8-Methods.pptx

Methods - Subprograms COM136 – Software Development 1 1 Learning Outcomes • At the end of this lecture you should • Understand the concepts associated with the method construct and their implementation in Java. • Understand the difference between action and value methods. • Learn how to use param...

Methods - Subprograms COM136 – Software Development 1 1 Learning Outcomes • At the end of this lecture you should • Understand the concepts associated with the method construct and their implementation in Java. • Understand the difference between action and value methods. • Learn how to use parameters in Java methods • Understand what is meant by call-by-value parameter passing • Understand what is meant by scope and duration of variables 2 Breaking larger programs down • Two popular approaches to program design, especially when writing larger programs • Structured programming – divide initial problem into smaller sub-problems. Implement algorithms for each sub-problem and combine into overall solution. • Object Oriented programming – modern approach which will be covered in module COM139 next semester. • We are using the structured approach, but the algorithm development skills we are learning are required in both approaches 3 Sub-programs and Methods • Algorithms we have created have used the basic programming constructs of • Sequence • Selection • Repetition • But larger programs may consist of multiple algorithms • We can use methods (sub-programs) to implement multiple algorithms in our solutions 4 Sub-programs and Methods • Advantages of methods • Allow us to provide solutions to larger problems. • Different people can work on different methods simultaneously. • Methods can be reused. • Using methods enhances the programs readability because it reduces complexity. • Methods are also integral to the object oriented approach covered next semester. 5 Some methods we’ve already seen • Which classes do each of these belong to? • What type of parameters can we give to them? • How do we use them? • nextInt() • getInt(“Enter an Integer”) • println(“Hello!”) • main(String args[]) • mars() • Pizza base recipe (Not Java!) 6 Answers • nextInt() • Class: Scanner • parameters: none • Action: returns integer value from text available to scanner or throws exception if number cannot be parsed • getInt (“Enter an integer”) • Class: Console • parameter: A String • Action: Writes String parameter to console and gets integer input from user 7 Answers (cont) • println (“Hello!”) • Class: System.out • parameters: a String • Action: Writes String to console • main(String args[]) • Class: Defined by the programmer • parameter: An array of strings • Action: Used to implement any algorithm you choose 8 Answers (cont) • mars() • Class: Defined by us • parameter: None • Action: Writes a message on the screen • Pizza base recipe (Not Java!) • Class: The recipe book (collection of algorithms) • parameter: List of ingredients • Action: Makes a pizza base 9 How do we use (call) these methods? • Following method is called automatically when you run the program main(String args[]) • Following methods are called by using their name (with parameters, if any) as a statement as shown below System.out.println(“Hello!”); mars(); 10 Note • When the method isn’t in the class from which it is being called, we must include its class name in the call, as with following: • System.out.println(“Hello”); • These are called static methods and we will learn more about static and non-static methods next semester 11 Calling methods (continued) • The following method call is different! int age = Console.getInt(“Enter your age”); integer returned by Console.getInt method is assigned to variable age • We call it by using its name, as with the others, but the call is an expression, not a statement. • To capture the result, we must either • assign it to a variable, as above • pass it to another method as a parameter 12 What do these method calls do? System.out.println( “Hello world” ); 1. Call to System.out.println(..) evaluates the parameter ‘a string’ 2. And result of method is to displays this string to the console System.out.println( Console.getString(“Name?”) ); 1. Call to System.out.println(..) evaluates the parameter, this time its ‘a call to a value returning method’ Console.getString(..) 2. Remember, getString(..) returns a user entered string and this 3. String is passed to System.out.println method which displays the string 13 So we’ve got two types of methods: • Ones that carry out an action (have a side-effect) these have return type void - indicates that they don’t return anything e.g. public static void main(String args[]) public static void mars() public static void println(String prompt) 14 … and ones that return a value • Ones that return a value have a non-void return type e.g. Console.getInt(“…”) • What is the return type of this method? - an int • How do we know? Well we read the documentation for this library method 15 Here’s an example of each type // action method – prints square public static void squareAction(int y) { System.out.println( y * y ); } // value method – returns square public static int squareValue(int y) { return y * y; } 16 Example of Usage • Calling action method System.out.print(“Square of 6 is “); squareAction(6);  prints 36 to console • Calling value method int sq = squareValue(6); System.out.println(“Square of 6 is “ + sq); • Alternative way to call value method System.out.println(“Square of 6 is “ + squareValue(6) ); squareValue returns 36 which is added to string “Square of 6 is “ and printed to console by println command 17 Note • A method with return type void (an action method) must not return a value; it may have a return statement without an expression return; • …but this is simply used to terminate the method. • A method with a return type other than void must contain a return statement with an expression of the return type. 18 Method Parameters • Method header indicates if the method accepts any parameters and returns a value (value method) or no value (action method) • For example public static int getInt(String prompt) One String parameter No parameters public static void mars() One int parameter public static void squareAction(int a) 19 Some examples • Consider the following method declarations public static void method1() public static void method2(int x, double y) public static int method3(char c) • Which of following are valid method calls method1(); method2(2); int a = method3(‘B’); method2(2, 4.0); method2(method3(‘A’),4.56); => => => => => Ok Error Ok Ok Ok 20 Parameters • Parameters declared in heading of the method are called formal parameters. • Parameters supplied when the method is called are actual parameters. • Actual and formal parameters must correspond in number, order and type. • ‘Correspond in type’ does not necessarily mean they must be the same type. For example an int parameter could be passed to a method that expects a double (but not vice a versa). 