Java Advanced

Front 1

What is package access, how does it differ to private, protected, and public access ?

Back 1

Instance variables or methods havingpackage access can be accessed by nameinside the definition of any class in the samepackage

However, neither can be accessed outside thepackage.

Front 2

List the three scope from which a protected member of a class can be accessed, eg: outside class, inside class etc.

Back 2

Inside its own class definition

Inside any class derived from it

In the definition of any class in the same package

Front 3

How does public access differ to package access ?

Back 3

Public access means anyone from anywhere without restriction can access the member or method. However with package access only things inside the same package has access.

Front 4

How would you accomplish the following ?

Call the base class version of an overriddenmethod of the base class.

Back 4

Using super. For example if toString was overriden in class B which inherited from class A. Class B can have a method like :

public String toString(){

return super.toString();

}

Front 5

True or False :

It is only valid to use super to invoke a methodfrom a direct parent

Back 5

True.

Repeating super will not invoke a method from some other ancestor class. For example the following is illegal : super.super.toString();

To accomplish this you would have to chain super calls.

Front 6

Explain the instanceof operator ?

Back 6

interface Domestic {}

class Animal {}

class Dog extends Animal implements Domestic {}

class Cat extends Animal implements Domestic {}

Imagine a dog object, created with Object dog = new Dog(),

then:

dog instanceof Domestic // true - Dog implements Domestic

dog instanceof Animal // true - Dog extends Animal

dog instanceof Dog // true Dog is Dog

dog instanceof Object // true - Object is the parent type of all objects

Front 7

Explain the getClass method and how it is used ?

Back 7

An invocation of getClass() on an objectreturns a representation only of the classthat was used with new to create the object

The results of any two such invocations can be compared with == or != to determine whether ornot they represent the exact same class

Front 8

What is binding ?

Back 8

The process of associating a methoddefinition with a method invocation is calledbinding

Front 9

What is early binding or static binding ?

Back 9

If the method definition is associated with itsinvocation when the code is compiled, thatis called early binding or static binding

Front 10

What is dynamic binding ?

Back 10

If the method definition is associated with itsinvocation when the method is invoked (atrun time), that is called late binding ordynamic binding

Front 11

What is upcasting ? Give an example ?

Back 11

Upcasting casting a derived class to its ancestor.

For example, assume B extends A. Then it is okay to do :

A aa = new B();

Front 12

What is down casting ? Give an example ?

Back 12

Down casting is when we explicitly cast a class to one of its decendents. For example assume B extends A. then B bb =new B();

A aa = (A) nn; is allowed.

Front 13

True or false ?

An abstract class can have any numberof abstract and/or fully defined methods

Back 13

True

Front 14

What is a class called which fully implements all abstract methods of an abstract class ?

Back 14

A Concerete class.

Front 15

Even though one cannot create instances of an abstract class is it okay to reference them by type ?

Back 15

Yes this is legal.

Front 16

When should exceptions be used and when should we check for errors ?

Back 16

Exceptions should be used when something that is not expected occurs. For example when we expect the user to input in a specific format and they do not.

Front 17

How does one access the message of an exception ?

Back 17

use the getMessage() method inherited from the exception class.

Front 18

Why is the following code wrong ?

catch (Exception e){ . . . }

catch (NegativeNumberException e)

{ . . . }

Back 18

Because NegativeNumberException is a of type Exception and therefore the last catch block will never be executed.

Front 19

How do you write the method header which thrown two exception: exception1 and exception1

Back 19

public void method() throws

exception1,exception2{}

Front 20

What are important point regarding overriding methods which throw exceptions ?

Back 20

When a method in a derived class isoverridden, it should have the sameexception classes listed in its throwsclause that it had in the base class– Or it should have a subset of them

A derived class may not add anyexceptions to the throws clause– But it can delete some

Front 21

Exceptions are an example of what kind of programming methodology ?

Back 21

Exception handling is an example of aprogramming methodology known as eventdrivenprogramming

Front 22

What does the finally block do ?

Back 22

The code in the finally block will execute last regardless of whether an exception is thrown and caught or not.

Front 23

Are runtime exceptions checked exception or unchecked exceptions ?

Back 23

Runtime exceptions are unchecked exceptions. Though the programmer may choose to catch them.

Front 24

What is the input stream from the console ?

Back 24

System.in

Front 25

What is the output stream from the console ?

Back 25

System.out

Front 26

What are some benefits of binary files over text files ?

Back 26

An advantage of binary files is that they aremore efficient to process than text files

Front 27

Write an abstract method header.

Back 27

public abstract void foo();

Front 28

How would you open a file test.txt and write the line "File" and then close it.

