Core Java Interview Questions Part 3
What is the most important feature of Java?
Java is a platform independent language.
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.
- 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.
- 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.
- Can a abstract class be declared final?
Not possible. An abstract class without being inherited is of no use and hence will result in compile time error.
- What is use of a abstract variable?
Variables can't be declared as abstract. only classes and methods can be declared as abstract.
- Can you create an object of an abstract class?
Not possible. Abstract classes can't be instantiated.
- Can a abstract class be defined without any abstract methods?
Yes it's possible. This is basically to avoid instance creation of the class.
- Class C implements Interface I containing method m1 and m2 declarations. Class C has provided implementation for method m2. Can i create an object of Class C?
No not possible. Class C should provide implementation for all the methods in the Interface I. Since Class C didn't provide implementation for m1 method, it has to be declared as abstract. Abstract classes can't be instantiated.
- 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.
- Can an Interface implement another Interface?
Intefaces doesn't provide implementation hence a interface cannot implement another interface.
- Can an Interface extend another Interface?
Yes an Interface can inherit another Interface, for that matter an Interface can extend more than one Interface.
- Can a Class extend more than one Class?
Not possible. A Class can extend only one class but can implement any number of Interfaces.
- 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.
- Can an Interface be final?
Not possible. Doing so so will result in compilation error.
- Can a class be defined inside an Interface?
Yes it's possible.
- Can an Interface be defined inside a class?
Yes it's possible.
- What is a Marker Interface?
An Interface which doesn't have any declaration inside but still enforces a mechanism.
- Which object oriented Concept is achieved by using overloading and overriding?
- Why does Java not support operator overloading?
Operator overloading makes the code very difficult to read and maintain. To maintain code simplicity, Java doesn't support operator overloading.
- Can we define private and protected modifiers for variables in interfaces?
- 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).
- What modifiers are allowed for methods in an Interface?
Only public and abstract modifiers are allowed for methods in interfaces.
- 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.
- What is an abstract method?
An abstract method is a method whose implementation is deferred to a subclass.
- 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.
- Can a Byte object be cast to a double value?
No, an object cannot be cast to a primitive value.
- 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.
- 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.
- 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.
- 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.
- Which class is extended by all other classes?
The Object class is extended by all other classes.
- 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.
- What restrictions are placed on method overloading?
Two methods may not have the same name and argument list but different return types.
- 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.
- What is the return type of a program's main() method?
- 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.
- 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.
- What is Downcasting ?
Downcasting is the casting from a general to a more specific type, i.e. casting down the hierarchy.
- 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.
- 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.
- 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).
- What is a native method?
A native method is a method that is implemented in a language other than Java.
- 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.
- 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.
- What is the range of the char type?
The range of the char type is 0 to 216 - 1 (i.e. 0 to 65535.)
- 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)
- Why isn't there operator overloading?
Because C++ has proven by example that operator overloading makes code almost impossible to maintain.
- 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 like System.out.println() work. out is a static field in the java.lang.System class.