DCIT 201 Programming I - Methods PDF

DCIT 201 PROGRAMMING I Session 2 – Methods Lecturer: Mr. Paul Ammah, CSD Contact Information: [email protected] Department of Computer Science School of Physical and Mathematical Science  Session Objectives Understand the advantages of using methods Be able to define and inv...

DCIT 201 PROGRAMMING I Session 2 – Methods Lecturer: Mr. Paul Ammah, CSD Contact Information: [email protected] Department of Computer Science School of Physical and Mathematical Science  Session Objectives Understand the advantages of using methods Be able to define and invoke methods To develop reusable code that is modular, easy to read, easy to debug, and easy to maintain Understand the scope of variables Understand, identify and apply method overloading Understand the concept of parameter passing Slide 2  Session Outline The key topics to be covered in the session are as follows: Methods Defining Methods Parameter passing Overloading methods The scope of variables Slide 3  Reading List Chapter 5 – Big Java Late Objects (Horstmann, 2012) Slide 4 Topic One METHODS Slide 5  Introduction A method packages a computation consisting of multiple steps into a form that can be easily understood and reused. A method is a sequence of instructions with a name. Every Java program has a method called main. You call a method in order to execute its instructions. Slide 6  Introduction cont’d A method is a collection of statements grouped together to perform an operation. In earlier slides we have used predefined methods such as System.out.println, JOption-Pane.showMessageDialog, JOptionPane.showInputDialog, Integer.parseInt, Double.parseDouble, System.exit, Math.pow, and Math.random. These methods are defined in the Java library. Slide 7 Topic Two DEFINING METHODS Slide 8  Defining a Method - Syntax The syntax for defining a method is as follows: modifier returnValueType methodName(list of parameters) { // Method body; } Slide 9  Understanding the syntax The method syntax specifies the modifiers, return value type, method name, and parameters of the method. The static modifier is used for all the methods for now. A method may return a value. The returnValueType is the data type of the value the method returns. Some methods perform desired operations without returning a value. In this case, the returnValueType is the keyword void. For example, the returnValueType is void in the main method, as well as in System.exit, System.out.println, and JOptionPane.showMessageDialog. Slide 10  Understanding the syntax cont’d If a method returns a value, it is called a value- returning method, otherwise it is a void method. The variables defined in the method header are known as formal parameters or simply parameters. A parameter is like a placeholder. When a method is invoked, you pass a value to the parameter. This value is referred to as an actual parameter or argument. The parameter list refers to the type, order, and number of the parameters of a method. Slide 11  Understanding the syntax cont’d The method name and the parameter list together constitute the method signature. Parameters are optional; that is, a method may contain no parameters. For example, the Math.random() method has no parameters. In order for a value-returning method to return a result, a return statement using the keyword return is required. The method terminates when a return statement is executed. In the method header, you need to declare a separate data type for each parameter. Slide 12 Implementing Methods Slide 13  Calling a Method In creating a method, you define what the method is to do. To use a method, you have to call or invoke it. There are two ways to call a method, depending on whether the method returns a value or not. If the method returns a value, a call to the method is usually treated as a value. For example, int larger = max(3, 4); calls max(3, 4) and assigns the result of the method to the variable larger. Another example of a call that is treated as a value is System.out.println(max(3, 4)); which prints the return value of the method call max(3, 4). Slide 14  Sample program using Method public class TestMax { public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2; return result; } public static void main(String[] args) { int i = 5, j = 2, k = max(i, j) ; System.out.println("The maximum between " + i + " and " + j + " is " + k); } } Slide 15  Call Stacks Each time a method is invoked, the system stores parameters and variables in an area of memory known as a stack, which stores elements in last-in, first-out fashion. When a method calls another method, the caller’s stack space is kept intact, and new space is created to handle the new method call. When a method finishes its work and returns to its caller, its associated space is released. Slide 16  void Method Sometimes, you need to carry out a sequence of instructions that does not yield a value. If that instruction sequence occurs multiple times, you will want to package it into a method. In Java, you use the return type void to indicate the absence of a return value. A void method returns no value, but it can produce output. Slide 17  void Method Example public class TestReturnGradeMethod { public static void main(String[] args) { System.out.print("The grade is "+ printGrade(78.5) ); System.out.print("\nThe grade is "+ printGrade(59.5) ); } public static void printGrade(double score) { if (score >= 90.0) System.out.println('A'); else if (score >= 80.0) System.out.println('B'); else if (score >= 70.0) System.out.println('C'); else if (score >= 60.0) System.out.println('D'); else System.out.println('F'); } } Slide 18 Topic Three PARAMETER PASSING Slide 19  Parameter passing When a method is called, variables are created for receiving the method’s arguments. These variables are called parameter