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java nullpointerexception

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

209

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?

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    4106

    There are two overarching types of variables in Java:

    1. Primitives: variables that contain data. If you want to manipulate the data in a primitive variable you can manipulate that variable directly. By convention primitive types start with a lowercase letter. For example variables of type int or char are primitives.

    2. References: variables that contain the memory address of an Object i.e. variables that refer to an Object. If you want to manipulate the Object that a reference variable refers to you must dereference it. Dereferencing usually entails using . to access a method or field, or using [ to index an array. By convention reference types are usually denoted with a type that starts in uppercase. For example variables of type Object are references.

    Consider the following code where you declare a variable of primitive type int and don’t initialize it:

    int x;
    int y = x+x;
    

    These two lines will crash the program because no value is specified for x and we are trying to use x‘s value to specify y. All primitives have to be initialized to a usable value before they are manipulated.

    Now here is where things get interesting. Reference variables can be set to null which means “I am referencing nothing“. You can get a null value in a reference variable if you explicitly set it that way, or a reference variable is uninitialized and the compiler does not catch it (Java will automatically set the variable to null).

    If a reference variable is set to null either explicitly by you or through Java automatically, and you attempt to dereference it you get a NullPointerException.

    The NullPointerException (NPE) typically 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. So you have a reference to something that does not actually exist.

    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 reference value yet. Since you have not yet said what to point to, Java sets it to null.

    In the second line, the new keyword is used to instantiate (or create) an object of type Integer, and the reference variable num is assigned to that Integer object.

    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 “num may not have been initialized,” 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, "obj must not be null");
    

    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 & 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.

    In Java 14, the following is a sample NullPointerException Exception message:

    in thread “main” java.lang.NullPointerException: Cannot invoke “java.util.List.size()” because “list” is null

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    • 607

      “The best way to avoid this type of exception is to always check for null when you did not create the object yourself.” If the caller passes null, but null is not a valid argument for the method, then it’s correct to throw the exception back at the caller because it’s the caller’s fault. Silently ignoring invalid input and doing nothing in the method is extremely poor advice because it hides the problem.

      – Boann

      Jul 29, 2014 at 13:32

    • 118

      I would add a remark about this post explaining that even assignments to primitives can cause NPEs when using autoboxing: int a=b can throw an NPE if b is an Integer. There are cases where this is confusing to debug.

      Sep 26, 2014 at 19:45


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      Is it possible to capture NPE thrown by a webapp from the web browser?like will it show in the view page source from the web browser..

      – Sid

      Apr 13, 2015 at 14:51

    • 85

      Yes check if the object equals null before you invoke a method on it or try to access a variable it might have. Some times structuring your code can help avoid null pointer exception. eg when checking an input string with a constant string you should start with the constant string like here: if (“SomeString”.equals(inputString)) {} //even if inputString is null no exception is thrown. So there are a bunch of things that you can do to try to be safe.

      – Rose

      Nov 11, 2015 at 4:39

    • 90

      An additional way of avoiding NullPointerException problems in your code is to use @Nullable and @NotNull annotations. The following answer has more information on this. Although this answer is specificially about the IntelliJ IDE, it is also applicable to other tools as is apparanet from teh comments. (BTW I am not allowed to edit this answer directly, perhaps the author can add it?)

      Jan 3, 2016 at 18:17


    940

    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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      I understood everything you wrote there, but only because I’ve been coding for a while and know what a ‘pointer’ and a ‘reference’ are (and what null is, for that matter). When I try to dive right into explanations like that, my students look at me crosseyed, because there’s not enough background.

      – mmr

      Feb 20, 2009 at 4:06

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      A more common way to get a NullPointerException in practice would be forgetting to explicitly initialize a member variable to something other than null before using it, like this. With local variables, the compiler would catch this error, but in this case it doesn’t. Maybe that would make a useful addition to your answer?

      Jan 18, 2014 at 9:28


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      @EJP I think your points are valid, so I’ve updated the answer to be clearer and to avoid saying ‘points to null’ where it did.

      Jul 6, 2016 at 22:42

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      @StevePowell I indicated a long time ago that I didn’t want my answer to change. Please respect the intent of the original author.

      Jul 6, 2016 at 23:07

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      Sorry, I was “improving the answer” as requested in the top of this stackoverflow item (This question’s answers are a collaborative effort: if you see something that can be improved, just edit the answer to improve it!) I disagree that your version is better, IMO @EJB has valid points; but you are quite welcome to keep your answer intact, confusing though it is.

      Jul 14, 2016 at 11:19

    736

    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:

    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).

    3

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      @RuchirBaronia A debugger allows you to step through a program line by line to see which methods are called and how variables are changed. IDEs should have some tools to do this. See vogella.com/tutorials/EclipseDebugging/article.html for example.

      – fgb

      Dec 10, 2015 at 10:22

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      @RuchirBaronia You set breakpoints on the methods around any NullPointerExceptions as seen in the stacktrace, and check the values of variables against what you expect them to be. If you know a variable is null when it shouldn’t be, then you can set breakpoints around any code that changes the value. There are also conditional breakpoints you can use which will tell you when a value changes.

      – fgb

      Dec 10, 2015 at 11:33

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      i suggest also static analysis tools, like FINDBUGS en.m.wikipedia.org/wiki/FindBugs

      – Massimo

      Jun 30, 2017 at 12:32