pre midterm

Question 1

Can you change the return type when overriding a method?

Answer 1

Not for primitive types.

Question 2

What is a covariant return type?

Answer 2

When the return type of an overriding method is changed to a descendent of the original method’s return type.

Question 3

In Java, a function with code that might throw an exception, and that is not caught in said function, must declare it by using the throws.

Answer 3

True

Question 4

If a parent class has a method int func(), a child class can reduce its access modifier (make it more restrictive).

Answer 4

False, the child can only make it less restrictive.

Question 5

A child class automatically overrides every method of its parent class so long as they have the same signature.

Answer 5

False, not “every” method can be overridden (private, static, final).

Question 6

The super() constructor can always be called in a child constructor as long as it is called first.

Answer 6

False, the superclass may not have a default constructor.

Question 7

In Java, a class can both inherit from multiple classes and implement multiple interfaces.

Answer 7

False, a class can only inherit from 1 other class.

Question 8

Is this line of code correct? : public class PartTimeEmployee implements Serializable extends Employee { … }

Answer 8

False, “extend” must come first.

Question 9

Java allows you to declare a variable of an abstract class.

Answer 9

True, you cannot create instances of it but you can declare pointers of it.

Question 10

All types of exceptions must follow the Catch or Declare rule.

Answer 10

False, this isn’t true for unchecked exceptions.

Question 11

Abstract classes can have contructors.

Answer 11

True.

Question 12

When using PrintWriter, there is no way to append to a file - it is always overriten.

Answer 12

False

Question 13

A grand-child class can easily call its grandparent class using super().super()

Answer 13

false

Question 14

Order of catch blocks are wirtten is unimportant

Answer 14

false - more specific first

Question 15

Super constructor is always called, either explicitily or implicitely.

Answer 15

True.

Question 16

A derived class with an overridden method can add exceptions to the list of thrown exceptions.

Answer 16

You can only add unchecked exceptions or checked exceptions that are children of the original method’s thrown exceptions. Generally, false.

Question 17

The finally block is executed after an exception is successfully caught.

Answer 17

True, but not only after an exception is caught, and not if a catch containing system.exit(0); executes.

Question 18

In java it is possible to throw an exception, catch it, then re-throw the same exception if desired.

Answer 18

True, but using another try-catch within the catch block that “re-throws” the exception.

Question 19

In order for polymorphism to work, it is sometimes crucial to create an abstract base class (parent) that has at least one private method.

Answer 19

False - nonsense.

Question 20

If a base class has a method such as: public int fun1(), an inherited class cannot reduce accessibility to this method by declaring it as protected.

Answer 20

True

Question 21

Assume class B is inherited from class A. The following statement, where a1 and b1 are references to Objects A and B, would result in compilation error:
a1 = (A)b1;

Answer 21

False, we’re allowed to up-cast. Even if we were down-casting, we wouldn’t get a compilation error because we are forcing the cast.

Question 22

A constructer of an inherited class has the right to call the this() constructor and the super() constructer, but super() must be called first.

Answer 22

False : this() and super() cannot be used in the same constructor under any conditions.

Question 23

Even if a class A is created as an abstract class, the following code will still be valid :
A a1;

Answer 23

True

Question 24

What’s the order of access modifiers from most restrictive to least?

Answer 24

Private, Default (package), Protected, Public

Question 25

What does the equals() method do when it is not defined by the object calling it?

Answer 25

It compares addresses.

Question 26

Which method from the BufferedReader may return -1?

Answer 26

read()

Question 27

How do you detect the end of a binary file?

Answer 27

When an exception is thrown.

Question 28

When does overriding occur in place of overloading?

Answer 28

When the new method has the same number and types of parameters as in the base class.

Question 29

When a subclass overloads a method from the base class, does the subclass still inherit the method of the same name from the baseclass?

Answer 29

Yes.

Question 30

A final method cannot be overridden.

Answer 30

True.

Question 31

A final class can be used to derive other classes.

Answer 31

False.

Question 32

If super is not used, a call to the default
constructor of the base class is always issued.

Answer 32

False, a (compilation) error occurs in cases where the base class has no default constructor.

Question 33

A derived type can be use in place of an ancestor type.

Answer 33

True.

Question 34

What three things can we change about a method definition to overload a method?

Answer 34

Change number of parameters, change type of parameters or change order of parameters to overload.

Question 35

The super constructor must be the first action of a constructor.

Answer 35

True.

Question 36

An instance variable can be used as an argument to super.

Answer 36

False.

Question 37

An abstract method can be final.

Answer 37

False (complier error).

Question 38

An abstract method can be static.

Answer 38

False.

Question 39

An abstract method can be private.

Answer 39

False.

Question 40

A private attribute is not accessible by a child class.

Answer 40

It is not accessible by name, but one can use setters/getters.

Question 41

Does “extends” or “implements” come first in a class definition?

Answer 41

extends.

Question 42

What does instanceof do that getClass() does not?

Answer 42

Unlike getClass(), instanceof also returns true when the first operand is a descendent of the second operand.

Question 43

What are OOP’s three main mechanisms?

Answer 43

Encapsulation, Inheritance, Polymorphism.

Question 44

Polymorphism is the ability to…

Answer 44

associate many meanings to a single method name.

Question 45

Polymorphism is made possible by what type of binding?

Answer 45

Late / dynamic binding.

