Chapter 6

Question 1

What does the Java class library documentation include

Answer 1

The interface of the class:
-class name
-class description
-list of constructors and methods
-return values & parameters for constructors & methods
-a description of purpose of each constructor & method

Question 2

The Java class library documentation does not include___

Answer 2

The implementation of the class:
-private fields
-private methods
-the bodies (source code) of methods

Question 3

Using library classes

Answer 3

• classes organized into packages
• classes from the library must be imported using an import statement (except classes from the java.lang package)
• can then be used like classes from the current project

Question 4

Methods from string

Answer 4

• contains
• ends with
• index of
• substring
• to UpperCase
• trim

Question 5

T or F: strings are mutable

Answer 5

F: strings are immutable

Question 6

Which is the correct way to modify a string?
input.toUpperCase();
or
input = input.toUpperCase();

Answer 6

2

strings cannot be modified
the correct one doesn’t modify but returns a new string

Question 7

What’s the difference?
if(input == “bye”)
and
if(input.equals(“bye”))

Answer 7

1 tests identity

#2 tests equality - whether 2 objects contain the same state
Always use .equals for text equality

Question 8

== vs .equals

Answer 8

1. The main difference between the .equals() method and == operator is that one is a method, and the other is the operator.
2. We can use == operators for reference comparison (address comparison) and .equals() method for content comparison. In simple words, == checks if both objects point to the same memory location whereas .equals() evaluates to the comparison of values in the objects.

Question 9

using random

Answer 9

• Class can be used to generate random numbers
  Random rand = new Random();

Question 10

Parameterized classes (generic)

Answer 10

• the documentation includes provision for a type parameter:
  ArrayList<E></E>
• These type names reappear in the parameters & return types
  E get (int index)
  boolean add(Ee)
• the types in the documentation are placeholders for the types we use in practice
  An Array List <Ticket> actually has methods:
  TicketMachine get(int index)
  boolean add(TicketMachine e)</Ticket>

Question 11

Packages and import

Answer 11

• single classes may be imported
  import java.util.ArrayList;
• whole packages can be imported:
  import java.util.*;
• importation does not involve source code insertion
• Some classes are used so frequently that almost every class would import them like class string

Question 12

maps

Answer 12

• collections that contains pairs & values
• pairs consist of a key and a value
• lookup works by supplying a key & removing a value
• ideal for one way lookup not reverse
  i.e. telephone book

Question 13

List, map and set

Answer 13

• alternative ways to group objects
• varying implementations available
• but hashMap is unrelated to Hashset

Question 14

Set

Answer 14

A collection that stores each individual element at most once
- does not maintain any specific order

Question 15

set vs ArrayList

Answer 15

• Identical to ArrayList except we used Hashset
• A list will keep all elements entered in the desired order but set doesn’t
• List provides access to elements by an index & can contain same element multiple times
• each element in a set at most once

Question 16

Situations to use wrapper classes

Answer 16

int, boolean & char
- separate from objects
- their values are not instances of classes
- so not normally possible to add them to a collection
- Wrapper classes are the solution
- wrapper class for int = Integer
Integer iwrap = new Integer(ix);

Question 17

Autoboxing

Answer 17

• performed automatically when a primitive type value is used in a context requiring a wrapper type
• It makes storing primitive values on a object collection easier
• applied whenever a primitive type value is passed as a parameter to a method that expects a wrapper type & when a primitive type value is stored in a wrapper type variable

Question 18

Unboxing

Answer 18

• reverse of autoboxing, also automatic
• applied when a wrapper type value is passed as a parameter to a method that expects a primitive type value & when stored in a primitive type variable

Question 19

Ways to use javadoc

Answer 19

1. switching the pop-up selector @ the top right of the editor window from source code to documentation (or by using toggle documentation view from the editor’s tools menu)
2. Use project documentation function from main windows tools menu to generate documentation for all classes in the project

Question 20

What should class documentation include?

Answer 20

• class name
• comment describing overall purpose & characteristics of the class
• a version #
• author’s names
• documentation for each constructor & each method

Question 21

What should constructor & method documentation include?

Answer 21

• method name
• return type
• parameter names & types
• descriptor of purpose & function of the method
• description of each parameter
• description of value returned

Question 22

public vs private

Answer 22

• public elements are accessible to objects of other classes- fields, constructors & methods
• fields should not be public
• private elements are accessible only to objects of the same class
• only methods that are intended for other classes should be public

Question 23

information hiding

Answer 23

• Data belonging to on object is hidden from other objects
• know what an object can do, not how it does it
• increases the level of independence
• independence of modules is important for large systems & maintenance
• private keyword enforces info hiding

Question 24

code completion

Answer 24

• the BlueJ editor supports lookup of methods
• use ctrl-space after a method-call dot to bring up a list of available methods
• use return to select a highlighted method

Question 25

class variables

Answer 25

• shared b/t all instances of the class
• it belongs to the class & exists independent of any instances
• designated by the static keyword
• public static variables are accessed via the class name

Question 26

Constants

Answer 26

• a variable, once set, can have its value fixed
• designated by final keyword
  final int max = list.size();
• final fields must be set in their declaration or the constructor
• combining static & final is common

Question 27

class constants

Answer 27

• static: class variable
• final: constant
• private static final int gravity = 3;
• Public visibility is less of an issue with final fields
• upper case names often used for class constants:
  public static final int BOILING_POINT = 100;

Question 28

class methods

Answer 28

• class methods are conceptually related to class variables & use a related syntax (the static keyword)
  public static int getNumberOfDaysThisMonth()

Question 29

limitations of class methods

Answer 29

1. can’t access any instances fields defined in the class
2. can’t call an instance method from the class

Question 30

main method

Answer 30

public static void main (String args [])

• to access program without BlueJ- user specifies class and Java will then invoke a method called Main in that class