CIS 270—Application Development II Chapter 8—Classes and Objects: A Deeper Look

Time Class Case Study Two classes: Time1 (Fig 8.1) and Time1Test (Fig. 8.2) Time1 has three private instance variables and three public methods (a.k.a., public services or the public __________). Time1 has a default ___________ that sets instance variables to 0. Note the code hour = ((h>=0 && h<24) ? h:0); It means if(h>=0 && h<24) hour=h; else hour=0; The Time1 object will always have __________ data (always in range), but not necessarily correct data. interface constructor consistent

Time Class Case Study Note the code String.format(“%02d:%02d:%02d", hour, minute, second); in the Time1 class. format is a method of the __________ class The %02d means “format is leading 0, two decimal integers” (this formatted output is new in J2SE 5.0) The three formats (in “” ) will be applied to hour, minute, second Likewise, note the code String.format( "%d:%02d:%02d %s", ( ( hour == 0 || hour == 12 ) ? 12 : hour % 12 ), minute, second, ( hour < 12 ? "AM" : "PM" ) ); The hour % 12 means hour ______ 12 String mod

Time Class Case Study Note Time1 time = new Time1(); in the Time1Test class. Variable time is assigned the address of a ________ object ( time is a reference variable). The code System.out.println( time.toUniversalString() ); prints the string returned when the object time calls the method toUniversalString() in the class _________. The code time.setTime( 13, 27, 6 ); is the object time calling the setTime method in Time1. The class MemberAccessTest in Fig. 8.3 shows that a client cannot directly access _________ members. Time1 private

5 8.4 The this Reference Note the file ThisTest.java in Fig ThisTest.java has two classes (but only one can be public, which contains the ________ method here). Compiling ThisTest.java will create two _______ files. Parameter names for a constructor should NOT be the same as the instance variable (______) names. Variables in a method with the same names as fields will shadow (hide) the fields. In that case, you will need to refer to fields with the keyword _______. Accessing members within the same class does not require the this reference (see Line 35). main.class field this

6 8.5 Overloaded Constructors Note class Time2 in Fig A class can have several (__________) constructors if the ________ (parameter list) of each is different. public Time2() is a no-__________ constructor. this can be used as the first statement in a constructor’s body to call another constructor. This use of this allows for code ______ with less maintenance if a constructor needs to be changed. The methods setTime, toUniversalString, and toString use set and get methods instead of accessing instance variables directly. overloaded signature argument reuse

7 8.6 Default and No-Argument Constructors If a class has no constructors, the compiler creates a default constructor that sets numeric fields to 0, Boolean fields to false, and reference fields to ____. If a class has constructors, the compiler will not create a default constructor. The class will need a no-argument constructor. A no-argument constructor with no _____ will set fields to their default values. A constructor is identified by Java because it has the same name as the class and has no _______ type. null body return

8 8.7 Set and Get Methods Classes provide public methods to allow clients to set and get ________ instance variables. Set and get methods should only be used as needed. Set methods are also called mutator methods and get methods are called _________ methods. Set and get methods make private fields accessible, but not _______. Get methods can hide the actual data format from the client when they return formatted data. Set methods can protect fields from careless attempts to modify values by using ______ checking. private accessor public validity

9 8.8 Composition A data member of a class can be a reference to an ______ of another class. This is known as composition or a _____ relationship. Note that the Employee class in Fig. 8.8 uses composition. Employee has two Date objects as instance variables. In UML (Unified _________ Language), composition would be expressed as follows: EmployeeDate object has-a Modeling

Enumerations An enumeration is a special kind of class which declares a set of _________. enum types are final and __________. Do not try to create an object of type enum. An enum can have instance variables, constructors, and methods, much like an ordinary class. Enhanced for loop (new in J2SE 5.0) int scores[] = { 87, 93, 92, 95, 88, 90 }; int var = 0; for(int var : scores) // var holds each array value totalScore += var; See Fig and Fig for example of enum. constants static

static Class Members A static field (class variable) is shared by all objects. Example: numOfItems in a class called Item : public static int numberOfItems = 0; In the constructor Item, include the code numberOfItems++; A public static field can be directly accessed by an object or with the _______ name: anItem.numberOfItems Item.numberOfItems A ________ static field can be accessed by calling a public static method using the class name: Item.aMethod() See Fig and Fig for examples. class private

static Import and final Instance Variables static Import By using code like import static java.lang.Math.*;, you can access the static methods of a class without using the class name. For example, sqrt(900.0) instead of ______.sqrt(900.0) final Instance Variables Principle of _______ Privilege: code should be granted only the amount of privilege and access that it needs. Instance variables can be made final —they can only be set in a constructor and cannot be changed otherwise. private final int INCREMENT; Math Least

Some OO Concepts Software reuse Java is a language, but it is also a ___________ for accessing the Java API (thousands of prewritten classes) Software can be created by using prewritten components Data abstraction and encapsulation Information ______: classes hide their implementation from clients Data abstraction: clients focus on data, not on implementation details An __________ data type (ADT) has data representation and operations that manipulate the data A class is an ADT—it has data and operations framework hiding abstract