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M250 > Exam > Flashcards

Exam Flashcards

1
Q

Is it legal to assign an object to a variable declared to be of the type of one of its superclasses?

A

Yes

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2
Q

Is it legal to assign an object to a variable declared to be of the type of one of its subclasses?

A

No

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3
Q

Are all methods abstract in an abstract class?

A

No

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4
Q

Can an abstract class include a constructor?

A

Yes

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5
Q

Can an abstract class be instanstiated?

A

No

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6
Q

Can an interface be instanstiated?

A

No

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7
Q

Can an interface include a constructor?

A

No

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8
Q

Are all methods in an interface abstract?

A

Yes

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9
Q

Is it legal to assign an object to a variable declared as the type of its superclass if the superclass is abstract?

A

Yes

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10
Q

Is it legal to suppy as a method’s actual argument an object which is a direct or indirect subclass of the type specified by the formal argument?

A

Yes

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11
Q

Is it legal to supply as a method’s actual argument an object whose class is a superclass of the formal argument?

A

No

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12
Q

Explain the concept of substitutability

A

Where a Java program expects an instance of a particular class, it is legal to substitute an instance of any of its direct or indirect subclasses.

sameColourAs(Amphibion)

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13
Q 
Frog kermit = new HoverFrog();
((HoverFrog) kermit).up();

What is the result?

A

The HoverFrog referenced by kermit will move up, and will NOT be cast to a HoverFrog reference variable.

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14
Q

Frog kermit = new HoverFrog();
HoverFrog h = (HoverFrog) kermit;
h.up();

What is the result?

A

The HoverFrog referenced by kermit will be cast and assigned to the HoverFrog reference variable h.

The HoverFrog now referenced by kermit and h will move up.

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15
Q

What is an interface?

A

An interface specifies a list of public abstract methods that a group of unrelated classes must implement so that their instances can receive a common set of messages.

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16
Q

Does a subclass of a class that implements an interface inherit the implementation?

A

Yes

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17
Q

Can interface methods return values?

A

Yes

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18
Q

If varA and varB are two variables, what needs to be true about their types if the statement

varA = varB;

is to compile?

A

If they are of the same type, or the variable on the right is a subtype of the one on the left, the assignment will work without a cast.

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19
Q

If varA and varB are two variables, under what circumstances can you cast a reference variable of one type to another type in an assignment?

A

The variable to which the cast is applied (the one on the right) must be of the same type as, or a supertype of, the variable on the left.

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20
Q

Explain the concept of polymorphism

A

A polymorphic message is a message understood by more than one class. Each class of object can behave in its own way in response to such a message. This occurs via overriding/interfaces.

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21
Q

Give an example of direct interaction between objects

A

frog1.sameColourAs(frog2)

frog1 sends a message to frog2 within the body of the method, which is then used to alter the state of frog1.

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22
Q

Give an example of indirect interaction

A

frog1.setPosition(frog2.getPosition());

A third party (e.g. OUWorkspace, coordinating object) is sending a message to frog2 and using the result to alter the state of frog1.

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23
Q

Can you use the keyword this in a class method?

A

No

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24
Q

Can you use the keyword super in a class method?

A

No

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25
Q

Give an example of static binding

A

The class method Collections.sort() can be resolved at compile time because it is not a message and therefore does not have a receiver.

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26
Q

Give an example of dynamic binding

A

The instance method anAmphibian.right() cannot be resolved at compile time because it could refer to one of several different right() methods depending on the class of the receiver

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27
Q

Are class methods and variables inherited?

A

Yes

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28
Q

Can an instance method override an inherited class method?

A

No - compilation error

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29
Q

Can an instance method override an inherited instance method?

A

Yes

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30
Q

Can a class method override an inherited instance method?

A

No - compilation error

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31
Q

Can a class method override an inherited class method?

A

No - it hides the inherited class method, which can still be accessed by qualifiying the method with the superclass name.

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32
Q

Is super(); always implied in a constructor?

A

Yes, if there is no explicit super() statement.

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33
Q

Is a zero-argument constructor always included by the compiler?

A

No - only when no other constructor is included.

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34
Q

Where should static variables be initialised?

A

Upon declaration

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35
Q

Where should final variables be initialised?

A

Upon declaration or in the constructor

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36
Q

Where should static final variables be initialised?

A

Upon declaration

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37
Q

What are the implications for declaring primitive data type variables and reference type variables as final?

A

For primitive data, the value assigned cannot be altered.

For references variables, the reference cannot be altered but the object it references can.

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38
Q

Are local variables given default values?

A

No

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39
Q

Are instance variables given default values?

A

Yes

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40
Q

Are static variables given default values?

