Java Classes

1. Java Classes Chapter 1

2. Chapter Contents • Objects and Classes • Using Methods in a Java Class • References and Aliases • Arguments and Parameters • Defining a Java Class • Passing Arguments • Constructors • Static Fields and Methods • Packages – The Java Class Library

3. Classes • A class is a type or kind of object • A class definition is a general description of • What the object is • What it can do • Objects of the same class have • The same kinds of data • The same methods Blueprint for constructing objects

4. Objects • An object is a program construct that • Contains data • Performs certain actions • The actions are called methods • The actions interact to form the solution to a given problem

5. An Example of a Class An outline of a class

6. Instantiation of the Class Three instances of the class Automobile

7. Create a object for a Java Class • Declare:Name joe; • Instantiate: joe = newName(); • Combine: Name joe = new Name(); • The new operator creates an instance of the class • Invokes the constructor method

8. Create a object for a Java Class A variable that references an object.

9. References and Aliases • Primitive types:(Built-in to Java) • A chunk of memory holding the data itself • byte, short, int, long float, double, char, boolean • int number1; • All other types are reference or class types • All objects defined from classes • The object “refers to” or “points to” the chunk of memory that actually holds the data • String greeting = "Howdy"; • greeting is a reference variable instead of the actual string • Set to null value for an reference-variable • When two variables reference the same instance, they are considered aliases

10. References and Aliases • Name jamie = new Name(); • jamie.setFirst(“Jamie”); • jamie.setLast(“Jones”); • Name friend = jamie; • Two references, one chunk of data • Is friend == jamie? Aliases of an object

11. Method Definitions • Given • This is a valued method • Returns a String • Given • This is a void method public String getFirst() { return first; } // end getFirst public void setFirst(String firstName) { first = firstName; } // end setFirst

12. Arguments and Parameters • Given statements:Name joe = new Name();joe.setFirst ("Joseph");joe.setLast ("Brown"); • "Joseph" and "Brown" are arguments sent to the methods • Arguments must correspond to the formal parameters in the method declaration public void setFirst(String firstName) { First = firstName; }

13. Defining a Java Class public class Name { private String first; // first nameprivate String last; // last name < Definitions of methods are here. > } // end Name • Given • Store class definition in a file whose name is the name of the class followed by .java. • public, private: access modifier • class Name is public, any other Java class can use this class • private data fields (instance variables) • Note data fields are private: only the methods inside this class can access them. All data fields should be private • They will require accessor and mutator methods

14. Coding Conventions • Naming conventions • conventions are things we suggest to make code more consistent and easier to understand • this is purely aesthetic; not enforced by the compiler • We use capitalization to distinguish an identifier’s purpose • class names begin with uppercase letters: Use noun or descriptive phrase • method, variable names begin with lower case: Use verb or action phrase • instance variables start with an underscore Good Name Student takeClass _cs220Student Poor Name Thing (no role, purpose) doStuff (not specific) c (too cryptic) Class Method object

15. Passing Arguments • Call by value • For primitive type, formal parameter initialized to value of corresponding argument in call. Method cannot change the value of the argument. • Call by reference • For a reference type, formal parameter is initialized to the memory address of the object in the call. Method can change the data in the object.

16. Example 1 public void giveLastNameTo(Name child) { child.setLast(last); } public static void main(String[] args) { Name jamie = new Name(); jamie.setFirst(“Jamie”); jamie.setLast(“Jones”); Name jane = new Name(); jane.setFirst(“Jane”); jane.setLast(“Doe”); jamie.giveLastNameTo(jane); }

17. Passing Arguments The method giveLastNameTo modifies the object passed to it as an argument.

18. Passing Arguments The method giveLastNameTo modifies the object passed to it as an argument.

19. Example 2 Public void giveLastNameTo(Name Child) { String firstName = child.getFirst(); child = new Name(); child.setLast(last); child.setFirst(firstName); } Public static void main(String[] args) { Name jamie = new Name(); jamie.setFirst(“Jamie”); jamie.setLast(“Jones”); Name jane = new Name(); jane.setFirst(“Jane”); jane.setLast(“Doe”); jamie.giveLastNameTo(jane); }

20. Passing Arguments A method cannot replace an object passed to it as an argument.

21. Passing Arguments A method cannot replace an object passed to it as an argument.

22. Constructors • A method that • Allocates memory for the object • Initializes the data fields • Properties • Same name as the class • No return type, not even void • Any number of formal parameters including no parameters • Default constructor has no parameters

23. Constructors public Name() public Name(String firstName, String lastName) { { setFirst(firstName); setLast(lastName); } } Name jill = new Name(“Jill”, “Jones”); jill = new Name(“Jill”, “Smith”); An object (a) after its initial creation; (b) after its reference is lost memory leak

24. Static Fields and Methods • A data field that does not belong to any one object • Adding reserved word static: • private static int numOfInvocation = 0; • Also called:static field, static variable, class variable • One copy of that data item exists to be shared by all the instances of the class

25. Static Fields • Static does not mean constant, shared by all objects of its class, but value can change. Objects can use a static field to communicate with each other to perform some joint action. • private static int numberOfInvocations = 0; A static PI versus a non static field

26. Static Methods • A method that does not belong to an object of any kind. • You use the class name instead of an object name to invoke the method. • Java predefined class math contains standard mathematical methods. • int maximum = Math.max(2,3); • double root = Math.sqrt(4.2); • Static method cannot access non-static fields, nor non-static methods. • Constructors cannot be static.

27. Packages • Multiple related classes can be conveniently grouped into a package • Begin each file that contains a class within the packagepackage myStuff; • Place all files within a directory • Give folder same name as the package • Package can contain packages • To use the package, begin the program with import myStuff.graphics.*; myStuff.graphics.Frame package names class name

28. The Java Class Library • Many useful classes have already been declared • Collection exists in Java Class Library • Example • The class Math is part of the package java.lang