Chapter 5 Methods 1. Motivations Method : groups statements that perform a function. Level of abstraction (black box) Code Reuse – no need to reinvent

Chapter 5 Methods

1

MotivationsMethod :groups statements that perform a function.

Level of abstraction (black box) Code Reuse – no need to reinvent the wheel!

2

Defining Methods3

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

public static int max(int num1, int num2) {

int result; if (num1 > num2) result = num1; else result = num2; return result;

}

modifier return value

type method name

formal parameters

return value

method body

method header

parameter list

Define a method Invoke a method

int z = max(x, y);

actual parameters (arguments)

method signature

Method Signature4

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



}


type method name

formal parameters

return value

method body

method header

parameter list


int z = max(x, y);


method signature

Formal Parameters5

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



}


type method name

formal parameters

return value

method body

method header

parameter list


int z = max(x, y);


method signature

Actual Parameters6

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



}


type method name

formal parameters

return value

method body

method header

parameter list


int z = max(x, y);


method signature

Return Value Type7

A method may return a value.

If the method does not return a value, the returnValueType is the keyword void. (i.e., main method)



}


type method name

formal parameters

return value

method body

method header

parameter list


int z = max(x, y);


method signature

Calling Methods8

Listing 5.1 Testing the max method

This program demonstrates calling a method max to return the largest of the int values

TestMax Run

Calling Methods, cont.9

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); }

public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2; return result; }

pass the value of i pass the value of j

animation

Trace Method Invocation10



i is now 5

animation




j is now 2

animation




invoke max(i, j)

animation




invoke max(i, j)Pass the value of i to num1Pass the value of j to num2

animation




declare variable result

animation




(num1 > num2) is true since num1 is 5 and num2 is 2

animation




result is now 5

animation




return result, which is 5

animation




return max(i, j) and assign the return value to k

animation




Execute the print statement

animation

CAUTIONA 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.

20

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.

public static int sign(int n) { if (n > 0) return 1; else if (n == 0) return 0; else if (n < 0) return –1; }

(a)

Should be

(b)

public static int sign(int n) { if (n > 0) return 1; else if (n == 0) return 0; else return –1; }

Call Stacks 21

(a) The main method is invoked.

Space required for the main method k: j: 2 i: 5

(b) The max method is invoked.

Space required for the max method num2: 2 num1: 5

(d) The max method is finished and the return value is sent to k.

(e) The main method is finished.

Stack is empty


Space required for the main method k: 5 j: 2 i: 5

(c) The max method is being executed.

Space required for the max method result: 5 num2: 2 num1: 5


Trace Call Stack

The main method is invoked.

i: 5

22

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); } public static int max(int num1, int num2) { int result; if (num1 > num2) result = num1; else result = num2; return result; }

i is declared and initialized

animation

Trace Call Stack


j: 2 i: 5

23


j is declared and initialized

animation

Trace Call Stack


Space required for the main method k:

j: 2 i: 5

24


Declare k

animation

Trace Call Stack



j: 2 i: 5

25


Invoke max(i, j)

animation

Trace Call Stack

The max method is invoked.

num2: 2 num1: 5


j: 2 i: 5

26


pass the values of i and j to num1 and num2

animation

Trace Call Stack


result:

num2: 2 num1: 5


j: 2 i: 5

27


pass the values of i and j to num1 and num2

animation

Trace Call Stack


result:

num2: 2 num1: 5


j: 2 i: 5

28


(num1 > num2) is true

animation

Trace Call Stack


Space required for the max method result: 5

num2: 2 num1: 5


j: 2 i: 5

29


Assign num1 to result

animation

Trace Call Stack


Space required for the max method result: 5

num2: 2 num1: 5

Space required for the main method k:5

j: 2 i: 5

30


Return result and assign it to k

animation

Trace Call Stack


Space required for the main method k:5

j: 2 i: 5

31


Execute print statement

animation

Overloading MethodsTwo methods can have the same namebut different argument lists.

Java examines the signatures to determine which method to call.

Unlike C++, operators cannot be overloaded by the user.

