java interview questions

                JAVA  Interview Questions

1.what  is a transient variable?

 A transient variable is a variable that may not be serialized.


2.which containers use a border Layout as their default layout?

 The window, Frame and Dialog classes use a border layout as their default layout.


3.Why do threads block on I/O?

 Threads block on i/o (that is enters the waiting state) so that other threads may execute while the i/o Operation is performed.


4. How are Observer and Observable used?

 Objects that subclass the Observable class maintain a list of observers. When an Observable object is updated it invokes the update() method of each of its observers to notify the observers that it has changed state. The Observer interface is implemented by objects that observe Observable objects.

5. What is the difference between an Inner Class and a Sub-Class?

 An Inner class is a class which is nested within another class. An Inner class

has access rights for the class which is nesting it and it can access all variables and

methods defined in the outer class.

A sub-class is a class which inherits from another class called super class. Sub-

class can access all public and protected methods and fields of its super class.

6. What are the various access specifiers for Java classes?

 In Java, access specifiers are the keywords used before a class name

which defines the access scope. The types of access specifiers for classes are:

1. Public : Class,Method,Field is accessible from anywhere.

2. Protected:Method,Field can be accessed from the same class to which they

belong or from the sub-classes,and from the class of same package,but not from

outside.

3. Default: Method,Field,class can be accessed only from the same package and

not from outside of it’s native package.

4. Private: Method,Field can be accessed from the same class to which they belong.

7. What’s the purpose of Static methods and static variables?

 When there is a requirement to share a method or a variable between multiple

objects of a class instead of creating separate copies for each object, we use static

keyword to make a method or variable shared for all objects.

8 What is data encapsulation and what’s its significance?

 Encapsulation is a concept in Object Oriented Programming for combining

properties and methods in a single unit.

Encapsulation helps programmers to follow a modular approach for software

development as each object has its own set of methods and variables and serves its

functions independent of other objects. Encapsulation also serves data hiding

purpose.

9. What is a singleton class? Give a practical example of its usage.

A singleton class in java can have only one instance and hence all its methods and

variables belong to just one instance. Singleton class concept is useful for the

situations when there is a need to limit the number of objects for a class.

Transistor

1. definition:-
   A transistor is a semiconductor device used to amplify and switch electronic signals and electrical power. It is composed of semiconductor material with at least three terminals for connection to an external circuit.
   

   • When it works as an amplifier, it takes in a tiny electric current at one end (an input current) and produces a much bigger electric current (an output current) at the other. In other words, it's a kind of current booster. That comes in really useful in things like hearing aids, one of the first things people used transistors for. A hearing aid has a tiny microphone in it that picks up  from the world around you and turns them into fluctuating electric currents. These are fed into a transistor that boosts them and powers a tiny loudspeaker, so you hear a much louder version of the sounds around you. William Shockley, one of the inventors of the transistor, once explained transistor-amplifiers to a student in a more humorous way: "If you take a bale of hay and tie it to the tail of a mule and then strike a match and set the bale of hay on fire, and if you then compare the energy expended shortly thereafter by the mule with the energy expended by yourself in the striking of the match, you will understand the concept of amplification."
   • Transistors can also work as switches. A tiny electric current flowing through one part of a transistor can make a much bigger current flow through another part of it. In other words, the small current switches on the larger one. This is essentially how all computer chips work. For example, a memory chip contains hundreds of millions or even billions of transistors, each of which can be switched on or off individually. Since each transistor can be in two distinct states, it can store two different numbers, zero and one. With billions of transistors, a chip can store billions of zeros and ones, and almost as many ordinary numbers and letters (or characters, as we call them). More about this in a moment.
   •  it is back to back connection of two diodes
   • transistors are of two types   1) PNP   2)NPN
   • every transistor has three terminals  EMITTER ,BASE,COLLECTOR.

   
   
   

Embedded Systems

• An embedded system can be thought of as a computer hardware system having software embedded in it. An embedded system can be an independent system or it can be a part of a large system.

1. Embedded System contains micro processor and micro controllers.
2. It has hard ware, software,Real Time operating System(RTOS).
3. RTOS controls the application software.
4. It is the basic unit that takes inputs and produces an output after processing the data.

                                                  control unit and execution unit.

• The Control Unit includes a fetch unit for fetching instructions from the memory
• The Execution Unit has circuits that implement the instructions pertaining to data transfer operation and data conversion from one form to another.

Embedded System is combination of Micro processors and Micro controllers.

Micro processor: