What Is a Nullpointerexception, and How to Fix It

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

There are two overarching types of variables in Java:

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.
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

How to fix java.lang.NullPointerException: null?

public Store()
{
   double[] sales=new double[10];
   String[] customerNames=new String[10];        
}

This declares an entirely new variable named sales, assigns a new double array to it, and then tosses the local variable it into the bin immediately, as is the fate of all local variables once its scope ends (local vars are scoped to the nearest set of braces, so, after these 2 lines, a closing brace appears: That is where all local vars declared inside, such as your sales and customerNames in this code, poof out of existence). The arrays will be garbage collected eventually; nobody has any reference to them anymore.

This does absolutely nothing whatsoever to your field named sales.

What you presumably want is:

public Store()
{
    sales=new double[10];
    customer=new String[10];        
}

java.lang.NullPointerException error how to fix?

Most likely you have CompatibleActivity declared in your class as

private Activity[] CompatibleActivity;

which declares a reference and initializes it to null by default. You need to assign it a real array with enough elements (e.g. in your constructor):

CompatibleActivity = new Activity[myBigNumber];

java.lang.NullPointerException

You get a NullPointerException when you try to call a method using a variable that is null. Simple example:

String s = null;
int len = s.length();  // NullPointerException because s is null

So you should check if the variable is null before calling any method on it, for example:

int len;
if (s == null) {
    len = 0;
}
else {
    len = s.length();  // Safe, s is never null when you get here
}

Note that a NullPointerException is usually easy to solve. Carefully look at the stack trace of the exception; it tells you exactly in which line of your code the exception happens. Check what could be null in that line of code, and check if you're calling a method on something that might be null. Add a check (or prevent that the relevant thing can ever be null in another way).

How to fix NullPointerException in For Loop

It's because of this line:

if (l==r) return null;

You are passing in 0 for both l and r meaning they are equal. Therefore, you are getting a null value passed in to your isSorted() call.

How to resolve null pointer exception in code?

You are trying to access the global variable passengers which is getting initialized only when the parameter otherArraySize is smaller than 0 as specified in the constructor that you are using. Try reversing the condition for that parameter. This way your array would initialize.

public Airplane(String otherAirplaneName, int otherArraySize){

        airplaneName = otherAirplaneName;

        if (otherArraySize > 0){
            otherArraySize = 0;
            passengers = new Passenger[otherArraySize];
        }

        numPassengers = 0;

    }