Core Java Interview Question Page 27

Modifiers

Question: What are access modifiers

Answer: These public, protected and private, these can be applied to class, variables, constructors and methods. But if you don't specify an access modifier then it is considered as Friendly

Question: Can protected or friendly features be accessed from different packages

Answer: No when features are friendly or protected they can be accessed from all the classes in that package but not from classes in another package

Question: How can you access protected features from another package

Answer: You can access protected features from other classes by subclassing the that class in another package, but this cannot be done for friendly features

Question: What are the rules for overriding

Answer: 

Private method can be overridden by private, friendly, protected or public methods
Friendly method can be overridden by friendly, protected or public methods
Protected method can be overridden by protected or public methods
Public method can be overridden by public method

Question: Explain modifier final

Answer: Final can be applied to classes, methods and variables and the features cannot be changed. Final class cannot be subclassed, methods cannot be overridden

Question: Can you change the reference of the final object

Answer: No the reference cannot be change, but the data in that object can be changed

Question: Can abstract modifier be applied to a variable

Answer: No it is applied only to class and methods

Question: Can abstract class be instantiated

Answer: No abstract class cannot be instantiated i.e you cannot create a new object of this class

Question: When does the compiler insist that the class must be abstract

Answer: 

If one or more methods of the class are abstract.
If class inherits one or more abstract methods from the parent abstract class and no implementation is provided for that method
If class implements an interface and provides no implementation for those methods

Question: How is abstract class different from final class

Answer: Abstract class must be subclassed and final class cannot be subclassed

Question: Where can static modifiers be used

Answer: They can be applied to variables, methods and even a block of code, static methods and variables are not associated with any instance of class

Question: When are the static variables loaded into the memory

Answer: During the class load time

Question: When are the non static variables loaded into the memory

Answer: They are loaded just before the constructor is called

Question: How can you reference static variables

Answer: Via reference to any instance of the class

Computer comp = new Computer ();
comp.harddisk where hardisk is a static variable
comp.compute() where compute is a method

Via the class name

Computer.harddisk
Computer.compute()

Question: Can static method use non static features of there class

Answer: No they are not allowed to use non static features of the class, they can only call static methods and can use static data

Question: What is static initializer code

Answer: A class can have a block of initializer code that is simply surrounded by curly braces and labeled as static e.g.
public class Demo{
static int =10;
static{
System.out.println(?Hello world');
}
}

And this code is executed exactly once at the time of class load

Where is native modifier used It can refer only to methods and it indicates that the body of the method is to be found else where and it is usually written in non java language

Question: What are transient variables

Answer: A transient variable is not stored as part of objects persistent state and they cannot be final or static

Question: What is synchronized modifier used for

Answer: It is used to control access of critical code in multithreaded programs

Question: What are volatile variables

Answer: It indicates that these variables can be modified asynchronously