Chapter 3: Developing Class Methods Object-Oriented Program Development Using Java: A Class-Centered Approach

2 Method Development: Algorithms Algorithm  Step-by-step set of instructions  Describes how data are to be processed to produce desired result Must clearly understand difference between algorithmic and intuitive commands  Computers do not understand intuitive commands Coding the algorithm  Writing in programming language

3 Method Development: Algorithms (continued) Pseudocode  English-like phrases used to describe algorithm Formulas  Mathematical equations are used Flowcharts  Diagrams that employ symbols are used

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5 Using Pseudocode Most commonly used method for developing algorithms Short English phrases Example:  Input three numbers into computer  Calculate average by adding numbers and dividing the sum by 3  Display average

6 Application: Swapping Values Swapping:  Exchanging data that are either stored or referenced by two variables  Commonly seen in programming Algorithm:  Store first variable’s value into temporary location  Store second variable’s value into first variable  Store temporary value into second variable

7 Application: Swapping Values (continued) How should two variables be made available to a method that will swap their values? Procedure:  Encapsulate two variables within a single object  Use class method to implement swap algorithm  Switch values in instance variables

8 static and final Variables So far, we’ve seen  local variable ( declared inside method)  instance variable ( declared inside class but outside method) Instance variable – term used because each instance of the class has its own version of the variable. Each object has its own memory place so it can have a distinct value for that class.

9 Static variable static variable (or class variable)  Created only once for each class  Shared by all objects created from class – only one copy of a static variable for all objects of the class  Changing the value of a static variable in one object changes it for all the other objects  Declared in same way as instance variables Except static reserved word is used in declaration Example static declaration:  private static int numEmployees = 125;

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11 Static or Class Method Invoked through the class name – don’t have to instantiate an object of the class to invoke the method.  e.g. all method of the Math class are static methods so  Math.sqrt(16); or Math.pow(5,3); These methods perform basic calculations based on values passed as parameters. No object state to maintain in these situations.

12 Static Methods (cont.) Java main method must be static so that main can be executed without instantiating an object from the class containing main. Static methods do not operate in the context of a particular object. Can only reference static variables

13 Summary of static methods or class methods Provide means of accessing static variables in absence of any specific object Restricted to using:  Static variables  Other static methods  Additional values and objects that are passed as arguments into method Include static reserved word in method’s header

14 static Methods (continued) Can also be used for constructing general-purpose methods Perform functions using only data passed to them as arguments at time of call  i.e. main() method

15 Scope Section of program within which an identifier is valid Defines portion of class where variable can be used Scopes in Java:  Local  Class

16 Scope (continued) Local scope:  Variables declared within a method  All parameters of a method Class scope:  All instance and static variables declared in a class’s variable declaration section  All methods contained within a class’s method definition section

17 Visibility Determines whether a member can be accessed from outside a class in which it is declared Modifiers:  Public  Protected  Private Visibility rules

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19 Values and Identities Identity  Every object has a unique identity Literal values do not have identities

20 final Variables final variables  Have initial value that cannot subsequently be changed  Also called: Named constants Symbolic constants Use final reserved word

21 final Variables (continued) Example:  public final static int NUMEMPS = 100; Blank final:  No initial value is provided  Value initialized later in program  final float DENSITY; Java convention:  Use uppercase letters for identifiers

22 Placement of Statements A variable must be declared before it can be used Proper placement: public class className { // declaration section final static variable declarations static variable declarations instance variable declarations continued …

23 Placement of Statements Proper placement: // methods section - example of one method method header line(parameter list) { final variable declarations variable declarations object declarations other Java statements }

24 Common Programming Errors Attempting to pass incorrect data types Declaring the same variable locally within both calling and called methods and assuming the change in one variable affects the other variable Forgetting to include the data type of a method’s parameters within the header line Attempting to alter an object’s private variables by passing an object into a non-class method as an argument

25 Summary A method is called by:  Giving its name  Passing any data to it in parentheses following the name A method’s return type declares the data type of the value returned by the method Methods can directly return at most a single value A called method cannot alter either a primitive data type argument’s value or a reference variable’s value

26 Summary (continued) Algorithm:  Step-by-step procedure  Describes how a single computation or task is to be performed Scope:  Section of a program within which an identifier is valid Static variables can be accessed without being referenced as a data member of a specific object