Back 28

import java.io.PrintWriter;

import java.io.FileOutputStream;

import java.io.FileNotFoundException;

try{

PrintWriter writer = new PrintWriter( new FileOutputStream(test.txt);

writer.println("File");

writer.close();

}catch(Exception e){

}

Front 29

What does the import java.io.FileNotFoundException exception mean in the context of opening a file for writing ?

Back 29

It means that the file could not be created or opened for some reason.

Front 30

What is the behavior of FileOutputStream when there is no textfile with the specified name to open ?

Back 30

A new textfile with that name is created.

Front 31

What is the behavior of FileOutputStream when there is a textfile with the specified name to open ?

Back 31

The textfile is opened and writes to the file will clear the current file and write the data.

Front 32

What java classes are required to read from a text file ?

Back 32

java.util.Scanner

java.io.FileInputStream

java.io.FileNotFoundException

Front 33

Write code to open a text file for reading ?

Back 33

import java.util.Scanner;

import java.io.FileInputStream;

import java.io.FileNotFoundException;

Scanner s = null;

try{

s = new Scanner( new FileInputStream("filename"));

}catch(FileNotFoundException e){

}

Remember that the Scanner class MUST be declared outside the try block !!!

Front 34

Once a file has been opened successfully how would one start reading from it ?

Back 34

Using the normal scanner methods :

Scanner.nextLine();

Scanner.nextInt();

Scanner.nextDouble();

etc.

Front 35

What will a Scanner Object do if nextLine() is invoked when the file has nothing left in it ?

Back 35

It will raise an exception.

Front 36

How can we check whether there are things that can be read in the file ?

Back 36

Use hasNextInt()

hasNextLine()

etc.

Front 37

Write code which will open and read a file until there are no more lines remaining without raising an exception. You can assume imports

Back 37

Scanner s = null;

try{

s = new Scanner(new FileInputStream("filename"))

}catch(FileNotFoundException e){

System.exit(0);

}

while(s.hasNextLine()){

System.out.println(s.nextLine());

}

s.close();

Front 38

How would you use BufferedReader to open a file for reading ?

Back 38

Using the BufferedReader and FileReader classes like so :

BufferedReader reader = null

try{

reader = new BufferedReader( new FileReader("fileName"))

}catch(Exception e){

System.exit(0);

}

Front 39

What methods of the BufferedReader class can be used to read a file which is opened ?

What do they normally return and what do they return when there is nothing to read.

Back 39

readLine and read.

readLine returns the line when there is a line to read and returns null otherwise.

read returns the integer value of the next char if present otherwise returns -1

Front 40

What are the objects which represent the System input stream, System output stream, and the System error stream ?

Back 40

System.in

System.out

System.err

Front 41

How would you redirect the system error stream to a file ?

Back 41

import java.io.PrintStream;

PrintStream s = null;

try{

s = new PrintStream(new FileOutputStream("Filename"));

System.setErr(s);

}catch(Exception e){

System.exit(0);

}

Front 42

How would you open a binary file for output and then write an object to the file ?

Back 42

import java.io.ObjectOutputStream;

import java.io.FileOutputStream;

import java.io.FileNotFoundException;

ObjectOutputStream out = null;

try{

s = new ObjectOutputStream( new FileOutputStream("Filename"));

s.writeObject(myObject);

}catch(Exception e){
System.exit(0);

}

Remember that the writeObject method may throw an io exception.

Front 43

What methods can be used with ObjectOutputStream to write Objects to file ? And what must each object implement ?

Back 43

All objects must implement java.io.Serializable.

methods include :

writeInteger(Integer);

writeObject(Object);

etc.

Front 44

How would you read an object from a binary file ?

Back 44

import java.io.ObjectInputStream;

import java.io.FileInputStream;

import java.io.FileNotFoundException;

ObjectInputStream s = null;

myObject ob = null;

try{

s = new ObjectInputStream( new FileInputStream("FileName"));

ob = (myObject) s.readObject();

}catch(exception e){

System.exit(0);

}

Front 45

What modifier must all instance variables and methods of an interface have ?

Back 45

All methods must be public.

All variables must be public final constant.

Front 46

Can interfaces inherit from classes or abstract classes ?

Back 46

No. Interfaces can only inherit from other interfaces.

Front 47

How would one implement an interface ?

Back 47

Use the syntax implements interface xx. And then fully define all methods in the interface.

Front 48

How would you implement the Comparable interface. Where does the Comparable interface live ?

Back 48

Comparable resides in the java.lang and therefore doesn't have to be imported.

Comparable has one require method

public int compareTo(Object)

Front 49

What do the returns of Comparables compareTo method mean ?

Back 49

-1 this precedes other

0 equals other

+1 procedes other