Static and final

What is the difference between Static and final?

Static variable is a global variable shared by all the instances of objects and it has only single copy.Final variable is a constant variable and it can't be changed.
Static variable can change their values final variables can not be changed they are constants . Static variable it attached to their class not with the object only one copy of static variable is exists and they can be called with reference of their class only final classed can not be extended and their are not static classes. Static variables are initialized when the Class Loader loads the class and hence can be access in the static block declaration of the class without declaring an object. Whereas the final variable is only accessible after the declaration of an instance of an object.


Static variables are marked final.
Static method can't be instantiated.
Static method can't be overloaded & overridden.


Final variable can't be modified.
Final variables are implicitly final.
Final methods can't be overriddern.
Final class can't be subclassed.


State the significance of public, private, protected, default modifiers both singly and in combination and state the effect of package relationships on declared items qualified by these modifiers.
A: public : Public class is visible in other packages, field is visible everywhere (class must be public too)
private : Private variables or methods may be used only by an instance of the same class that declares the variable or method, A private feature may only be accessed by the class that owns the feature.
protected : Is available to all classes in the same package and also available to all subclasses of the class that owns the protected feature.This access is provided even to subclasses that reside in a different package from the class that owns the protected feature.
default :What you get by default ie, without any access modifier (ie, public private or protected).It means that it is visible to all within a particular package.


Q: What if the main method is declared as private?
A: The program compiles properly but at runtime it will give "Main method not public." message.


Q: What if the static modifier is removed from the signature of the main method?
A: Program compiles. But at runtime throws an error "NoSuchMethodError".


Q: What if I write static public void instead of public static void?
A: Program compiles and runs properly.


Q: What if I do not provide the String array as the argument to the method?
A: Program compiles but throws a runtime error "NoSuchMethodError".


Q: What is the first argument of the String array in main method?
A: The String array is empty. It does not have any element. This is unlike C/C++ where the first element by default is the program name.


Q: If I do not provide any arguments on the command line, then the String array of Main method will be empty or null?
A: It is empty. But not null.


Q: How can one prove that the array is not null but empty using one line of code?
A: Print args.length. It will print 0. That means it is empty. But if it would have been null then it would have thrown a NullPointerException on attempting to print args.length.


Q: What environment variables do I need to set on my machine in order to be able to run Java programs?
A: CLASSPATH and PATH are the two variables.


Q: Can an application have multiple classes having main method?
A: Yes it is possible. While starting the application we mention the class name to be run. The JVM will look for the Main method only in the class whose name you have mentioned. Hence there is not conflict amongst the multiple classes having main method.


Q: Can I have multiple main methods in the same class?
A: No the program fails to compile. The compiler says that the main method is already defined in the class.


Q: Do I need to import java.lang package any time? Why ?
A: No. It is by default loaded internally by the JVM.


Q: Can I import same package/class twice? Will the JVM load the package twice at runtime?
A: One can import the same package or same class multiple times. Neither compiler nor JVM complains abt it. And the JVM will internally load the class only once no matter how many times you import the same class.


Q: Are the imports checked for validity at compile time? e.g. will the code containing an import such as java.lang.ABCD compile?
A: Yes the imports are checked for the semantic validity at compile time. The code containing above line of import will not compile. It will throw an error saying,can not resolve symbol
symbol : class ABCD
location: package io
import java.io.ABCD;


Q: Does importing a package imports the subpackages as well? e.g. Does importing com.MyTest.* also import com.MyTest.UnitTests.*?A: No you will have to import the subpackages explicitly. Importing com.MyTest.* will import classes in the package MyTest only. It will not import any class in any of it's subpackage.


Q: What is the difference between declaring a variable and defining a variable?
A: In declaration we just mention the type of the variable and it's name. We do not initialize it. But defining means declaration + initialization.
e.g String s; is just a declaration while String s = new String ("abcd"); Or String s = "abcd"; are both definitions.


Q: What is the default value of an object reference declared as an instance variable?
A: null unless we define it explicitly.


Q: Primitive data types are passed by reference or pass by value?
A: Primitive data types are passed by value.


Q: Objects are passed by value or by reference?
A: Java only supports pass by value. With objects, the object reference itself is passed by value and so both the original reference and parameter copy both refer to the same object .