variables. (Another commonly used term is formal parameters.) The values that are supplied to the method when it is called are the arguments of the call. Each parameter variable is initialized with the corresponding argument. Slide 20  Passing Parameters by Values The power of a method is its ability to work with parameters. When calling a method, you need to provide arguments, which must be given in the same order as their respective parameters in the method signature. When you invoke a method with a parameter, the value of the argument is passed to the parameter. This is referred to as pass-by-value. If the argument is a variable rather than a literal value, the value of the variable is passed to the parameter. The variable is not affected, regardless of the changes made to the parameter inside the method. Slide 21  Passing-by-value example public class Increment { public static void main(String[] args) { int x = 1; System.out.println("Before the call, x is " + x); increment(x); System.out.println("after the call, x is " + x); } public static void increment(int n) { n++; System.out.println("n inside the method is " + n); } } Slide 22  Program Output The previous program produces the following output: The value of x (1) is passed to the parameter n to invoke the increment method. n is incremented by 1 in the method , but x is not changed no matter what the method does. Slide 23 Topic Four OVERLOADING METHODS Slide 24  Overloading Methods The max method that was used earlier works only with the int data type. But what if we need to determine which of two floating-point numbers has the maximum value? The solution is to create another method with the same name but different parameters. The concept where methods having the same name but different parameters is known as method overloading. You cannot overload methods based on different modifiers or return types. Slide 25  Example: Overloading Methods public class TestMethodOverloading { public static void main(String[] args) { // Invoke the max method with int parameters System.out.println("The maximum between 3 and 4 is “ + max(3, 4) ); // Invoke the max method with the double parameters System.out.println("The maximum between 3.0 and 5.4 is “ + max(3.0, 4.5)); // Invoke the max method with three double parameters System.out.println("The maximum between 3.0, 5.4, and 10.14 is " + max(3.0, 5.4, 10.14) ); } Dr. Jamal-Deen Abdulai, CSCD Slide 26  Example: Overloading Methods cont’d public static int max(int num1, int num2) { if (num1 > num2) return num1; else return num2; } public static double max(double num1, double num2) { if (num1 > num2) return num1; else return num2; } public static double max(double num1, double num2, double num3){ return max(max(num1, num2), num3); } } Slide 27  Understanding the code When calling max(3, 4) , the max method for finding the maximum of two integers is invoked. When calling max(3.0, 5.4) , the max method for finding the maximum of two doubles is invoked. When calling max(3.0, 5.4, 10.14), the max method for finding the maximum of three double values is invoked. Both max(double, double) and max(int, int) are possible matches for max(3, 4). The Java compiler finds the most specific method for a method invocation. Since the method max(int, int) is more specific than max(double, double), max(int, int) is used to invoke max(3, 4). Slide 28 Topic Five THE SCOPE OF VARIABLES Slide 29  The scope of variables The scope of a variable is the part of the program where the variable can be referenced. A variable defined inside a method is referred to as a local variable. The scope of a local variable starts from its declaration and continues to the end of the block that contains the variable. A local variable must be declared and assigned a value before it can be used. Slide 30  The scope of variables cont’d A parameter is actually a local variable. The scope of a method parameter covers the entire method. A variable declared in the initial-action part of a for-loop header has its scope in the entire loop. But a variable declared inside a for-loop body has its scope limited in the loop body from its declaration to the end of the block that contains the variable. Do not declare a variable inside a block and then attempt to use it outside the block. It will result in an error. Slide 31  The scope of variables cont’d Do not declare a variable inside a block and then attempt to use it outside the block. It will result in an error. See below: for (int i = 0; i < 10; i++) { } System.out.println(i); The last statement would cause a syntax error, because variable i is not defined outside of the for loop. Slide 32  Method Abstraction and Stepwise Refinement The key to developing software is to apply the concept of abstraction. Method abstraction is achieved by separating the use of a method from its implementation. The client can use a method without knowing how it is implemented. The details of the implementation are encapsulated in the method and hidden from the client who invokes the method. This is known as information hiding or encapsulation. If the programmer decide to change the implementation, the client program will not be affected, provided the method signature remains unchanged. The implementation of the method is hidden from the client in a “black box”. Slide 33  Divide-and-Conquer The concept of method abstraction can be applied to the process of developing programs. When writing a large program, you can use the divide-and-conquer strategy, also known as stepwise refinement, to decompose it into subproblems. The subproblems can be further decomposed into smaller, more manageable problems. Slide 34  References Big Java Late Objects (Horstmann, 2012) An Introduction to Problem Solving and Programming (Savitch, 2012) Introduction to Java Programming(Liang, 2011) Slide 35