inside of it   String   phrases   new String 10   String keyPhrase    Bird   for String phrase   phrases        System out println phrase equals keyPhrase        This particular NPE can be avoided if the comparison order is reversed  namely  use  equals on a guaranteed non-null object   All elements inside of an array are initialized to their common initial value  for any type of object array  that means that all elements are null   You must initialize the elements in the array before accessing or dereferencing them   String   phrases   new String     The bird    A bird    My bird    Bird    String keyPhrase    Bird   for String phrase   phrases        System out println phrase equals keyPhrase

[java] What is a NullPointerException, and how do I fix it?

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