Core Java Interview Questions -2

Core Java Interview Questions -2

1. Can a class be declared as static?

We can not declare top level class as static, but only inner class can be declared static.
public class Test
    static class InnerClass
        public static void InnerMethod()
        { System.out.println("Static Inner Class!"); }
    public static void main(String args[])
//output: Static Inner Class!

2. What modifiers are allowed for methods in an Interface?

Only public and abstract modifiers are allowed for methods in interfaces.

3. What is a Marker Interface?

An Interface which doesn't have any declaration inside but still enforces a mechanism.

4. Why is an Interface be able to extend more than one Interface but a Class can't extend more than one Class?

Basically Java doesn't allow multiple inheritance, so a Class is restricted to extend only one Class. But an Interface is a pure abstraction model and doesn't have inheritance hierarchy like classes(do remember that the base class of all classes is Object). So an Interface is allowed to extend more than one Interface.

5. What is Externalizable?

Externalizable is an Interface that extends Serializable Interface. And sends data into Streams in Compressed Format. It has two methods, writeExternal(ObjectOuput out) and readExternal(ObjectInput in)

6. Can a method inside a Interface be declared as final?

No not possible. Doing so will result in compilation error. public and abstract are the only applicable modifiers for method declaration in an interface.

7. What is a local, member and a class variable?

Variables declared within a method are "local" variables.
Variables declared within the class i.e not within any methods are "member" variables (global variables).
Variables declared within the class i.e not within any methods and are defined as "static" are class variables.

8. Which object oriented Concept is achieved by using overloading and


9. What is an abstract method?

An abstract method is a method whose implementation is deferred to a subclass.

10. Can we define private and protected modifiers for variables in interfaces?


11. Which non-Unicode letter characters may be used as the first character of an identifier?
The non-Unicode letter characters $ and _ may appear as the first character of an identifier

12. What is the % operator?
It is referred to as the modulo or remainder operator. It returns the remainder of dividing the first operand by the second operand.

13. What is an object's lock and which object's have locks?
An object's lock is a mechanism that is used by multiple threads to obtain synchronized access to the object. A thread may execute a synchronized method of an object only after it has acquired the object's lock. All objects and classes have locks. A class's lock is acquired on the class's Class object.

14. What do you understand by private, protected and public?
These are accessibility modifiers. Private is the most restrictive, while public is the least restrictive. There is no real difference between protected and the default type (also known as package protected) within the context of the same package, however the protected keyword allows visibility to a derived class in a different package.

15. What value does read() return when it has reached the end of a file?
The read() method returns -1 when it has reached the end of a file.

16. What restrictions are placed on method overloading?
Two methods may not have the same name and argument list but different return types.

17. How many bits are used to represent Unicode, ASCII, UTF-16, and UTF-8 characters?

Unicode requires 16 bits and ASCII require 7 bits Although the ASCII character set uses only 7 bits, it is usually represented as 8 bits.
UTF-8 represents characters using 8, 16, and 18 bit patterns.
UTF-16 uses 16-bit and larger bit patterns.

18. What is the difference between a static and a non-static inner class?
A non-static inner class may have object instances that are associated with instances of the class's outer class. A static inner class does not have any object instances.

19. What is a native method?
A native method is a method that is implemented in a language other than Java.

20. What is the return type of a program's main() method?

21. Can an anonymous class be declared as implementing an interface and extending a class?
An anonymous class may implement an interface or extend a superclass, but may not be declared to do both.

22. If a variable is declared as private, where may the variable be accessed?
A private variable may only be accessed within the class in which it is declared.

23. When can an object reference be cast to an interface reference?
An object reference be cast to an interface reference when the object implements the referenced interface.

24. What are order of precedence and associativity, and how are they used?
Order of precedence determines the order in which operators are evaluated in expressions. Associatity determines whether an expression is evaluated left-to-right or right-to-left.

25. Which characters may be used as the second character of an identifier, but not as the first character of an identifier?

The digits 0 through 9 may not be used as the first character of an identifier but they may be used after the first character of an identifier.

26. What is Downcasting?

Downcasting is the casting from a general to a more specific type, i.e. casting down the hierarchy

27. When does the compiler supply a default constructor for a class?

The compiler supplies a default constructor for a class if no other constructors are provided.

28. What is the range of the char type?
The range of the char type is 0 to 216 - 1 (i.e. 0 to 65535.)

29. Can a Byte object be cast to a double value?
No, an object cannot be cast to a primitive value.

30. What is casting?
There are two types of casting, casting between primitive numeric types and casting between object references. Casting between numeric types is used to convert larger values, such as double values, to smaller values, such as byte values. Casting between object references is used to refer to an object by a compatible class, interface, or array type reference.

31. What restrictions are placed on the location of a package statement within a source code file?
A package statement must appear as the first line in a source code file (excluding blank lines and comments).

32. Is the ternary operator written x : y? z or x? y : z?
It is written x? y : z.

33. What happens when you add a double value to a String?
The result is a String object.

34. Which class is extended by all other classes?
The Object class is extended by all other classes.

35. What modifiers can be used with a local inner class?
A local inner class may be final or abstract.

36. Does a class inherit the constructors of its superclass?
A class does not inherit constructors from any of its superclasses.

37. What does it mean that a method or field is "static"?
Static variables and methods are instantiated only once per class. In other words they are class variables, not instance variables. If you change the value of a static variable in a particular object, the value of that variable changes for all instances of that class. Static methods can be referenced with the name of the class rather than the name of a particular object of the class (though that works too). That's how library methods likeSystem.out.println() work. out is a static field in the java.lang.System class.

38. Is null a keyword?

The null value is not a keyword.

39. Are true and false keywords?
The values true and false are not keywords.

40. How is rounding performed under integer division?
The fractional part of the result is truncated. This is known as rounding toward zero.

41. What modifiers may be used with an inner class that is a member of an outer class?

A (non-local) inner class may be declared as public, protected, private, static, final, or abstract.

42. What are the legal operands of the instanceof operator?
The left operand is an object reference or null value and the right operand is a class, interface, or array type.

43. If a class is declared without any access modifiers, where may the class be accessed?
A class that is declared without any access modifiers is said to have package access. This means that the class can only be accessed by other classes and interfaces that are defined within the same package.

44. Name the eight primitive Java types.

The eight primitive types are byte, char, short, int, long, float, double, and boolean.

45. What is the difference between inner class and nested class?
When a class is defined within a scope od another class, then it becomes inner class. If the access modifier of the inner class is static, then it becomes nested class.

46. What is the difference between a while statement and a do while statement?

A while statement checks at the beginning of a loop to see whether the next loop iteration should occur. A do while statement checks at the end of a loop to see whether the next iteration of a loop should occur. The do whilestatement will always execute the body of a loop at least once.

47. Why isn't there operator overloading?

Because C++ has proven by example that operator overloading makes code almost impossible to maintain.

48. What restrictions are placed on the values of each case of a switch statement?

During compilation, the values of each case of a switch statement must evaluate to a value that can be promoted to an int value.

49. If a method is declared as protected, where may the method be accessed?

A protected method may only be accessed by classes or interfaces of the same package or by subclasses of the class in which it is declared.

50. What is the range of the short type?

The range of the short type is -(215) to 215 - 1. (i.e. -32,768 to 32,767)

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