Core Java



21. Does the order of public and static declaration matter in main() method?  

No. It doesn't matter but void should always come before main().


22. Can a source file contain more than one class declaration?  

Yes a single source file can contain any number of Class declarations but only one of the class can be declared as public.


23. What is a package?  

Package is a collection of related classes and interfaces. package declaration should be first statement in a java class.


24. Which package is imported by default?  

java.lang package is imported by default even without a package declaration.



25. What is classloader ?  

The classloader is a subsystem of JVM that is used to load classes and interfaces.There are many types of classloaders e.g. Bootstrap classloader, Extension classloader, System classloader, Plugin classloader etc.



26. Can a class be declared as protected?  

The protected access modifier cannot be applied to class and interfaces. Methods, fields can be declared protected, however methods and fields in a interface cannot be declared protected.


27. What is the access scope of a protected method?  

A protected method can be accessed by the classes within the same package or by the subclasses of the class in any package.


28. What is the purpose of declaring a variable as final?  

A final variable's value can't be changed. final variables should be initialized before using them.


29. What is the impact of declaring a method as final?  

A method declared as final can't be overridden. A sub-class can't have the same method signature with a different implementation.


30. I don't want my class to be inherited by any other class. What should i do?  

You should declared your class as final. But you can't define your class as final, if it is an abstract class. A class declared as final can't be extended by any other class.


31. Can you give few examples of final classes defined in Java API?  

java.lang.String, java.lang.Math are final classes.


32. How is final different from finally and finalize()?  

Final is a modifier which can be applied to a class or a method or a variable. final class can't be inherited, final method can't be overridden and final variable can't be changed.

Finally is an exception handling code section which gets executed whether an exception is raised or not by the try block code segment.

Finalize() is a method of Object class which will be executed by the JVM just before garbage collecting object to give a final chance for resource releasing activity.



33. When will you define a method as static?  

When a method needs to be accessed even before the creation of the object of the class then we should declare the method as static.


34. What are the restriction imposed on a static method or a static block of code?  

A static method should not refer to instance variables without creating an instance and cannot use "this" operator to refer the instance.


35. Is Empty .java file name a valid source file name ?  

Yes, save your java file by .java only, compile it by javac .java and run by java yourclassname Let's take a simple example:

    //save by .java only  
    class A{  
    public static void main(String args[]){  
    System.out.println("Hello java");  
    }  
    }  
    //compile by javac .java  
    //run by     java A  


36. What is the importance of static variable?  

static variables are class level variables where all objects of the class refer to the same variable. If one object changes the value then the change gets reflected in all the objects.


37. Can we declare a static variable inside a method?  

Static varaibles are class level variables and they can't be declared inside a method. If declared, the class will not compile.


38. What is an Abstract Class and what is it's purpose?  

A Class which doesn't provide complete implementation is defined as an abstract class. Abstract classes enforce abstraction.


39. Can a abstract class be declared final?  

Impossible. An abstract class without being inherited is of no use and hence will result in compile time error.


40. What is use of a abstract variable?  

Variables can't be declared as abstract. only classes and methods can be declared as abstract.


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