Be Prepared Ch 3-4 - Java Features

Question 1

arguments and parameters

Answer 1

the values passed in a method call (in the parentheses) The words Argument and parameters are used interchangeably

Question 2

The concept of “privacy” applies to the ____________ and not to individual objects. Different objects of the same class have full access to each other’s fields and methods.

Answer 2

class as a whole

Question 3

• Requierement on ap exam to make all instance variables _____

Answer 3

private

Question 4

• Instance vs static variables

Answer 4

object vs class

Question 5

static method rules

Answer 5

§ Static methods cannot access or modify any instanec variables and cannot refer to this (a reference to a particular object) because this is undefined when a static method is running. Static variables can be initialized in a class ocnstructor and can be accessed and modified in instance methods.

Question 6

Should you print the return value using System.out from inside the method (when it is not requested)?

Answer 6

NO. • It is a minor error (-1/2) to print the return value to a System.out from inside the method (when it is not requested)

Question 7

Parameters of primitive data types are passed to methods ___________

Answer 7

“by value”

Question 8

• Given § Public void fun(int a, int b){ □ a+=b; □ b+=a; § } • The output from the following code: § Int x = 3, y = 5; § Fun(x,y); § System.out.println(x + “ “ + y); • Will print ____________

Answer 8

“3 5” • This is beause x and y are ints, so they are passed to fun by value.

Question 9

§ All objects are passed to methods as _____. A method receives a _______ to (the address of ) the object

Answer 9

references a copy of a reference

Question 10

immutable objects

Answer 10

objects without modifier methods String, Integer, and Double classes For example, there is no way in Java to write a method □ Public void toUpperCase(String s) { . . . } § Because the method has no way to change the string passed to it. For immutable objects, the method has to create and return a new object with the desired properties: □ Public String toUpperCase(String s) { . . .}

Question 11

§ A method whose return type is a class can also return

Answer 11

a null (a reference with a zero value that indicates that it does not refer to any valid object.

Question 12

§ If a method returns an ArrayList, write the

Answer 12

full ArrayList type, including its elements’ type in angle brackets, as the method’s return type.

Question 13

• The compiler treats overloaded methods as ________.

Answer 13

different methods It figures out whichh one to call depending on the number and types of the parameters.

Question 14

Random Numbers [1,n]

Answer 14

• Int r = (int) (n * Math.random()) + 1;

Question 15

Input and Output

Answer 15

• Ap subset does not include any classes or methods for data input. The Scanner class is not in the AP subset. • If a question ivolves user input it may be stated as follows: § Double x = < call to a method that reads a floating-point number> § Or § Int x = IO.readInt(); //reads user input

Question 16

• An exception

Answer 16

is a run-time event that signals an abnormal condition in the program.

Question 17

Checked Exceptions

Answer 17

• Some run-time errors, such as invalid user input or an attempt to read past the end of a file, are considered fixable,. The try-catch-finally syntax allows the programmer to catch and process the exception and have the program recover. This type of exception is called a checked exception § Checked exceptions and the try-catch statements are not in the AP subset.

Question 18

Unchecked exceptions

Answer 18

• Other errors, such as an array index out of bounds or an attempt to call a method of a non-existing objet (null reference) are considere fataL: the program displays an error message with information about where the error occurred, then quits. This type of exception is called an unchecked exception.

Question 19

• In Java, an exception is ____________

Answer 19

an object. The Jaba library impletments many types of exceptions, and if necessary you can derive your own exception class from one of the library classes.

Question 20

Exceptions to understand

Answer 20

§ (ArithemeticException), § IndexOutOfBoundsException, - what ArrayList methods throw § (ArrayIndexOutOfBoundsException) - what Arrays throw § (NullPointerException), § IllegalArgumentException, and § (ClassCastException) § Java automatically throws the bolded exception listed in parentheses, when the trigegering condition occurs, (meaning your code does not need to explicitly throw an ArithemeticException)

Question 21

• Throwing your own exceptions

Answer 21

§ Sometimes you need to “throw” your own exceptions, e.g. such as NoSuchElementException or IllegalStateException § If (items.size() ==0) □ “throw new NoSuchElementException();” § If an object is not ready for a particular method call □ Public void stop() □ { ® If (!isMoving()) ◊ Throw new IllegalStateException(); //displays a message and quits ® Speed = 0; ® … § }

Question 22

• Procedural abstraction

Answer 22

• eg #1. Java isolates a developer from a particular platform. e.g #2 : an algorithm: can be described using pseudocode, flowcharts, or other tools independent of any particular programming language

Question 23

• You can write ____class object = new ___class();

Answer 23

super; sub § e.g. □ School sch - new HighSchool();

Question 24

• A superclass’s private methods ______ callable in subclass

Answer 24

are not

Question 25

• Constructors _____ inherited: a subclass ______ provide its own

Answer 25

are not has to

Question 26

• A subclass’s constructors can explicity call the superclass’s constructors using the keyword ____

Answer 26

super

Question 27

§ If super is used, it must be the ___________________ in the subclass’s constructor

Answer 27

first statement

Question 28

• A subclass inherits all the class (static) variables and instance variables of its supercalss. However, the instance variables are usually private, thus they are ____

Answer 28

>>not directly accessible in its subclass, so you have to use the public accessors and modifiers

Question 29

• Superclass public constants (public static final) are

Answer 29

directly accessible everywhere

Question 30

• Abstract classes can have constructors and methods; however, some of its methods may be declared abstract and left without code

Answer 30

§ Public abstract class Solid{ □ …public abstract double getVolume(); □ …}

Question 31

• A class in which all the methods are defined is called

Answer 31

concrete.

