Since static methods can be called directly from the class (i.e. ClassName.methodName), why it is required to call a static method with the object of the class?
If someone knows then, elaborate with example.
public static void methodA(){
}
4 Answers
The following code contains an example, in which a static method is called via a null reference.
public class Test {
public static void main(String... args) {
Test test = null;
test.greeting(); // call with null reference
}
public static void greeting() {
System.out.println("Hello World");
}
}
Because Test::greeting is a static method, the expression test.greeting() is identical to Test.greeting(). For that reason, there is no NullPointerException thrown at runtime.
7 Comments
LMK
sir, could please explain why NPE is not throwing ?
Suresh Atta
@LMK Attached the full reason in my post which is explained in spec. Please have a look.
JamesB
This is fairly self explanatory. The call to greeting is not using the instance (which is null) so no NPE occurs.
vermaraj
If i change static method to instance method above, then it will throw NullPointerException.
LMK
String s = null; System.out.println(""+s.trim()); throws NPE why ?
|
There is no need for an instance while invoking static member or method.
Since static members belongs to class rather than instance.
Example 15.11.1-2. Receiver Variable Is Irrelevant For static Field Access
The following program demonstrates that a null reference may be used to access a class (static) variable without causing an exception:
The example from spec it self.
class Test3 {
static String mountain = "Chocorua";
static Test3 favorite(){
System.out.print("Mount ");
return null;
}
public static void main(String[] args) {
System.out.println(favorite().mountain);
}
}
And the analysis of why it is happening
Even though the result of favorite() is null, a NullPointerException is not thrown. That "Mount " is printed demonstrates that the Primary expression is indeed fully evaluated at run time, despite the fact that only its type, not its value, is used to determine which field to access (because the field mountain is static).
4 Comments
LMK
String s = null; System.out.println(""+s.trim()); throws NPE why ?
Suresh Atta
@LMK
trim() is not a static method here :) it needs specific instance to perform trim operation.Suresh Atta
@LMK There is no silly questions. Only silly people are there who afraid to ask a question :) Happy coding.
Kanad
Perfect answer using exact reference! That's what I was looking for. This should be marked as correct answer to the question.
Very well you can call a static method with null object.
See the example below.
public class Hashing {
public static void Hash() {
System.out.println("hello");
}
public static void main(String[] args) {
Hashing h = null;
h.Hash();
}
}
Above code snippet will print hello
Because at the compile time h.hash() will be converted to Hashing.hash() since hash() is a static method.
When I de-compiled .class file I got this code.
/*
* Decompiled with CFR 0_114.
*/
import java.io.PrintStream;
public class Hashing {
public static void Hash() {
System.out.println("hello");
}
public static void main(String[] args) {
Object h = null;
Hashing.Hash();
}
}
As you can see in the above snippet h.Hash(); is converted to Hashing.Hash();
HTH!!
1 Comment
Thakur Amit
perfect explanation! :thumbsup
Yes,and this is a strange situation. Below is the example.
public class Test{
public static String foo(){
System.out.println("foo");
return "";
}
public static void main(String args[]){
Test t=null;
t.foo();//no error will be thrown
}
}
Output: foo
Explanation : it is expected this should throw a null pointer exception but it just gives you a warning that "The static method foo() from the type Test should be accessed in a static way". But when executing it will work.
((YourClass)null).methodA()?