32

Ambiguous Invocation Two or more possible matches for an invocation of a method Compiler cannot determine the most specific match. Compilation error.

33

Ambiguous Invocationpublic class AmbiguousOverloading { public static void main(String[] args) { System.out.println(max(1, 2)); } public static double max(int num1, double num2) { if (num1 > num2) return num1; else return num2; } public static double max(double num1, int num2) { if (num1 > num2) return num1; else return num2; }}

34

Scope of Local VariablesLocal variable: defined inside a method.

Scope: part of program where variable can be referenced.

The scope of a local variable: from declaration until end of the block that contains it

35

Scope of Local Variables, cont.

Can you declare a local variable with the same name multiple times in a method?

If the statements are in different non-nesting blocks of code

36

Scope of Local Variables, cont.Variable declared in the initial action part of for loop:

has scope in the entire loop Variable declared inside for loop body:

has scope from its declaration to the end of the block

37

public static void method1() { . . for (int i = 1; i < 10; i++) { . . int j; . . . } }

The scope of j

The scope of i

Scope of Local Variables, cont.

38

public static void method1() { int x = 1; int y = 1;

for (int i = 1; i < 10; i++) {

x += i; }

for (int i = 1; i < 10; i++) {

y += i; } }

It is fine to declare i in two non-nesting blocks public static void method2() {

int i = 1; int sum = 0;

for (int i = 1; i < 10; i++) {

sum += i; } }

It is wrong to declare i in two nesting blocks

Scope of Local Variables, cont.// Fine with no errorspublic static void correctMethod() { int x = 1; int y = 1; // i is declared for (int i = 1; i < 10; i++) { x += i; } // i is declared again for (int i = 1; i < 10; i++) { y += i; }}

39

Scope of Local Variables, cont. 40

// With no errorspublic static void incorrectMethod() { int x = 1; int y = 1; for (int i = 1; i < 10; i++) { int x = 0; x += i; }}

Method AbstractionYou can think of the method body as a black box that contains the detailed implementation for the method.

41

Method Header

Method body Black Box

Optional arguments for Input

Optional return value

Benefits of Methods42

• Write a method once and reuse it anywhere.

• Information hiding. Hide the implementation from the user.

• Reduce complexity.

Stepwise RefinementMethod abstraction in program development:“Divide and conquer”

ORStepwise refinement

Decompose program into subproblems.

Subproblems can be further decomposed.

43

PrintCalender Case Study Let us use the PrintCalendar example to demonstrate the stepwise refinement approach.

44

PrintCalendar Run

Design Diagram45

printCalendar (main)

readInput printMonth

getStartDay

printMonthTitle printMonthBody

getTotalNumOfDays

getNumOfDaysInMonth

getMonthName

isLeapYear

Design Diagram46



getStartDay


getTotalNumOfDays

getNumOfDaysInMonth

getMonthName

isLeapYear

Design Diagram47



getStartDay


getTotalNumOfDays

getNumOfDaysInMonth

getMonthName

isLeapYear

Design Diagram48



getStartDay


getTotalNumOfDays

getNumOfDaysInMonth

getMonthName

isLeapYear

Design Diagram49



getStartDay


getTotalNumOfDays

getNumOfDaysInMonth

getMonthName

isLeapYear

Design Diagram50



getStartDay


getTotalNumOfDays

getNumOfDaysInMonth

getMonthName

isLeapYear

Implementation: Top-Down51

A Skeleton for printCalendar

Top-down approach:Work your way down the structure chart

Invoke an incomplete method using method stubs.

Implement the main method first and then use a stub for the printMonth method. For example, let printMonth display the year and the month in the stub. Thus, your program may begin like this:

Implementation: Bottom-Up52

Bottom-up approach: Work your way up the structure chart

Write program to test each method implemented.

Stepwise refinement in either direction implements the methods incrementally helps isolate programming errors

Documents

Chapter 5 Methods 1. Motivations Method : groups statements that perform a function. Level of abstraction (black box) Code Reuse – no need to reinvent