CMSC 132

What do interfaces do?

Facilitate polymorphism

True or False: you can declare static final variables in an interface?

True

Give an example of how to write an interface?

public interface CanEat{

public void locateFood(); //abstract method

public default void eat(){ //default method

syso(nom nom);

}

}

True or False: there cannot be constructors in an interface?

True

True or False: you can put a static method in an interface along with its implementation?

True

When should you use @Override?

When rewriting any method that has already been inherited. The names of the method and the type and order of the parameters match.

When should you use @Overload?

When there are multiple methods have the same name in the same class but a different order and types of parameters

True or False: you can implement multiple interfaces?

True

Give an example of how to implement multiple interfaces?

public class Owl implements CanEat, CanFly{…}

What is dynamic dispatch?

Call to the method is custom to the type during runtime

What are polymorphic collections?

Collections that can consist of different types

Give an example of a Polymorphic collection?

public interface FlyingThing{…}

public class Airplane implements FlyingThing{…}

public class Helicopter implements FlyingThing{…}

public class Squirrel implements FlyingThing{…}

FlyingThing\[\] thingsThatFly = new FlyingThing\[7\];

//thingsThatFly can consist of anything that implements the flyingThing class

What does API stand for?

Application Programming Interface

What is an API?

Features that are available to you as a programmer when you use that class, usually all the public stuff

What is encapsulation?

Anything that might change later is wrapped in a shell so no one is relying on it

What is a wrapper class?

Classes that “wrap” primitives so that they are used as objectives in the Java Collections

What is caching?

Not making multiple of the same class and value, reduces garbage, returns reference to the Object that already exists

What is auto-boxing and auto-unboxing?

Java automatically wraps and unrwraps it based on what is needed at the time

What is generic notation?

It allows the compiler to check what we have put into the the collection is the right type of object

What is the syntax for a for each loop?

for (Cat c: collection){

//process

}

What are enumerated types (enums)?

An enum is like a class with a small pre-defined number of instances that are identified with symbolic constants

Give an example of how to declare an enum?

public enum Day {

SUN, MON, TUE, WED, THU, FRI, SAT;

}

How do you access an enum?

nameOfEnum.SPECIFICELEMENT

Can you use == to compare enums? .equals()?

yes you can use both

What are some of the methods in the java collections class?

Collections.shuffle(list);

//next 3 must implement comparable

Collections.max(list);

Collections.min(list);

Collections.sort(list);

//must already be sorted

Collections.binarySearch(list, value);

Collections.reverse(list)

How do you go through all the different values in an enum?

enumType.values()

How do you access a specific value enum going through with index?

enumType.values()\[index\]

How do you write a for each loop iterating through enum values?

for (Day currentDay : Day.values())

How do you compare two enums in the same enum?

Day.WED.compareTo(Day.SUN);

How do you get the index of an enum?

Day.TUE.ordinal();

How do return something given the index in the enum?

Day.values()\[index\];

True or False: Enums can have instance variables and instance methods?

True

True or False: Enums cannot have static methods?

False

True or False: Constructors for enums are always private?

True

Give an example of how to create an enum with instances that have different values?

public enum Day{

//declaring using constructor

SUN(“sunday”, 0), MON(“monday”, 8), TUE(“tuesday”,6);

\
//instance variables

private String name;

private int workHours;

\
//constructor

private Day(String name, int workHours){

this.name=name;

this.workHours = workHours;

}

}

What is an Iterator?

An iterator is an object that can sequentially access the elements of a collection

What does the Iterable interface ensure?

That the class knows how to give you an iterator that goes through itself

True or False: most collections in java are iterable?

True

Give an example of how to get the iterator?

ArrayList list = new ArrayList<>();

…

Iterator iterator = list.iterator();

A collection implements what interface: Iterator or iterable?

Iterable

A iterator implements what interface: iterable or iterator?

Iterator

True or false: for each loops are actually iterators?

True

Why do we use iterators?

1\. Its the only way to cycle through elements of ANY java collection

2\. Removing elements without an iterator is tricky, whereas with it, it is implied that it can keep itself in check

True or False: In a for-each loop, you cannot add or remove elements?

True

Give an example of how to use an iterator in a loop?

Iterator iterator = list.iterator();

while(iterator.hasNext()){

Integer value = iterator.next();

if (value

True or False: when you call the remove method on the iterator, it can only remove what was last returned?

True

What is the difference between immutable and mutable?

Immutable means that after the instance variables are first instantiated, mutable means that they can change

Where do static variables go on a memory diagram?

In a little box in the corner of the heap

What is the Comparable interface?

public int compareTo(Other x);

How do we determine/define the natural order of things?

Using the comparable interface

Give an example of implementing the Comparable interface?

public class Car implements Comparable {

public int compareTo(Car other){

return weight-other.weight;

}}

When implementing an interface, what must you make sure to do?