A

Yes

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41
Q

True or false:

Methods that may throw a checked exception must either handle the exception or declare that it throws an exception

A

True

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42
Q

True or false:

Methods that may throw a unchecked exception must either handle the exception or declare that it throws an exception

A

False

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43
Q

Which classes of exception are checked?

A

Exception and its subclasses, excluding RuntimeException and its subclasses.

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44
Q

Which classes of exception are unchecked?

A

Error and its subclasses

RuntimeException and its subclasses

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45
Q

Explain the Design by Contract approach

A

Preconditions and postconditions are established before writing a method. The method is then written to guarantee that if the precondition is met, the postcondition will be guaranteed.

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46
Q

What does the Design by Contract approach require to happen if a method’s precondition is not met?

A

Throw an exception

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47
Q

How do you throw an exception?

A

throw new IllegalArgumentException(“debit “
+ anAmount + is inappropriate. Balance “
+ this.balance);

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48
Q

Write an assert statement that will cause an AssertionError with the argument “oops” if the boolean variable b is not true

A

assert b : “oops”;

49
Q

Assert statements are usually found in what kind of methods?

A

Private helper methods

50
Q

What is the main difference between using an assert statement and throwing an AssertionError or Exception?

A

assert statements are ignored at during the normal running of the program. They are used for testing and documenting assumptions about the program.

51
Q

Do or Don’t?

Use assertions in public methods to check the validity of any arguments received

A

Don’t - instead throw IllegalArgumentsException

52
Q

Do or Don’t?

Use assertions to write code that changes the state of the object

e.g. assert condition : doSomething();

A

Don’t - the behaviour will change when assertions are disabled

53
Q

Do or Don’t?

Use assertions in private helper methods to check the validity of any arguments received

A

Do

54
Q

Do or Don’t?

Use assertions in private methods to test your assumptions about the state of the object receiving the message (this)

A

Do

55
Q

Do or Don’t?

Throw an IllegalArgumentsException in a private method

A

Don’t - we should be in control of any arguments passed to a private method so we should be confident that it will not receive any illegal arguments

56
Q

True or false?

An assert statement must be placed within a try-catch block because it can throw an AssertionError

A

False - AssertionError is unchecked. In fact, such errors should not be caught.

57
Q

How do you declare an array variable?

A

String[ ] names;

Frog[ ] frogPond;

58
Q

How do you create an empty array object?

A

(String[ ] names;)

names = new String[7];

59
Q

How do you find the size of an array?

A

anArray.length

60
Q

Are array components given default values?

A

Yes

61
Q

How do you declare and initialise an array in one line?

A 
String[ ] names = {"Jay", "John", "Ali", "Alan"};
Frog[ ] frogPond = {new Frog(), new Frog()};
62
Q

How do you declare and initialise an array on separate lines?

A

Frog[ ] frogPond;
…
frogPond = new Frog[ ] {new Frog(), new Frog()};

63
Q

How do you access an array element?

A

anArray[5]

64
Q

How do you change the colour of a Frog object stored in an array of Frog objects?

A

frogPond[5].setColour(OUColour.RED);

65
Q

How do you use a for loop to iterate through an array?

A

for (int i = 0; i

66
Q

Can you iterate through an array using a for-each loop?

A

Yes

67
Q

What is the main limitation of for-each loops?

A

It cannot be used to assign values to components.

68
Q

How do you search for a value in an array using a loop?

A

String searchTerm = “term”;
int i = 0; boolean found = false;
while ((i

69
Q

Can arrays be sorted?

A

Yes, provided they are of a type with a natural ordering, by using the Arrays class method: Arrays.sort()

70
Q

How do you search for a value in an array using a Class method?

A

Arrays.sort(myArray);

Arrays.binarySearch(myArray, 9);
Arrays.binarySearch(myArray, “Matthew”);

71
Q

How do you get the size of a String?

A

aString.length()

72
Q

What is string interning?

A

Each time a string literal is created the Java compiler looks to see if it already exists. If it does, it simply creates a new reference to the existing String object rather than creating a new one.

73
Q

What is the difference between the size() and capacity() of a StringBuilder object?

A

The size() is the number of characters it currently holds. The capacity() is the total number of characters it has space to hold.

74
Q

How do you find the size of a Set/Map/List?

A

myCollection.size()

75
Q 
What are the wrapper classes for:
int
long
float
double
short
byte
char
boolean
A 
Integer
Long
Float
Double
Short
Byte
Character
Boolean
76
Q

What is auto-boxing and auto-unboxing?

A

Java uses auto-boxing and auto-unboxing to automate the process of wrapping primitive data into a wrapper class (e.g. int into Integer) and unwrapping it back to primitive data. This allows us to perform arithmetic operations on objects stored in collections and add primitive data to a collection without having to handle the wrapping and unwrapping manually.