Question 46

Static methods can be overridden.

Answer 46

True (compiler allows it), but this may not work as intended because static methods bind statically. So no.

Question 47

What is upcasting?

Answer 47

When an object of a derived class is assigned to a variable of a base class. Upcasting is allowed.

Question 48

What is downcasting?

Answer 48

When an object of a base class is assigned to a variable of a derived class. Downcasting is mostly illegal (you can force it).

Question 49

String S1 = “abc”; String S2 = “abc”;
What does S1 == S2 return?

Answer 49

True, since the Java compiler is smart enough to know that their values are the same, and so it saves space to put them in the same location in memory.

Question 50

Abstract classes must have at least 1 abstract method.

Answer 50

False.

Question 51

A class with an abstract method must be abstract.

Answer 51

True.

Question 52

Can you call super in a class derived from an abstract class?

Answer 52

Yes.

Question 53

How do you define a checked exception?

Answer 53

By extending “Exception”

Question 54

How do you define an unchecked exception?

Answer 54

extend RuntimeException.

Question 55

Can you have zero catch blocks?

Answer 55

Yes if you have a finally block (compile wise).

Question 56

Will try block with zero catch blocks crash?

Answer 56

Yes if an exception is thrown.

Question 57

An incorrect order of catch blocks is a logic error (most specific isn’t placed first).

Answer 57

False, it is a compiler error.

Question 58

Subclasses inherit private methods.

Answer 58

They do, but they will never be invoked directly.

Question 59

Subclasses can invoke private inherited methods.

Answer 59

Only by calling a public method of the parent class that then calls the private method in question.

Question 60

Subclasses can use private inherited methods within their own method definition.

Answer 60

False.

Question 61

Instance variables should be marked with what access modifier?

Answer 61

Generally, private.

Question 62

Package access attributes and methods can be accessed by name by other classes in the same package.

Answer 62

True.

Question 63

Package access attributes/methods can be accessed by name by descendent classes in other packages.

Answer 63

False.

Question 64

Protected access attributes can be accessed by name by unrelated classes in the same package.

Answer 64

True.

Question 65

Casting a reference changes the object held in memory.

Answer 65

False, it only changes what the compiler thinks of the object.

Question 66

instanceof looks at the variable type, not the type in memory.

Answer 66

False.

Question 67

public function(Int… a) { return a; }
is valid code.

Answer 67

(Int… a) in the parameters of a method will allow the method user to input any number of arguments, and these arguments will be stored as an array called a.

Question 68

Java’s File object can point to directories as well as files.

Answer 68

True.

Question 69

The try block scope connects with the catch block scope.

Answer 69

False, the try and catch block scopes are separate.

Question 70

What exception is thrown when the Scanner attempts to read at the end of a file? Is it a checked exception?

Answer 70

“NoSuchElementException” is thrown. This is exception is unchecked.

Question 71

Can you use variable of father class A to invoke method unique to child class B if the variable holds an instance of B?

Answer 71

No, compiler won’t allow it.

Question 72

the getClass() method returns the declared class, rather than the actual class in memory.

Answer 72

False, getClass() returns the actual class in memory.

Question 73

How can you check whether a File object’s file exists?

Answer 73

using .exists()

Question 74

How can you check whether a File object’s file is readable?

Answer 74

using .canRead()

Question 75

In what way are serializable classes restricted if they wish to have instance variables of a class type?

Answer 75

The instance variable classes must also be serializable.

Question 76

What classes can you use to write to a file? (ASCII)

Answer 76

PrintWriter and FileOutputStream

Question 77

What’s more restrictive, default access or protected access?

Answer 77

Default is more restrictive.

Question 78

What methods are used to write to a file using PrintWriter?

Answer 78

print, println and printf.

Question 79

What classes can you use to read from a file? (ASCII)

Answer 79

Scanner with FileInputStream
or
BufferedReader with FileReader

Question 80

Scanner’s useDelimiter() method is a void method.

Answer 80

False, it returns the calling object.

Question 81

What are the 3 common exceptions thrown by a scanner’s methods.

Answer 81

NoSuchElementException (no more tokens), InputMismatchExecption (next token doesn’t match), and IllegalStateException if scanner stream is closed.

Question 82

What are the buffered reader’s 3 methods (besides constructors and close)? What are their return types?

Answer 82

read() returns int, readLine returns a string, and skip() returns a long.

Question 83

List the methods in the class File (14).

Answer 83

exists(), canRead(), setReadOnly(), canWrite(), delete(), createNewFile(), getName(), getPath(), renameTo(File newname), isFile(), isDirectory(), mkdir(), mkdirs(), length().

Question 84

What can you use to write to a binary file?

Answer 84

ObjectOutputStream and FileOutputStream.

Question 85

List the methods in the class ObjectOutputStream.

Answer 85

writeInt() (or any primitive type), writeUTF(str), writeObject(obj), close() and flush().

Question 86

What can you use to read from a binary file?

Answer 86

ObjectInputStream and FileInputStream.

Question 87

What are the methods of the class ObjectInputStream?

Answer 87

readInt() (or any primitive type), readUTF(), readObject(), skipBytes(int) and close().

Question 88

How do you check for end of file using ObjectInputStream?

Answer 88

Waiting for EOFException to be thrown.

Question 89

Can you change the contents of a final array.

Answer 89

Yes