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 that 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.
Tóm Tắt
Java
public class Test
{
public static void main (String[] args) throws java.lang.Exception
{
Object obj = NULL;
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 a default value null.
Java
public class Test
{
private static Object obj;
public static void main(String args[])
{
System.out.println("Value of object obj is : " + obj);
}
}
Output:
Value of object obj is : null
3. Type of null: Unlike the 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.
Java
public class Test {
public static void main(String[] args)
throws java.lang.Exception
{
Integer i = null;
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.
Java
public class Test {
public static void main(String[] args)
throws java.lang.Exception
{
Integer i = null;
Integer j = 10;
System.out.println(i instanceof Integer);
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.
Java
public class Test {
public static void main(String args[])
{
Test obj = null;
obj.staticMethod();
obj.nonStaticMethod();
}
private static void staticMethod()
{
System.out.println(
" static method,
can be called by null reference & quot;);
}
private void nonStaticMethod()
{
System.out.print("
Non - static method - ");
System.out.println(
"
cannot be called by null reference & quot;);
}
}
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.
Java
public class Test {
public static void main(String args[])
{
System.out.println(null == null);
System.out.println(null != null);
}
}
Output:
true false
8. “null” can be passed as an argument in the method :
We can pass the null as an argument in java and we can print the same. The data type of argument should be Reference Type. But the return type of method could be any type as void, int, double or any other reference type depending upon the logic of program.
Here, the method “print_null” will simply print the argument which is passed from the main method.
Java
import java.io.*;
class GFG {
public static void print_null(String str)
{
System.out.println("Hey, I am : " + str);
}
public static void main(String[] args)
{
GFG.print_null(null);
}
}
Output :
Hey, I am : null
9. ‘+’ operator on null :
We can concatenate the null value with String variables in java. It is considered a concatenation in java.
Here, the null will only be concatenated with the String variable. If we use “+” operator with null and any other type(Integer, Double, etc.,) other than String, it will throw an error message.
Integer a=null+7 will throw an error message as “bad operand types for binary operator ‘+’ “
Java
import java.io.*;
class GFG {
public static void main(String[] args)
{
String str1 = null;
String str2 = "_value";
String output = str1 + str2;
System.out.println("Concatenated value : "
+ output);
}
}
Output
Concatenated value : null_value
This article is contributed by Gaurav Miglani. If you like GeeksforGeeks and would like to contribute, you can also write an article using write.geeksforgeeks.org or mail your article to [email protected]. See your article appearing on the GeeksforGeeks main page and help other Geeks.
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