77
Q

What is the effect of this code?

Map m = new Map();

m. put(1, “A”);
m. put(1, “B”);

A

m has one entry: (1, “B”).

78
Q

How do you iterate through the key-value pairs in a Map?

e.g. Map cities

A 
for (String eachCity : cities.keySet())
{
  System.out.println(eachCity 
    \+ this.cities.get(eachCity));
}
79
Q

How do you iterate through the keys in a Map?

e.g. Map cities

A

for (String eachCity : cities.keySet())
{
System.out.println(eachCity);
}

80
Q

How do you iterate through the values in a Map?

e.g. Map cities

A

int total = 0;

for (Integer eachPopulation : cities.values())
{
total = total + eachPopulation;
}

81
Q

What happens if you remove an entry from the set returned by keySet() or values()?

A

The key-value pair will be removed from the Map

82
Q

How do you obtain the union of two sets?

A

Set Union = new HashSet(set1);

union.addAll(set2);

83
Q

How do you obtain the intersection of two sets?

A

Set intersection = new HashSet(set1);

intersection.retainAll(set2);

84
Q

How do you obtain the difference between two sets?

A

Set differenceA = new HashSet(set1);

differenceA.removeAll(set2);

Set differenceB = new HashSet(set2);

differenceB.removeAll(set1);

85
Q

What are the features of a List?

Fixed-size/Dynamic
Primitive/Reference values
Ordering
Indexing
Duplicates
A 
Dynamic size
Stores reference values
Ordered by index (can be sorted)
Indexable
Duplicates allowed
86
Q

Arrays consist of several ______, each of which holds an ______.

A

Arrays consist of several COMPONENTS, each of which holds an ELEMENT.

88
Q

What are the features of a Map?

Fixed-size/Dynamic
Primitive/Reference values
Ordering
Indexing
Duplicates
A 
Dynamic size
Stores reference values
Not ordered (SortedMap is)
Keys-values
No duplicate keys allowed
Duplicate values allowed
89
Q

How do you insert an element to a certain position in a List?

A

myList.add(5, “Element Six”);

90
Q

How do you access an element in a List?

A

myList.get(5)

91
Q

How do you search for an element in a list?

A

myList.indexOf(“Matthew”);

92
Q

How do you replace an element of a List?

A

myList.set(5, “Matthew”);

93
Q

How do you convert an array to a List?

A

String[ ] sArray = {“The”, “cat”, “sat”};

List stringList = Arrays.asList(sArray);

94
Q

How do you swap two elements in a List?

A

Collections.swap(herbList, 2, 4);

95
Q

How do you find a sublist in a List?

A

Collections.indexOfSublist(myList, subList)

96
Q

How do you find the minimum and maximum values in a List?

A

String maxScore = Collections.max(scoreList);

String minScore = Collections.min(scoreList);

97
Q

How do you implement Comparable?

A

public class Test implements Comparable

…

public int compareTo(Test obj)
{
return this.getAttribute() - obj.getAttribute();
}

98
Q

How do you override the equals() method?

A 
public boolean equals(Object obj)
{
  RowOfStars row = (RowOfStars) obj;
  return this.getLength() == row.getLength();
}
99
Q

How do you write a hashCode method?

A

public int hashCode()
{
return this.getLength()
}

To match equals()

109
Q

How do you read from a file using BufferedReader?

A

String pathname = OUFileChooser.getFilename();

File aFile = new File(pathname);

BufferedReader bufferedFileReader = null;

try
{
  String currentLine;
  bufferedFileReader = new BufferedReader(new FileReader(aFile));

currentLine = bufferedFileReader.readLine();

  while (currentLine != null)
  {
    System.out.println(currentLine);
    currentLine = bufferedFileReader.readLine();
  }
}
catch (Exception anException)
{
  System.out.println("Error: " + anException);
}
finally
{
  try
  {
    bufferedFileReader.close();
  }
  catch (Exception anException)
  {
    System.out.println("Error: " + anException);
  }
}
110
Q

How do you create an object from a text file using a Scanner?

e.g. an Account object

A

String pathname = OUFileChooser.getFilename();

File aFile = new File(pathname);

Scanner bufferedScanner = null;
Set accountSet = new HashSet();
try
{
  String accountHolder;
  String accountNumber;
  double accountBalance;
  Scanner lineScanner;
  String currentLine;
  bufferedScanner = new Scanner(new BufferedReader(new FileReader(aFile)));
  while (bufferedScanner.hasNextLine())
  {
    currentLine = bufferedScanner.nextLine();
    lineScanner = new Scanner(currentLine);
    lineScanner.useDelimiter(",");
    accountHolder = lineScanner.next();
    accountNumber = lineScanner.next();
    accountBalance = lineScanner.nextDouble();
    accountSet.add(new Account(accountHolder, accountNumber, accountBalance));
  }
}
catch (Exception anException)
{
  System.out.println("Error: " + anException);
}
finally
{
  try
  {
    bufferedScanner.close();
  }
  catch (Exception anException)
  {
    System.out.println("Error: " + anException);
  }
}

return accountSet

111
Q

What are the features of a Set?

