Java Section 5: Methods and Variables - AY25 PDF

AY25a Java Section 5 MORE ON METHODS AND VARIABLES You will learn more details about methods and variables, and also review examples to reenforce much of the learning to date. Java Section 5...

AY25a Java Section 5 MORE ON METHODS AND VARIABLES You will learn more details about methods and variables, and also review examples to reenforce much of the learning to date. Java Section 5 1  Java Section 5 More about Methods  Java Programs and Classes  Static methods  Static variables  Method calls  Method overloading  Variable-length argument lists  Command-line arguments  JOptionDialog  Wrapper Classes  Case studies Lectures will emphasize the most important! It is strongly recommended that you also read the recommending readings and complete the associated exercises. JN Java Section 5 2  BlueJ: Java5_Examples[MoreMethodsVariables] Examples discussed in this section and more: Go Explore! Note: Normally classes would be grouped in separate projects. README.TXT: The classes are collected here for Click on page ease of opening ICON to open a and comparison. brief description of classes Java Section 5 3  BlueJ: Java5_Examples[MoreMethodsVariables] Examples discussed in this section: Go Explore! README.TXT: Click on page ICON to open a brief description Java Section 5 4  Java Programs and Classes (more details in Java2 and Java5)  The challenge is to break a large problem through divide and conquer into relatively small and simple building blocks.  The building blocks of Object Oriented (OO) Systems are classes.  Java programs are made up of classes which contain fields and methods.  The fields maintain features or properties and are stored in variables.  The methods implement the tasks to be completed (the behaviour) using statements and variables.  The design and implementation effort is to construct the solution from existing and new building blocks (components).  OO Systems are developed through several iterative life-cycle stages of:  Specification: identify and document the Requirements and Use Cases  Design: specify the classes and detailed application behaviour  Code and Test: the new classes (unit test) and the application (system test).  Software reusability  Use existing well-tested components where possible. Create reluctantly! Java Section 5 5  Instance Variables and Instance Methods Recall  An instance variable is used to store an attribute of an object. private String name; private int quantity;  An object is an instance of a class.  Each created object has its own set of instance variables.  An instance method normally accesses/modifies instance variables.  Get methods to access e.g. getName(), getQuantity()  Set methods to modify e.g. setName(), setQuantity()  An instance method is called through an object of its class. objectReference.methodName( arguments ) Java Section 5 6  static Methods  When a method does not depend on any instance variables, it is known as a class method or static method.  The static keyword is used to identify them, hence known as static methods.  As static methods don’t use instance variables, we do not have to create an object of the class (using new) to call a static method.  Classes may contain static methods to perform general tasks, e.g.  Java class Math methods to implement mathematical formulae  Java class Arrays methods to perform tasks on arrays.  To declare place keyword static before the return type. public static returnType methodName ( parameters )  To call a static method methodName( arguments ) if called from in same class ClassName.methodName( arguments ) if called in a different class then prefix with the ClassName Java Section 5 7  Static Methods max of 4 real numbers FindMax4.java To call the static max method: max( 1.2, 3.4, 5.6, 7.8) // if called from method in same class FindMax4.max( 1.2, 3.4, 5.6, 7.8) // if called from different class Java Section 5 8  static Methods The main method  The main method is always declared as static public static void main ( String[] args ) { }  When you run an application e.g., with the java command, the Java Virtual Machine (JVM) locates the main method.  As it is static, the JVM can call it without creating an instance of the class  Hence, initially no object creation is required.  When a main method is executed:  main can accept one parameter, an array of type String called args  The array elements contain 0 or more command line arguments (CLAs)  CLAs are specified when starting the application. Java Section 5 9  static variables (aka class variables and static fields)  Previously we met instance variables where each object (instance) of the class has its own instance of the variable.  A class Module representing a university module with a title attribute would declare an instance variable: public class Module { private String title; // instance variable, one for each module created  static variables, also known as class variables  are declared for variables of a class which do not require a separate instance of the variable for each object.  