Chapter 5 Methods

Chapter 5 Methods Spring 2013

Opening Problem Find the sum of integers from 1 to 10, from 20 to 30, and from 35 to 45, respectively.

Problem int sum = 0; for (int i = 1; i <= 10; i++) sum += i; System.out.println("Sum from 1 to 10 is " + sum); sum = 0; for (int i = 20; i <= 30; i++) sum += i; System.out.println("Sum from 20 to 30 is " + sum); sum = 0; for (int i = 35; i <= 45; i++) sum += i; System.out.println("Sum from 35 to 45 is " + sum);

Solution public static int sum(int i1, int i2) { int sum = 0; for (int i = i1; i <= i2; i++) sum += i; return sum; } public static void main(String[] args) { System.out.println("Sum from 1 to 10 is " + sum(1, 10)); System.out.println("Sum from 20 to 30 is " + sum(20, 30)); System.out.println("Sum from 35 to 45 is " + sum(35, 45)); }

Objectives • To define methods, invoke methods, and pass arguments to a method (§5.2-5.5). • To develop reusable code that is modular, easy-to-read, easy-to-debug, and easy-to-maintain. (§5.6). • To use method overloading and understand ambiguous overloading (§5.7). • To design and implement overloaded methods (§5.8). • To determine the scope of variables (§5.9). • To know how to use the methods in the Math class (§§5.10-5.11). • To learn the concept of method abstraction (§5.12). • To design and implement methods using stepwise refinement (§5.12).

Defining Methods A method is a collection of statements that are grouped together to perform an operation.

Method Signature Method signature is the combination of the method name and the parameter list.

Formal Parameters The variables defined in the method header are known as formal parameters.

Actual Parameters When a method is invoked, you pass a value to the parameter. This value is referred to as actual parameter or argument.

Return Value Type A method may return a value. The returnValueType is the data type of the value the method returns. If the method does not return a value, the returnValueType is the keyword void. For example, the returnValueType in the main method is void.

Example Return Value Types • Return type int • public static int max(int a, int b) { • Returns type integer • public static double max(int a, int b) { • Returns type double • public static void printn(String c) { • Does not return a value, so no return statement in this method.

Method body The method body is the code executed to produce the results.

Modifiers • Identifies how the method is used: • public: any other program has access to this method • static: static methods can be called without creating an instance of the class

Modifiers • Java modifiers • public: all methods have access to the method • protected: only methods in the same package have access to the method • private: only methods in the same class has access to the method • Java structure • Package: contains one or more classes • Class: contains one or more methods • Subclass: derived from a class

Calling Methods Testing the max method This program demonstrates calling a method max to return the largest of the int values TestMax

TestMax public class TestMax { /** Main method */ public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j); System.out.println("The maximum between " + i + " and " + j + " is " + k); } /** Return the max between two numbers */ public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2; return result; } }

animation Calling Methods, cont.

animation Trace Method Invocation i is now 5 i 5

animation Trace Method Invocation j is now 2 i j 5 2

animation Trace Method Invocation invoke max(i, j) i j 5 2

animation Trace Method Invocation invoke max(i, j) Pass the value of i to num1 Pass the value of j to num2 i j 5 2 num1 num2 5 2

animation Trace Method Invocation declare variable result i j 5 2 num1 num2 result 5 2

animation Trace Method Invocation (num1 > num2) is true since num1 is 5 and num2 is 2 i j 5 2 num1 num2 result 5 2

animation Trace Method Invocation result is now 5 i j 5 2 num1 num2 result 5 2 5

animation Trace Method Invocation return result, which is 5 i j 5 2 num1 num2 result 5 2 5

animation Trace Method Invocation return max(i, j) and assign the return value to k i j k 5 2 5 num1 num2 result 5 2 5

animation Trace Method Invocation Execute the print statement i j k 5 2 5 num1, num2, and result no longer exist

CAUTION A return statement is required for a value-returning method. The method shown below in (a) is logically correct, but it has a compilation error because the Java compiler thinks it possible that this method does not return any value. To fix this problem, delete if (n < 0) in (a), so that the compiler will see a return statement to be reached regardless of how the if statement is evaluated.

Reuse Methods from Other Classes NOTE: One of the benefits of methods is for reuse. The max method can be invoked from any class besides TestMax. If you create a new class Test, you can invoke the max method using ClassName.methodName (e.g., TestMax.max). publicclass Test { publicstaticvoid main(String[] args) { int a = 4; int b = 8; int c = TestMax.max(a,b); System.out.println("The largest of " + a + " and " + b + " is " + c); } }

Call Stacks

animation Trace Call Stack i is declared and initialized

animation Trace Call Stack j is declared and initialized

animation Trace Call Stack Declare k

animation Trace Call Stack Invoke max(i, j)

animation Trace Call Stack pass the values of i and j to num1 and num2

animation Trace Call Stack (num1 > num2) is true

animation Trace Call Stack Assign num1 to result

animation Trace Call Stack Return result and assign it to k

animation Trace Call Stack Execute print statement

void Method Example This type of method does not return a value. The method performs some actions. TestVoidMethod

TestVoidMethod public class TestVoidMethod { public static void main(String[] args) { System.out.print("The grade is "); printGrade(78.5); } 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'); } } }

Passing Parameters public static void nPrintln(String message, int n) { for (int i = 0; i < n; i++) System.out.println(message); } Suppose you invoke the method using • nPrintln(“Welcome to Java”, 5); What is the output? Suppose you invoke the method using • nPrintln(“Computer Science”, 15); What is the output? Welcome to Java Welcome to Java Welcome to Java Welcome to Java Welcome to Java

Pass by Value • Pass the value of the variable, not the memory location • Make changes to value will not change the value in the original variable • Listing 5.2 Testing Pass by value • This program demonstrates passing values to the methods. This program demonstrates passing values to the methods. Increment

Pass by Value Testing Pass by value This program demonstrates passing values to the methods. TestPassByValue

TestPassByValue public class TestPassByValue { /** Main method */ public static void main(String[] args) { // Declare and initialize variables int num1 = 1; int num2 = 2; System.out.println("Before invoking the swap method, num1 is " + num1 + " and num2 is " + num2); // Invoke the swap method to attempt to swap two variables swap(num1, num2); System.out.println("After invoking the swap method, num1 is " + num1 + " and num2 is " + num2); } /** Swap two variables */ public static void swap(int n1, int n2) { System.out.println("\tInside the swap method"); System.out.println("\t\tBefore swapping n1 is " + n1 + " n2 is " + n2); // Swap n1 with n2 int temp = n1; n1 = n2; n2 = temp; System.out.println("\t\tAfter swapping n1 is " + n1 + " n2 is " + n2); } }

Pass by Value, cont. After the swap, n2 = 1 and n1 = 2 But the values in num1 and num2 do not change

Modularizing Code Methods can be used to reduce redundant coding and enable code reuse. Methods can also be used to modularize code and improve the quality of the program. GreatestCommonDivisorMethod PrimeNumberMethod

Chapter 5 Methods