Ch 7 Designing Java Classes & Class structure. Fields have data values define/describe an instance

Transcription

1 Ch 7 Designing Java Classes & Class structure Classes comprise fields and methods Fields have data values define/describe an instance Constructors values are assigned to fields when an instance/object is created (special kind of method) Methods what does an instance know? What can an instance do? getters setters equals tostring other ACS

2 UML Diagram of a Class fields Class name A class has fields and methods (including constructors) methods ACS

3 e.g. Math class Math has two static fields Math has a private constructor You cannot instantiate a Math object Math has many static methods To use π you write Math.PI +E +PI To use the static method max you write Math.max(n1, n2) Math -Math() +abs(double a) +abs(float a) + abs(int a) + max(double a, double b) + max(int a, int b) ACS

4 e.g. Random class must instantiate an object to get a random sequence Random gen = new Random(84854); instance fields seed multiplier = 0x5DEECE66DL -seed -multiplier Random constructors instance methods gen.nextboolean() gen.nextint() +Random() +Random(long seed) +nextboolean() +nextint() ACS

5 Fields Fields may be primitive variables Or, they may be of some other type (e.g. String, ) May be public or private public - anyone can use it private limited access ACS

6 Methods Methods are either: value-returning must have a return statement e.g. getters naming convention is void no return statement e.g. setters naming convention is ACS

7 Methods public vs private public - anyone can use it private - special cases Math constructor is private you cannot instantiate a Math object try to do it nextid() in Student is private It has a special purpose used to generate the next id. It is only called from within the Student class ACS

8 equals Method equals( ) Value-returning Returns a boolean Usually an equals method is designed for a class. Designer must determine the condition when two objects are considered equal. E.g. String class has an equals method string1.equals(string2) ACS

9 tostring Method tostring() Value-returning Returns a string A method automatically called when an object is displayed E.g. System.out.println(myObject); The designer of a class determines what it returns E.g. ArrayList has a tostring() method result is of the form: [ object 1, object 2, object n ] ACS

10 Example - Student Consider the student class ACS

11 Class Diagram for Student -id -firstname -lastname -gender -active show 1, 2, or 3 compartments/ info as needed getters + means there is public access to the method - means there is no public access to the field or method setters ACS

12 Java code for Student - fields instance vs class e.g. consider Student class Which fields are class? Which fields are instance? ACS

13 Java code for Student - fields private vs public private: only directly accessible from within the class/object, and from outside the class via getters/setters public : accessible from anywhere A design principle is to make fields private but give public access to the getters and setters ACS

14 Java code for Student - constructors The no-arg constructor Constructor with 4 parameters ACS

15 Java code for Student - getters Notice Getters (accessors) for most private fields Naming convention: Start with get followed by the field name but this starts with a capital letter ACS

16 Java code for Student - setters Notice Setters (mutators) for most private fields Naming convention: Start with set followed by the field name but this starts with a capital letter ACS

17 Java code for Student other methods Private method used to control the id assigned to a new student object tostring Executes when a student is printed equals Tests two student objects to see if they are equal ACS

18 Java Classes Class is a template for objects How are these shown in UML? (Figures 7.2, 7.4, 7.5 ACS

19 Objects Objects: instantiated/created via new lots of examples also called an instance so we can speak of instance fields/methods How are these shown in UML? (Figures 7.3, 7.4) ACS

20 Associations Classes will have relationships with other classes. When designing you must decide whether to implement an association, and how to implement it. Figure 7.1 A student is majoring in a subject A student will have at most one major The student class has a private field with getter/setter Implementation in Student: Private field getter setter ACS

21 Associations Classes will have relationships with other classes. When designing you must decide whether to implement an association, and how to implement it. Figure 7.1 A subject has majors (students) A subject may have many majors The Subject class has an ArrayList with getter/setter & addmajor( ) Implementation in Subject: ACS

22 Java Classes making a connection between objects Associations: Figure 7.1 Consider SamDeclaresMathMajor.java 1. Instantiate a subject math 2. Instantiate a student sam 3. Set Sam s major to be math 4. Add Sam to the list of math majors ACS

23 Java Classes- reusing code Methods are used for two purposes 1. To make a program more readable through decomposition 2. To reuse code instead of duplicating code Consider that the code in SamDeclaresMathMajor.java could be replicated for every student declaring a major jill.setmajor (math) ; math.addmajor (jill) ; sam.setmajor (math) ; acs.addmajor (sam) ; bob.setmajor (math) ; acs.addmajor (bob) ; For jill For sam For bob We can replace this kind of code by writing a method that sets a student s major and adds the student to a subject ACS

24 Java Classes- reusing code Methods are used for two purposes 1. To make a program more readable through decomposition 2. To reuse code instead of duplicating code Consider that the code in SamDeclaresMathMajor.java could be replicated for every student declaring a major Three calls to the method below A method to handle declaring a major ACS

25 Java Classes - Parameters and arguments Arguments passed to a method Parameters defined for a method Arguments are copied into the parameters on entry, but there is no copying on return. Arguments must match parameters by type. ACS

26 Java Classes - Parameters and arguments Parameter Lists / Arguments A parameter list defines the type of data that will be passed in to a method Arguments appear in the call statement. Arguments are copied into the parameters on entry, but there is no copying on return. But for objects its possible to modify them in the called method See ObjectModifiedByCalledMethod.java ACS