All objects of a class containing static variables share one copy.  Assuming all modules of class Module are in the same university private static String university;  Together the static (class) variables and instance variables represent the fields of a class. Java Section 5 10  static variable count number of times a method is called SequenceStatic.java Use a static variable to count the number of times a method has been called. Method generates a String representation of an arithmetic sequence < a a+d a+2d... a+(n-1)d > Java Section 5 11  static variable count number of times a method is called SequenceStatic.java a= 0, n= 4, d= 2 < 0 2 4 6 > a= 0, n= 4, d=-2 < 0 -2 -4 -6 > < 1 4 7 10 > < 10 15 20 25 30 35 > Number of sequence Strings 5 Java Section 5 12  static methods and variables ExploreStatic.java class ExploreStatic provides a collection of static methods from the MyMath challenge.  factorial  biCoeff factorial(6) is 720  power biCoeff( 4, 2) i.e. 4 choose 2 is 6 - uses a for loop biCoeff( 6, 3) i.e. 6 choose 3 is 20 biCoeff(24, 2) i.e. 24 choose 2 is 3???  pow - uses Math.pow() Different calls to power methods for 2.0 to power of 4 Demonstrates 16.0 16.0  argument passing 16.0 to methods The power and pow methods differ by 1.35525e-20!  static (aka class) We have a power(0.1,4)=0.00010000000000000003 variables difference! pow(0.1,4)=0.00010000000000000002  static (aka class) factorial() was called 14 times methods Java Section 5 13  Java class Math  class Math provides a collection of static methods that enable you to perform common mathematical calculations.  Many of the methods are overloaded to support different primitive types  Two math constants for mathematical constants π and e Math.PI 3.141592653589793 Math.E 2.718281828459045  Are declared in class Math with the modifier keywords as follows: public static final double E = 2.718281828459045; public static final double PI = 3.141592653589793;  Modifier public allows you to use these fields in other classes.  Modifier static ensures one version per class  Modifier final declares the field as constant  class Math is in the java.lang package so you don’t need to import it. Java Section 5 14  Java class Math Example methods  To call a method, prefix with the Math class name, see Example column. Math.methodName( arguments ) Method Description Example abs(a) Absolute value of a Math.abs(-12.2) is 12.2 cos(a) Trigonometric cosine of a (radians) Math.cos(Math.PI) is 1.0 exp(a) Exponential ea Math.exp(1) is 2.7182818… log(a) Natural logarithm of a Math.log(Math.E) is 1.0 min(a,b) smaller of a and b Math.min(1.5, 2.4) is 1.5 max(a,b) greater of a and b Math.min(1.5, 2.4) is 2.4 pow(a,b) a raised to the power b (ab) Math.pow(2.0, 16.0) is 65536 sin(a) Trigonometric sine of a (radians) Math.sin(MathPI/4) is 0.7071… sqrt(a) Square root of a Math.sqrt(625.0) is 25.0 tan(a) Trigonometric tangent of a (radians) Math.tan(MathPI/4) is 1.0 Java Section 5 15  Methods Declaration, formal parameters and actual arguments.  Each method must be declared within a class.  A method cannot be declared inside another method.  The parameters for a static or non-static method, are given in the method declaration and enclosed in ( ). public static returnType methodName ( parameters ) public returnType methodName ( parameters )  Multiple parameters are specified in a comma separated list.  Each parameter is known as a formal parameter to the method.  When calling a method, there must be an actual argument for each formal parameter. Note: The terms parameter and argument are often used interchangeably. Java Section 5 16  Methods Three ways of calling 1. Using a method name by itself to call another method of the same class methodName( arguments ) 2. Using a variable that contains a reference to an object, followed by a dot. and the method name to call any method of the referenced object objectReference.methodName( arguments ) 3. Using the class name and a dot (.) to call a static method of a class ClassName.methodName( arguments )  An instance (non-static) method can call any method of the same class directly and can manipulate any of the class’s fields directly.  