Fixed-size/Dynamic
Primitive/Reference values
Ordering
Indexing
Duplicates
A 
Dynamic size
Stores reference values
Not ordered (SortedSet is)
Not indexed
No duplicates
129
Q

What are the features of an array?

Fixed-size/Dynamic
Primitive/Reference values
Ordering
Indexing
Duplicates
A 
Fixed-size
Stores primitive or reference values
Ordered by index
Indexable
Duplicates allowed
130
Q

What is a stream?

A

A stream is the object we use to link a data source to a date sink, enabling data to be transferred from the source to the sink.

131
Q

What do objects of the File class represent?

A

File objects represent the name of a particular file or folder in a platform-independent way

132
Q

How do you write a platform-dependent line break?

A

System.getProperty(“line.separator”)

133
Q

How do write to a file using FileWriter?

A

OUFileChooser.setPath(“C:/Users/Kyuzo/Documents”);

String pathname = OUFileChooser.getFilename();

File aFile = new File(pathname);
File aFileWriter = null:
try
{
  aFileWriter = new FileWriter(aFile);
  aFileWriter.write("To be or not to be");
}
catch (Exception anException)
{
  System.out.println("Error: " + anException);
}
finally
{
  try
  {
    aFileWriter.close();
  }
  catch (Exception anException)
  {
    System.out.println("Error: " + anException);
  }
}
134
Q

How do you append a file using FileWriter?

A

FileWriter aFileWriter = new FileWriter(aFile, true);

135
Q

How do you read from a file using FileReader?

A

String pathname = OUFileChooser.getFilename();

File aFile = new File(pathname);
FileReader aFileReader = null;
try
{
  aFileReader = new FileReader(aFile);

int ch = aFileReader.read();

  while (ch != -1)
  {
    System.out.print((char) ch);
    ch = aFileReader.read();
   }
}
catch (Exception anException)
{
  System.out.println("Error: " + anException);
}
finally
{
  try
  {
    aFileWriter.close();
  }
  catch (Exception anException)
  {
    System.out.println("Error: " + anException);
  }
}
136
Q

How do you write to a file using BufferedWriter?

A

String pathname = OUFileChooser.getFilename();

File aFile = new File(pathname);

BufferedWriter bufferedFileWriter = null;

try
{
  bufferedFileWriter = new BufferedWriter(new FileWriter(aFile));
  bufferedFileWriter.write("Writing to a file");
  bufferedFileWriter.newLine();
  bufferedFileWriter.write("More text");
}
catch (Exception anException)
{
  System.out.println("Error: " + anException);
}
finally
{
  try
  {
    bufferedFileWriter.close();
  }
  catch (Exception anException)
  {
    System.out.println("Error: " + anException);
  }
}
137
Q

Why should a BufferedWriter variable be declared and assigned null before the try block in which it is assigned a BufferedWriter object?

A

So that it is accessible from the scope of the finally block, where it must be closed.

138
Q

What is a variable?

A

A variable is a named block of the computer’s memory that can hold either a value of some primitive type or the address of an object.

139
Q

What is the difference between a reference type variable and a value type variable?

A

A reference type variable holds the address of an object, whereas a value type variable holds a value of some primitive data type.

140
Q

What does the pseudo-variable ‘this’ represent?

A

‘this’ is used within a method to reference the receiver of the message that activated the method.

141
Q

What is a class?

A

A class is a template that serves to describe all instances of that class. It defines the attributes and protocol for objects belonging to it.

142
Q

What is a subclass?

A

A subclass is a class defined in terms of an existing class (its superclass). A subclass is used to create a more specialised version of its superclass, and might include extra variables or methods.

143
Q

Explain the concept of inheritance

A

In a class hierarchy, classes that are lower in the hierarchy (subclasses) inherit all of the methods and variables from classes higher in the hierarchy (their direct and indirect superclasses).

144
Q

Explain the concept of data hiding

A

Java classes can use access modifiers to restrict access to its members (methods and variables), thereby hiding its inner workings and forcing objects of other classes to interact via a limited set of methods.

145
Q

Explain the concept of encapsulation

A

Encapsulation is the bundling together of state and behaviour. Classes achieve this by defining the variables (state) and methods (behaviour) that each of its objects will consist of.

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