Implement all non-default instance methods

What is instanceOf?

an operator that determines whether or not the instance is the instanceOf the class in question

How do you write an equals method that overrides the equals method in the Object class?

@Override

public boolean equals(Object other){

if (this == other){

return true;

}

if (! (other instanceOf Dog)){

return false;

}

Dog dog = (Dog) other;

return (dog.name.equals(name) && dog.size==size);

}

What is this a reference to?

The current object as the class that it is

What is super a reference to

a reference to the current object as its superclass

What is the first thing you do in a subclass to construct it?

call a constructor in the super class

Give an example of how to write a constructor for a subclass?

public Student (String name, int ID, double GPA, int admitYear){

super(name,ID);

this.GPA = GPA;

this.admitYear = admitYear;

}

If you do not call the super constructor, what will java do?

It will implicitly call the super constructor with no arguments. If there is no super constructor with no arguments, the code will not compile.

Give an example of how to write a copy constructor in a subclass?

public Student(Student other){

super(other);

GPA = other.GPA;

admitYear = other.admitYear;

}

Give an example of how to write a toString method in a subclass?

@Override

public String toString(){

return super.toString()+ “ GPA: “ + gpa + “ Admit Year: “ + admitYear;

}

Give an example of how to write an equals method in a subclass?

@Override

public boolean equals (Object other){

if (!(other instanceOf Student)){

return false;

}

Student s = (Student) other;

return super.equals(s) && GPA == s.GPA && admitYear == s.admitYear;

}

True or False: If your class doesn’t directly extend something, it will implicitly extend Object?

True

What is always at the root of an inheritance tree?

Object

How do you diagram an interface in a UML diagram?

A dashed line with italics

Explain these 3 statements:

1) The type of variable

2) The class

3) The type of object

type of variable is exactly what it is (1)

the class of a variable is exactly what it is (1)/it is an instance of exactly 1 class

the type of object is where “is a” comes into play (an object may satisfy many types)

What is overload?

Two different methods have the same name with different parameters and are in no way related

What is override?

A subclass “replaces” inherited methods. They have the same parameter types. The return types do not have to be the same but it must satisfy the type (is a)

True or False: you can change the privacy type in an override as long as it is more public and not less public?

True

What is early(static) binding?

It will run whatever type it is declared as. The compiler makes the decision immediately.

ex. Person p = new Student s;

will run person version

What is late (dynamic) binding?

It will run the version it literally is, decision is made at runtime.

ex. Person p = new Student s;

will run student version

True or False: You can override more than once?

False

When will a cast run successfully?

When you cast either up or down the tree (must be reached by moving exclusively up or down)

Which type of casting will always run? Why?

An upcast will always run because the lower levels “is a” higher level

What cast do you have to be careful with? Why?

Downcasting because it is not necessarily that type

When can you implicitly cast? When can you not?

You can always implicitly upcast, you can never implicitly downcast

What type of error would you get if you downcast incorrectly?

ClassCastException

When will a cast compile?

As long as it is above or below the type on the tree

What is the safe practice in downcasting so that it doesn’t throw exceptions?

if (p instanceof Student){

Student s = (Student) p;\`

}

Is multiple inheritance allowed in Java?

No

What is inheritance, what is composition?

Because you can only have 1 superclass, it is important to make sure that you really want make sure that is it is an absolutely necessary thing while in composition you can place an object of what you need inside the class and therefore you can still access all the functionalities

True or false: composition allows for polymorphism?

False

If given the choice, what do you usually favor, composition or inheritance?

Composition

What does it mean to have a final primitive variable?

It cannot change, the value must remain the same

What does it mean to have a final object?

The object itself can change (if it is mutable) it just cannot be assigned to a new object

What does having a final class mean?

No one can extend the class

What doe having a final method mean?

You cannot override the method

What is protected visibility restricted to?

The class, package, and its subclasses

What is shadowing?

Overloading a variable that is inherited

What is the purpose of an abstract class?

To be extended in various ways (like an interface)

True or false: Abstract classes can be instantiated?

False

True or false: Abstract classes can have state, instance variables, and constructors?

True

What is better about Abstract classes?

You can include instance variables and constructors

What is better about interfaces?

A class can implement more than 1 interface

When given the option, do you favor abstract classes or interfaces?

interfaces

How do you write an abstract class?

public abstract class Shape{

private int xCoord, yCoord;

private Color color;

private int width;

public Shape(int x, int y, Color color, int width){

xCoord=x;

yCoord=y;

this.color=color;

this.width=width;

}

}

How do you write an abstract method in an abstract class?

public abstract void drawShape();

How do you write a try catch block?

try {

//code

} catch (NullPointerException e) {

//handler

} catch (RuntimeException (e) {

//handler

} finally {

…

}

What is the difference between checked and unchecked exceptions?

Unchecked exceptions means that there is something wrong with your code, fix it. Checked exceptions means that at runtime something goes wrong, it is beyond your control.

For what type of exceptions do you use a try catch?

Checked exceptions