A static method can directly call only other static methods and directly manipulate only static fields of the same class. Java Section 5 17  Methods Returning from a method  Recall: methods must be declared within the body of a class. public class ClassName { return-value-type methodName( method-parameters ) { // Statements to realise the method behaviour return return-value-expression; } }  Optional method-parameters return return-value; required if the return-value-type is not void  Three ways to return control to the statement that calls a method: 1. When the program flow reaches the method-ending right brace } 2. Using just return; applies only when return-value-type is void 3. When the method returns a result: return return-value-expression; Java Section 5 18  static vs instance (non-static) fields and methods Summary Static/class variable(s) One of each per class Fields private static int objectCount; Instance variable(s) One of each per instantiated object Java private String name; Class Class method(s) Access to static variables/methods Methods public static int max(int a, int b) … Instance method(s) Access to all fields and methods public String toString( ) … Java Section 5  Methods Overloading SumOverload.java Methods can have the same name if they have different parameter lists  e.g. sum to find the sum of the parameter values public static int sum( int a, int b, int c ) public static double sum( double a, double b, double c ) The compiler will select the appropriate method by comparing the number of parameters and their types. The return type is ignored. int sum1 = sum( 4, 8, 12 ); // will select first sum above double sum2 = sum( 4.2, 5.6, 7.2 ); // will select second sum above double sum3 = sum( 4.2, 5, 7 ); // will select second sum above Overloaded methods can have different numbers of parameters If the compiler cannot resolve, it will generate an error message. Recall: the challenge MyMath int and double min methods were overloaded. Java Section 5 20  Argument Promotion  Argument promotion among primitive types takes place to ensure that a method receives its corresponding parameter where possible.  Java has a set of promotion rules to specify allowed conversions  Applied when expression operators have more than one primitive type,  And when passing primitive-type values arguments to a method.  Values will be promoted when necessary to the “highest” type. Primitive Type Allowed promotions Examples double none 1. Method Math.cos(4) float double Since Math.cos() requires a long double float parameter of type double, the int double float long int 4 is promoted to double 4.0 before calling. char double float long int short double float long int 2. Expression: 4.2 * 5 byte double float long int short becomes 4.2*5.0 before multiplication, where int 5 is boolean none promoted to double 5.0 Java Section 5 21  Casting Explicit conversion  Converting values to types from higher to lower in the table could result in loss, and hence is not implicitly done (compilation error)  Converting a floating-point value to integer could lose the fractional part.  Converting a long to an integer may result in integer overflow (type) value will cast value to type (aka type convert) Primitive Type Allowed promotions Examples double none float double 1. int result = (int) (3.1+2.8); long double float result is 5 as (int) truncates (floors or rounds towards 0). int double float long char double float long int 2. (int) (Math.sqrt(x) + 0.5); short double float long int Math.sqrt() returns double byte double float long int short hence above will truncate to boolean none the nearest integer. Java Section 5 22  Passing Arguments to Methods Consider a MyMath average method public class MyMathTest 1. Call MyMath.average where { formal parameters a, b, c and d public static void main(String[ ] args) become, after argument promotion: { double result = MyMath.average( 1.8, 2, 7.2/2 + 4, 3*2 - 1 ); } } double double double double double 4.1 public class MyMath a 1.8 b 2.0 c 7.6 d 5.0 { public static double average(double a, double b, double c, double d) { return (a+b+c+d)/4; 2. average method then executes and evaluates the expression } 3. The average method return return (1.8+2.0+7.6+5.0)/4; } value will then be assigned i.e. return double value 4.1 to the local variable result Java Section 5  Methods Scope of Declarations  We declare entities such as variables and methods.  The scope of a declaration, when it said to be “in scope”, is the part of a class or method that can refer to the entity.  Each block of statements enclosed in { } can have its own variable declarations.  Local variables and parameter names can hide instance or class variables while “in scope”, known as shadowing.  The scope of a parameter or local variable  Scope of a parameter is the entire method body.  Scope of a local variable is from its declaration to the end of that block.  Scope of a variable declared in a for statement initialisation, is the entire for.  The scope of a method or field is the entire class body.  Except in static methods which can only access other static methods. Java Section 5 24  Variable length argument lists  Variable length argument lists are used to create methods that can receive an unspecified number of arguments  The last parameter type is followed by an ellipsis (…)  To indicate the variable number of arguments of that type can follow  The ellipsis can only occur in the last parameter.  