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49 Cards in this Set
- Front
- Back
What is package access, how does it differ to private, protected, and public access ? |
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. |
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List the three scope from which a protected member of a class can be accessed, eg: outside class, inside class etc. |
Inside its own class definition Inside any class derived from it In the definition of any class in the same package |
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How does public access differ to package access ? |
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. |
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How would you accomplish the following ? Call the base class version of an overriddenmethod of the base class. |
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(); } |
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True or False : It is only valid to use super to invoke a methodfrom a direct parent |
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. |
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Explain the instanceof operator ? |
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 |
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Explain the getClass method and how it is used ? |
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 |
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What is binding ? |
The process of associating a methoddefinition with a method invocation is calledbinding |
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What is early binding or static binding ? |
If the method definition is associated with itsinvocation when the code is compiled, thatis called early binding or static binding |
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What is dynamic binding ? |
If the method definition is associated with itsinvocation when the method is invoked (atrun time), that is called late binding ordynamic binding |
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What is upcasting ? Give an example ? |
Upcasting casting a derived class to its ancestor. For example, assume B extends A. Then it is okay to do : A aa = new B(); |
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What is down casting ? Give an example ? |
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. |
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True or false ? An abstract class can have any numberof abstract and/or fully defined methods |
True |
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What is a class called which fully implements all abstract methods of an abstract class ? |
A Concerete class. |
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Even though one cannot create instances of an abstract class is it okay to reference them by type ? |
Yes this is legal. |
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When should exceptions be used and when should we check for errors ? |
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. |
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How does one access the message of an exception ? |
use the getMessage() method inherited from the exception class. |
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Why is the following code wrong ? catch (Exception e){ . . . } catch (NegativeNumberException e) { . . . } |
Because NegativeNumberException is a of type Exception and therefore the last catch block will never be executed. |
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How do you write the method header which thrown two exception: exception1 and exception1 |
public void method() throws exception1,exception2{} |
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What are important point regarding overriding methods which throw exceptions ? |
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 |
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Exceptions are an example of what kind of programming methodology ? |
Exception handling is an example of aprogramming methodology known as eventdrivenprogramming |
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What does the finally block do ? |
The code in the finally block will execute last regardless of whether an exception is thrown and caught or not. |
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Are runtime exceptions checked exception or unchecked exceptions ? |
Runtime exceptions are unchecked exceptions. Though the programmer may choose to catch them. |
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What is the input stream from the console ? |
System.in |
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What is the output stream from the console ? |
System.out |
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What are some benefits of binary files over text files ? |
An advantage of binary files is that they aremore efficient to process than text files |
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Write an abstract method header. |
public abstract void foo(); |
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How would you open a file test.txt and write the line "File" and then close it. |
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){ } |
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What does the import java.io.FileNotFoundException exception mean in the context of opening a file for writing ? |
It means that the file could not be created or opened for some reason. |
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What is the behavior of FileOutputStream when there is no textfile with the specified name to open ? |
A new textfile with that name is created. |
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What is the behavior of FileOutputStream when there is a textfile with the specified name to open ? |
The textfile is opened and writes to the file will clear the current file and write the data. |
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What java classes are required to read from a text file ? |
java.util.Scanner java.io.FileInputStream java.io.FileNotFoundException |
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Write code to open a text file for reading ? |
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 !!! |
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Once a file has been opened successfully how would one start reading from it ? |
Using the normal scanner methods : Scanner.nextLine(); Scanner.nextInt(); Scanner.nextDouble(); etc. |
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What will a Scanner Object do if nextLine() is invoked when the file has nothing left in it ? |
It will raise an exception. |
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How can we check whether there are things that can be read in the file ? |
Use hasNextInt() hasNextLine() etc. |
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Write code which will open and read a file until there are no more lines remaining without raising an exception. You can assume imports |
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(); |
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How would you use BufferedReader to open a file for reading ? |
Using the BufferedReader and FileReader classes like so : BufferedReader reader = null try{ reader = new BufferedReader( new FileReader("fileName")) }catch(Exception e){ System.exit(0); } |
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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. |
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 |
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What are the objects which represent the System input stream, System output stream, and the System error stream ? |
System.in
System.out System.err |
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How would you redirect the system error stream to a file ? |
import java.io.PrintStream; PrintStream s = null; try{ s = new PrintStream(new FileOutputStream("Filename")); System.setErr(s); }catch(Exception e){ System.exit(0); } |
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How would you open a binary file for output and then write an object to the file ? |
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){ } Remember that the writeObject method may throw an io exception. |
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What methods can be used with ObjectOutputStream to write Objects to file ? And what must each object implement ? |
All objects must implement java.io.Serializable. methods include : writeInteger(Integer); writeObject(Object); etc. |
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How would you read an object from a binary file ? |
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); } |
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What modifier must all instance variables and methods of an interface have ? |
All methods must be public. All variables must be public final constant. |
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Can interfaces inherit from classes or abstract classes ? |
No. Interfaces can only inherit from other interfaces. |
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How would one implement an interface ? |
Use the syntax implements interface xx. And then fully define all methods in the interface. |
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How would you implement the Comparable interface. Where does the Comparable interface live ? |
Comparable resides in the java.lang and therefore doesn't have to be imported.
public int compareTo(Object) |
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What do the returns of Comparables compareTo method mean ? |
-1 this precedes other 0 equals other +1 procedes other |