Core Java Question List #5

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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 return -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 a modulo or remainder operator. It returns the remainder of piding the first operand by the second operand.

When can an object reference be case to an interface reference?An object reference is case to an interface reference when the object implements the referred 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 by 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 which it is declared.

What do you understand by private, protected and public?
These are accessibility modifier. Private is the most restrictive, which 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 hierachy

What modifiers may be used with an inner class that is a member of an outer class?
A (non-static) 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.