The system will create an array of type to hold the parameters public static int min(int … numbers) declares a method which can accept any number of parameters of type int. Java Section 5 25  Variable length argument lists VariableLengthArgs.java Java Section 5 26 " " Command line arguments  Arguments can be passed to an application from an Operating System (OS) command line, or in an IDE.  In OS : java MyApplication argA argB "argC in quotes"  In IDE, there is usually a run option where they can be specified.  They are separated by white space  In Java, these argument are presented to the main method as an array of Strings public static void main(String[] args)  The number of arguments is args.length  args is argA, args is argB, args is "argC in quotes"  Print using for (String arg : args) System.out.println( arg ); Java Section 5 27  Command line arguments CLAs CommandLineArgs.java >java CommandLineArgs -h -v "Hello there" -h Use the command line java -v command to run the application, Hello there args = -h with CLAs args = -v args = Hello there Java Section 5 28  Graphical User Interface (GUI) using JOptionPane  Graphical User Interfaces (GUI) improve user interaction  GUI Applications use windows or dialog boxes (also called dialogs) to interact with the user.  Many applications can be developed using two simple GUI dialog boxes for inputting and outputting information.  available in the class JOptionPane in package javax.swing.  The JOptionPane dialogs input and output a String  The input String is typically converted to another type e.g. a primitive such as int or double Input Dialog Message Dialog Java Section 5  Example JOptionPane Dialogs Dialog.java Java Section 5  JOptionPane.showInputDialog conversion to a primitive type  JOptionPane.showInputDialog always returns a String  Must convert from String to the required primitive type  Typically use the Wrapper class method parseType. Primitive Type Conversion method int Integer.parseInt(String s) double Double.parseDouble(String s) float Float.parseFloat(String s) boolean Boolean.parseBoolean(String s) char Use the String charAt(int index) method byte Byte.parseByte(String s) short Short.parseShort(String s) long Long.parseLong(String s) Java Section 5  Java Primitive Wrapper Classes  A Java primitive wrapper class is one of eight classes provided to provide object methods for the primitive types  An extensive set of methods are provided, see Java API  Available in the java.lang package so no need to import Primitive Type Wrapper Class Constructor Arguments int Integer int, String double Double double, String float Float float, String boolean Boolean boolean, String char Character char byte Byte byte, String short Short short, string long Long long, String Java Section 5  JOptionPane Exercise  Using the code from the Dialog.java class, modify to find the volume of a cone of radius r and height h, which are input by the user using a showInputDialog.  Display the volume V using a showMessageDialog.  Explore the JOptionPane class in the Java API  Explore the various methods e.g. showMessageDialog, showInputDialog, showConfirmDialog, showOptionDialog Java Section 5 33  Application Case Studies  You are encourage to explore the source code of these classes to get more familiar with source code and Java concepts.  Start by running the code and observe the output  Then read the code  Modify and extend the code  Start by duplicating the class so you have a backup nearby if necessary.  In BlueJ, right mouse click on class icon and select Duplicate Java Section 5 34  Deck of Cards Case Study TestCardDeck, CardDeck, Card Card.java Java Section 5 35  Deck of Cards Basic Card Shuffler: Random select/swap CardDeck.java The class CardDeck creates decks of standard cards, extended decks, supports: shuffling, dealing, peeking and displaying, and more This is the code for a basic shuffle operation. Do explore all the classes: Card CardDeck TestCardDeck Improvements welcome! Java Section 5 36  static and instance methods and variables Exercise: bringing it all together  Create a class Student  Instance variable and methods  Add a single instance variable called name, of appropriate type.  Code a set and get method for the name instance var.  Static variable and method  Add a static/class variable count to maintain a count of the number of Student objects constructed.  Create a method getCount that returns the value of count  Create a no argument Student constructor that increments count each time it is called.  Execute and explore Java Section 5  Next Semester More skills, confidence, work, freedom, fun...  Explore more Java API classes And there’s still more afterwards!  Enumerated Types  Generic Collections  Lambdas and Streams  Exceptions  Concurrency  Inheritance  Database Access  Polymorphism  Mobile Apps  Enterprise Apps  Java GUIs  Networking/Cloud  Java Graphics +++  Strings and Regular Expressions  Recursion Signing off here, and wishing you all the best in life’s journey,  Searching and Sorting with or without Java! + using a Professional IDE John Java Section 5 38