Almost all the programming languages are bonded with null. There is hardly a programmer, who is not troubled by null.
In Java, null is associated java.lang.NullPointerException. As it is a class in java.lang package, it is called when we try to perform some operations with or without null and sometimes we don’t even know where it has happened.
Below are some important points about null in java which every Java programmer should know:
1. null is Case sensitive: null is literal in Java and because keywords are case-sensitive in java, we can’t write NULL or 0 as in C language.
public class Test
{
public static void main (String[] args) throws java.lang.Exception
{
// compile-time error : can't find symbol 'NULL'
Object obj = NULL;
//runs successfully
Object obj1 = null;
}
}
Output:
5: error: cannot find symbol
can't find symbol 'NULL'
^
variable NULL
class Test
1 error
2. Reference Variable value: Any reference variable in Java has default value null.
public class Test
{
private static Object obj;
public static void main(String args[])
{
// it will print null;
System.out.println("Value of object obj is : " + obj);
}
}
Output:
Value of object obj is : null
3. Type of null: Unlike common misconception, null is not Object or neither a type. It’s just a special value, which can be assigned to any reference type and you can type cast null to any type
Examples:
// null can be assigned to String
String str = null;
// you can assign null to Integer also
Integer itr = null;
// null can also be assigned to Double
Double dbl = null;
// null can be type cast to String
String myStr = (String) null;
// it can also be type casted to Integer
Integer myItr = (Integer) null;
// yes it's possible, no error
Double myDbl = (Double) null;
4. Autoboxing and unboxing : During auto-boxing and unboxing operations, compiler simply throws Nullpointer exception error if a null value is assigned to primitive boxed data type.
public class Test
{
public static void main (String[] args) throws java.lang.Exception
{
//An integer can be null, so this is fine
Integer i = null;
//Unboxing null to integer throws NullpointerException
int a = i;
}
}
Output:
Exception in thread "main" java.lang.NullPointerException
at Test.main(Test.java:6)
5. instanceof operator: The java instanceof operator is used to test whether the object is an instance of the specified type (class or subclass or interface). At run time, the result of the instanceof operator is true if the value of the Expression is not null.
This is an important property of instanceof operation which makes it useful for type casting checks.
public class Test
{
public static void main (String[] args) throws java.lang.Exception
{
Integer i = null;
Integer j = 10;
//prints false
System.out.println(i instanceof Integer);
//Compiles successfully
System.out.println(j instanceof Integer);
}
}
Output:
false
true
6. Static vs Non static Methods: We cannot call a non-static method on a reference variable with null value, it will throw NullPointerException, but we can call static method with reference variables with null values. Since static methods are bonded using static binding, they won’t throw Null pointer Exception.
public class Test
{
public static void main(String args[])
{
Test obj= null;
obj.staticMethod();
obj.nonStaticMethod();
}
private static void staticMethod()
{
//Can be called by null reference
System.out.println("static method, can be called by null reference");
}
private void nonStaticMethod()
{
//Can not be called by null reference
System.out.print(" Non-static method- ");
System.out.println("cannot be called by null reference");
}
}
Output:
static method, can be called by null referenceException in thread "main"
java.lang.NullPointerException
at Test.main(Test.java:5)
7. == and != The comparison and not equal to operators are allowed with null in Java. This can made useful in checking of null with objects in java.
public class Test
{
public static void main(String args[])
{
//return true;
System.out.println(null==null);
//return false;
System.out.println(null!=null);
}
}
Output:
true
false