21 Parameter Passing • What happens when a method is called? int x = 4; squareAction(x); • The value of the actual parameter (variable x (4)) is copied into the formal parameter (variable y)within squareAction method overleaf. • This mechanism is known as call by value • Variable y behaves similarly to an initialised local variable within squareAction. • Any changes to y do not affect x 22 Call By Value parameter passing public static int squareValue( int y ) { y = y * y; return y; } public static void main(String[] args) { int x = 2; Memory x 2 y 24 System.out.println( “Square of “ + x + “ is “ + squareValue(x) ); } 23 More on Parameters • What will squareAction write to the console? squareAction(5); => 25 squareAction(2 + 4); => 36 squareAction(Console.getInt(“Num?”)); => ?? 24 Method Composition • The squareValue method returns a value which may itself be used as a parameter to another method. • What is the value of r in following expressions? 81 9 int r = squareValue( squareValue(3) ); 256 16  81 4 int r = squareValue( squareValue( squareValue(2) ) );  256 25 Duration of identifiers • The duration of an identifier is the time during which it exists in memory. • Parameters and local variables declared in a method body have automatic duration; • created when their declaration is reached, • destroyed when the block they are in, delimited by { } terminates. 26 Scope of identifiers • The scope of an identifier is that part of the program in which it may be referenced. • A variable declared in a block, that is, between any pair of { } has block scope - it can only be used between its declaration and the end of that block. 27 Duration and Scope public static void sample(int x) { int y = 0; while (y < x) { int z = y; System.out.println(z); y = y + 1 ; } Scope of z } Scope of x and y 28 Variable Scope and Duration Guidelines • Always declare a variable as locally as possiblethus limiting likelihood of being accidentally changed • Consider following swap algorithms Ok Better int temp; ... if (a > b) { temp = a; a = b; b = temp; } if (a > b) { int temp = a; a = b; b = temp; } Temp only declared in block where it is required 29 Flow of Execution • When a method is called, execution of the current method is halted while the called method is executing. • When the called method terminates, execution of the calling method resumes with next statement main() { statement1; method1(); statement3; } method1() { statement1; method2(); statement3; } method2() { statement1 statement2 statement3; } 30 Guidelines for creating methods • Each method should perform one defined task. • Typically a method should contain no more than a page of code. • A program will typically consist of a collection of methods including the main method. • Methods should be tested separately and then added to the main program. • Using methods makes a program easier to develop, debug and understand. 31 An Example  We define a value method to prompt a user to return a vat code 0-2 which we can use in our main method public class VatCalculator { // input number in range bound by low and high public static int getVatCode() { int code = Console.getInt(“Enter vat code 0-2”); while (code < 0 || code > 2) { num = Console.getInt(“Enter vat code 0-2”); } return code; } // PTO ... 32 An Example  We can now call this method ... public static void main(String args[]) { // accept a valid vat code (0-2) int code = getVatCode(); double price = Console.getDouble(“Enter price £”); // now apply the vat based on the code if (code == 1) { // } ... ... } // end of main } // end of class 33 An Improved version  We could make this method more “reusable” (less specific) as follows public class NumInput { // input number in range bound by low and high public static int getInRange(int low, int high, String prompt) { int num = Console.getInt(prompt); while (num < low || num > high) { num = Console.getInt(prompt); } return num; } … other input methods can go here } 34 An Example  To use this new class and method we would call the method as follows int num = NumInput.getInRange(0,2,”Enter Code 02”);  Note:  If class is in same directory as calling class then we can simply call it  Otherwise we need to add the directory containing the class to the Java CLASSPATH and import (like uulib)  Topic for next semester! 35 An Example  The Vat program calls getInRange which exists in the separate NumInput class public class VatCalculator { public static void main(String[] args) { // ensure value of code is 0,1 or 2 int code = NumInput.getInRange(0,2,”Enter(0-2)”); double price = Console.getDouble(“Enter price £”); ... // note how we call this method as its in this class file double vat = calcVatBasedOnCode(code, price); System.out.println(“Vat £ “ + vat); } // ... PTO 36 An Example // sub-program (method) to calc vat public static double calcVatBasedOnCode(int code, double price) { double vat = 0; // default is 0.0 vat if (code == 2) { vat = price * 0.2; // ideally use constant not 0.2 } else if (code == 1) { vat = price * 0.05; // ideally use constant not 0.05 } return vat; } } // end of class 37 Summary • In this section we have covered: • The concepts associated with the method construct and their implementation in Java. • The difference between action and value methods. • How to use parameters in Java methods • What is meant by call-by-value parameter passing • What is meant by scope and duration of variables 38 An extreme “functional” Example public class VatCalc { public static void main(String[] args) { double vat = calcVatBasedOnCode( NumInput.getInRange(0, 2, ”Enter(0-2)” ), Console.getDouble(“Enter price £”) ); System.out.println(“Vat £ “ + vat); } 39 An extreme “functional” Example 2 public class VatCalc { public static void main(String[] args) { System.out.println(“Vat £ “ + calcVatBasedOnCode( NumInput.getInRange(0, 2,”Enter Code 0-2”), Console.getDouble(“Enter price £”) ) ); } 40