Question 32

• You cannot instantiate an abstract class, but you can ___________

Answer 32

declare variables or arrays of its type § Solid s1 = new sphere(radius); § Solid s2 = new cube(side); § Solid[] solids = {new Sphere(100), new cube(100)};

Question 33

§ it seems pointless to have a pure abstract class (e.g. an abstract class where every method and variable is abstract). It would make more sense to use an ____

Answer 33

interface

Question 34

§ Also interfaces can extend _______

Answer 34

interfaces, but not other classes (not even abstract classes, or purely abstract classes)

Question 35

• Polymorphism

Answer 35

§ Is a mechanism that ensures that the ocrrect method is called for an object disguised as a more generic type. § For(Solid solid :solids) □ totalVolume+= solid.getVolume(); //for different shapes. spheres pyramids boxes etc.

Question 36

an interface is different from a class in several ways, including:

Answer 36

You cannot instantiate an interface. An interface does not contain any constructors. All of the methods in an interface are abstract. An interface cannot contain instance fields. The only fields that can appear in an interface must be declared both static and final. An interface is not extended by a class; it is implemented by a class. An interface can extend multiple interfaces.

Question 37

§ Public interface fillable{ □ Void fill(int x); □ Int getCurrentAmount(); □ Int getMaximumCapacity(); § } are those methods public abstract or something else?

Answer 37

They are implicitly understood as public abstract // no need to repeat public abstract for each method in an interface - it is understood

Question 38

§ A class implements an interface by

Answer 38

supplying code for all the interface methods. □ Public class Car implements Fillable{ □ …. □ Public void fill(int gallons) { fuelAmount += gallons;} □ Public int getCurrentAmount() { return fuelAmount;} □ Public int getMaximumcapacity() {return fuelTankCapacity;} □ }

Question 39

§ A class that claims it implements an interface but does not define some of the interface methods must be declared

Answer 39

abstract

Question 40

§ A class can extend only one class, but it can implement

Answer 40

several interfaces. □ Public class Car extends Vehicle implements Fillable, Sellable { . . .}

Question 41

□ Each interface adds a secondary data type to the objects of the class that implements it. If a class C implements interface I, objects of C can

Answer 41

be disguised as type I objects and polymorphism applies ( the same way as for subclasses). For example, we can have a method: ® Public void fillUp(fillable[] a) ® { ◊ For (Fillable f:a) } f.fill (f.getMaximumCapacity() - f.getCurrentAmount()); ® } ® The formula works polymorphically for any types of different Fillable objects tha might be stored in the arra a ®

Question 42

If class C implements interface I,

Answer 42

all subclasses of C automatically implement I.

Question 43

§ Comparable interface

Answer 43

□ Built in library: java.lang.Comparable □ Used to compare objects for various methods □ Specifies only one method: ® Int compareTo(T other);

Question 44

Comparable interface ® Int compareTo(T other);

Answer 44

◊ Returns a positive integer if this object is greater than the other, 0 if same, - if less ◊ Up to programmer to decide what smaller and greater mean when comparing objects of his class ◊ Takes one parameter of the type T - and it is usually assumed that the parameter belongs to the same class that implements Comparable } Public class flight implements Comparable } { – … – Public int compareTo(Flight other) – { – Return getDegartureTime() - other.getDepartureTime(); – } } } ◊ String, Integer, and bouble all implement Comparable, but naturally do so in different ways such as lexicographically vs or numerically

Question 45

◊ The comparable interface does not specify an equals method, and a class that implements comparable does not need an equals method to complie. But it is better to override the equals method inherited from Object and make it consistent with compareTo, to avoid possible errors later. For example:

Answer 45

Public boolean equals(Object x) } { } Return x instanceof Flight && compareTo((Flight)x) == 0; } } } The boolean operator instanceof is not in the AP subset. } Note that the argument to your equals method should have the type Object, so that your equals indeed overrides Object’s equals.

Question 46

• For each loop

Answer 46

§ For(T x: a){ □ System.out.println(x); § } § Where t is the data type of the elements of a

Question 47

If a constructor does not explicitly invoke a superclass constructor,

Answer 47

the Java compiler automatically inserts a call to the no-argument constructor of the superclass. If the super class does not have a no-argument constructor, you will get a compile-time error. Object does have such a constructor, so if Object is the only superclass, there is no problem.

Question 48

Breaking out of nested loops using labels

Answer 48