pre midterm Flashcards
Can you change the return type when overriding a method?
Not for primitive types.
What is a covariant return type?
When the return type of an overriding method is changed to a descendent of the original method’s return type.
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.
True
If a parent class has a method int func(), a child class can reduce its access modifier (make it more restrictive).
False, the child can only make it less restrictive.
A child class automatically overrides every method of its parent class so long as they have the same signature.
False, not “every” method can be overridden (private, static, final).
The super() constructor can always be called in a child constructor as long as it is called first.
False, the superclass may not have a default constructor.
In Java, a class can both inherit from multiple classes and implement multiple interfaces.
False, a class can only inherit from 1 other class.
Is this line of code correct? : public class PartTimeEmployee implements Serializable extends Employee { … }
False, “extend” must come first.
Java allows you to declare a variable of an abstract class.
True, you cannot create instances of it but you can declare pointers of it.
All types of exceptions must follow the Catch or Declare rule.
False, this isn’t true for unchecked exceptions.
Abstract classes can have contructors.
True.
When using PrintWriter, there is no way to append to a file - it is always overriten.
False
A grand-child class can easily call its grandparent class using super().super()
false
Order of catch blocks are wirtten is unimportant
false - more specific first
Super constructor is always called, either explicitily or implicitely.
True.
A derived class with an overridden method can add exceptions to the list of thrown exceptions.
You can only add unchecked exceptions or checked exceptions that are children of the original method’s thrown exceptions. Generally, false.
The finally block is executed after an exception is successfully caught.
True, but not only after an exception is caught, and not if a catch containing system.exit(0); executes.
In java it is possible to throw an exception, catch it, then re-throw the same exception if desired.
True, but using another try-catch within the catch block that “re-throws” the exception.
In order for polymorphism to work, it is sometimes crucial to create an abstract base class (parent) that has at least one private method.
False - nonsense.
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.
True
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;
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.
A constructer of an inherited class has the right to call the this() constructor and the super() constructer, but super() must be called first.
False : this() and super() cannot be used in the same constructor under any conditions.
Even if a class A is created as an abstract class, the following code will still be valid :
A a1;
True
What’s the order of access modifiers from most restrictive to least?
Private, Default (package), Protected, Public
What does the equals() method do when it is not defined by the object calling it?
It compares addresses.
Which method from the BufferedReader may return -1?
read()
How do you detect the end of a binary file?
When an exception is thrown.
When does overriding occur in place of overloading?
When the new method has the same number and types of parameters as in the base class.
When a subclass overloads a method from the base class, does the subclass still inherit the method of the same name from the baseclass?
Yes.
A final method cannot be overridden.
True.
A final class can be used to derive other classes.
False.
If super is not used, a call to the default
constructor of the base class is always issued.
False, a (compilation) error occurs in cases where the base class has no default constructor.
A derived type can be use in place of an ancestor type.
True.
What three things can we change about a method definition to overload a method?
Change number of parameters, change type of parameters or change order of parameters to overload.
The super constructor must be the first action of a constructor.
True.
An instance variable can be used as an argument to super.
False.
An abstract method can be final.
False (complier error).
An abstract method can be static.
False.
An abstract method can be private.
False.
A private attribute is not accessible by a child class.
It is not accessible by name, but one can use setters/getters.
Does “extends” or “implements” come first in a class definition?
extends.
What does instanceof do that getClass() does not?
Unlike getClass(), instanceof also returns true when the first operand is a descendent of the second operand.
What are OOP’s three main mechanisms?
Encapsulation, Inheritance, Polymorphism.
Polymorphism is the ability to…
associate many meanings to a single method name.
Polymorphism is made possible by what type of binding?
Late / dynamic binding.
Static methods can be overridden.
True (compiler allows it), but this may not work as intended because static methods bind statically. So no.
What is upcasting?
When an object of a derived class is assigned to a variable of a base class. Upcasting is allowed.
What is downcasting?
When an object of a base class is assigned to a variable of a derived class. Downcasting is mostly illegal (you can force it).
String S1 = “abc”; String S2 = “abc”;
What does S1 == S2 return?
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.
Abstract classes must have at least 1 abstract method.
False.
A class with an abstract method must be abstract.
True.
Can you call super in a class derived from an abstract class?
Yes.
How do you define a checked exception?
By extending “Exception”
How do you define an unchecked exception?
extend RuntimeException.
Can you have zero catch blocks?
Yes if you have a finally block (compile wise).
Will try block with zero catch blocks crash?
Yes if an exception is thrown.
An incorrect order of catch blocks is a logic error (most specific isn’t placed first).
False, it is a compiler error.
Subclasses inherit private methods.
They do, but they will never be invoked directly.
Subclasses can invoke private inherited methods.
Only by calling a public method of the parent class that then calls the private method in question.
Subclasses can use private inherited methods within their own method definition.
False.
Instance variables should be marked with what access modifier?
Generally, private.
Package access attributes and methods can be accessed by name by other classes in the same package.
True.
Package access attributes/methods can be accessed by name by descendent classes in other packages.
False.
Protected access attributes can be accessed by name by unrelated classes in the same package.
True.
Casting a reference changes the object held in memory.
False, it only changes what the compiler thinks of the object.
instanceof looks at the variable type, not the type in memory.
False.
public function(Int… a) { return a; }
is valid code.
(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.
Java’s File object can point to directories as well as files.
True.
The try block scope connects with the catch block scope.
False, the try and catch block scopes are separate.
What exception is thrown when the Scanner attempts to read at the end of a file? Is it a checked exception?
“NoSuchElementException” is thrown. This is exception is unchecked.
Can you use variable of father class A to invoke method unique to child class B if the variable holds an instance of B?
No, compiler won’t allow it.
the getClass() method returns the declared class, rather than the actual class in memory.
False, getClass() returns the actual class in memory.
How can you check whether a File object’s file exists?
using .exists()
How can you check whether a File object’s file is readable?
using .canRead()
In what way are serializable classes restricted if they wish to have instance variables of a class type?
The instance variable classes must also be serializable.
What classes can you use to write to a file? (ASCII)
PrintWriter and FileOutputStream
What’s more restrictive, default access or protected access?
Default is more restrictive.
What methods are used to write to a file using PrintWriter?
print, println and printf.
What classes can you use to read from a file? (ASCII)
Scanner with FileInputStream
or
BufferedReader with FileReader
Scanner’s useDelimiter() method is a void method.
False, it returns the calling object.
What are the 3 common exceptions thrown by a scanner’s methods.
NoSuchElementException (no more tokens), InputMismatchExecption (next token doesn’t match), and IllegalStateException if scanner stream is closed.
What are the buffered reader’s 3 methods (besides constructors and close)? What are their return types?
read() returns int, readLine returns a string, and skip() returns a long.
List the methods in the class File (14).
exists(), canRead(), setReadOnly(), canWrite(), delete(), createNewFile(), getName(), getPath(), renameTo(File newname), isFile(), isDirectory(), mkdir(), mkdirs(), length().
What can you use to write to a binary file?
ObjectOutputStream and FileOutputStream.
List the methods in the class ObjectOutputStream.
writeInt() (or any primitive type), writeUTF(str), writeObject(obj), close() and flush().
What can you use to read from a binary file?
ObjectInputStream and FileInputStream.
What are the methods of the class ObjectInputStream?
readInt() (or any primitive type), readUTF(), readObject(), skipBytes(int) and close().
How do you check for end of file using ObjectInputStream?
Waiting for EOFException to be thrown.
Can you change the contents of a final array.
Yes
