What are Null Pointer Exceptions (java.lang.NullPointerException) and how to avoid them?

What are Null Pointer Exceptions (java.lang.NullPointerException) and how to avoid them?

A java.lang.NullPointerException is thrown there’s an attempt to use null anywhere an object is actually required, such as trying to directly modify a null object.

All Java errors implement the java.lang.Throwable interface, or are extended from another inherited class therein.
java.lang.Exception inherits from java.lang.Throwable.
java.lang.RuntimeException inherits from java.lang.Exception.
Finally, java.lang.NullPointerException inherits from the java.lang.RuntimeException class.

As we saw in the hierarchical breakdown, the java.lang.NullPointerException inherits from java.lang.RuntimeException, so quite clearly this is an issue that will pop up during execution of an application. Specifically, virtually anytime an attempt is made to directly access a field, method, or the like of a null object a java.lang.NullPointerException is thrown.

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 it to 10 in the second line your value 10 is written into the memory location pointed 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 contain a primitive value. Instead, it contains a pointer (because the type is Integer which is a reference type). Since you did not say as yet what to point to Java sets it to null, meaning “I am pointing at nothing”.

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 this object. You can now reference the object using the dereferencing operator . (a dot).

The Exception that you asked about occurs when you declare a variable but did not create an 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 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
}
In which case you are not creating the object obj, rather assuming that it was created before the doSomething method was called. Unfortunately, it is possible to call the method like this:

doSomething(null);
In which case obj is null. If the method is intended to do something to the passed-in object, it is appropriate to throw the NullPointerException because it’s a programmer error and the programmer will need that information for debugging purposes.

To avoid the Null Pointer Exception
String comparison with literals
String str = null;
if(str.equals(“Test”)) {
/* The code here will not be reached, as an exception will be thrown. */
}
Check the arguments of a method
public static int getLength(String s) {
if (s == null)
throw new IllegalArgumentException(“The argument cannot be null”);

return s.length();
}
Prefer String.valueOf() method instead of toString()
Use the Ternary Operator
boolean expression ? value1 : value2;
Create methods that return empty collections instead of null
public class Example {
private static List numbers = null;

public static List getList() {
if (numbers == null)
return Collections.emptyList();
else
return numbers;
}
}
Make use of Apache’s StringUtils class
if (StringUtils.isNotEmpty(str)) {
System.out.println(str.toString());
}
Use the contains(), containsKey(), containsValue() methods
if(map.containsKey(key)) {
String value = map.get(key);
System.out.println(value.toString()); // No exception will be thrown.
}
Check the return value of external methods
Use Assertions
The instanceof operator
Accessing static members or methods of a class