Chapter 4 Methods F Introducing Methods –Benefits of methods, Declaring Methods, and Calling...
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Transcript of Chapter 4 Methods F Introducing Methods –Benefits of methods, Declaring Methods, and Calling...
Chapter 4 Methods Introducing Methods
– Benefits of methods, Declaring Methods, and Calling Methods
Passing Parameters– Pass by Value
Overloading Methods– Ambiguous Invocation
Scope of Local Variables Method Abstraction The Math Class Case Studies Recursion (Optional)
Introducing Methods
Method StructureA method is a collection of statements that are grouped together to perform an operation.
Introducing Methods, cont.•parameter profile refers to the type, order, and number of the parameters of a method.
•method signature is the combination of the method name and the parameter profiles.
•The parameters defined in the method header are known as formal parameters.
•When a method is invoked, its formal parameters are replaced by variables or data, which are referred to as actual parameters.
Declaring Methods
public static int max(int num1, int num2) {
if (num1 > num2) return num1; else return num2;}
Calling Methods
Example 4.1 Testing the max method
This program demonstrates calling a method max to return the largest of the int values
TestMaxTestMax Run
Calling Methods, cont.
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 i pass j
Calling Methods, cont.
The main method i: j: k:
The max method num1: num2: result:
pass 5
5
2
5
5
2
5
pass 2 parameters
CAUTION
A return statement is required for a nonvoid method. The following method is logically correct, but it has a compilation error, because the Java compiler thinks it possible that this method does not return any value. public static int xMethod(int n) { if (n > 0) return 1; else if (n == 0) return 0; else if (n < 0) return –1; }
To fix this problem, delete if (n<0) in the code.
Passing Parameterspublic static void nPrintln(String message, int n) {
for (int i = 0; i < n; i++) System.out.println(message);}
Pass by Value
Example 4.2 Testing Pass by value
This program demonstrates passing values to the methods.
TestPassByValueTestPassByValue Run
Pass by Value, cont.
swap(num1, num2)
swap( n1, n2)
Pass by value
num1
Swap
1
2
n1
n2
1
2
n1
n2
2
1
temp 1
Execute swap inside the swap body
num2
Invoke swap The values of num1 and num2 are passed to n1 and n2. Executing swap does not affect num1 and num2.
Overloading Methods
Example 4.3 Overloading the max Method
public static double max(double num1, double num2) {
if (num1 > num2) return num1; else return num2;}
TestMethodOverloadingTestMethodOverloading Run
Ambiguous Invocation
Sometimes there may be two or more possible matches for an invocation of a method, but the compiler cannot determine the most specific match. This is referred to as ambiguous invocation. Ambiguous invocation is a compilation error.
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; }}
Scope of Local VariablesA local variable: a variable defined inside a
method.Scope: the part of the program where the variable
can be referenced.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 before it can be used.
Scope of Local Variables, cont.
You can declare a local variable with the same name multiple times in different non-nesting blocks in a method, but you cannot declare a local variable twice in nested blocks. Thus, the following code is correct.
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; }}
Scope of Local Variables, cont.
// 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 Abstraction
You can think of the method body as a black box that contains the detailed implementation for the method.
Method Signature
Method body
Black Box
Optional Input Optional returnvalue
Benefits of Methods
• Write once and reuse it any times.
• Information hiding. Hide the implementation from the user.
• Reduce complexity.
The Math Class Class constants:
– PI– E
Class methods: – Trigonometric Methods – Exponent Methods– Rounding Methods– min, max, abs, and random Methods
Trigonometric Methods
sin(double a)
cos(double a)
tan(double a)
acos(double a)
asin(double a)
atan(double a)
Exponent Methods exp(double a)
Returns e raised to the power of a.
log(double a)
Returns the natural logarithm of a.
pow(double a, double b)
Returns a raised to the power of b.
sqrt(double a)
Returns the square root of a.
Rounding Methods double ceil(double x)
x rounded up to its nearest integer. This integer is returned as a double value.
double floor(double x)x is rounded down to its nearest integer. This integer is returned as a double value.
double rint(double x)x is rounded to its nearest integer. If x is equally close to two integers, the even one is returned as a double.
int round(float x)Return (int)Math.floor(x+0.5).
long round(double x)Return (long)Math.floor(x+0.5).
min, max, abs, and random
max(a, b)and min(a, b)Returns the maximum or minimum of two parameters.
abs(a)Returns the absolute value of the parameter.
random()Returns a random double valuein the range [0.0, 1.0).
Example 4.4 Computing Mean and Standard Deviation
Generate 10 random numbers and compute the mean and standard deviation
ComputeMeanDeviationComputeMeanDeviation Run
n
xmean
n
ii
1
1
)(
1
2
12
nn
xx
deviation
n
i
n
ii
i
Example 4.5 Obtaining Random Characters
Write the methods for generating random characters. The program uses these methods to generate 175 random characters between ‘!' and ‘~' and displays 25 characters per line. To find out the characters between ‘!' and ‘~', see Appendix B, “The ASCII Character Set.”
RandomCharacterRandomCharacter Run
Example 4.5 Obtaining Random Characters, cont.
Appendix B: ASCII Character Set
Case Studies
Example 4.6 Displaying Calendars
The program reads in the month and year and displays the calendar for a given month of the year.
PrintCalendarPrintCalendar Run
Design Diagram
printCalendar (main)
readInput printMonth
getStartDay printMonthTitle printMonthBody
getTotalNumOfDays
getNumOfDaysInMonth
getMonthName
isLeapYear
Recursion (Optional)
Example 4.7 Computing Factorial
factorial(0) = 1;
factorial(n) = n*factorial(n-1);
ComputeFactorialComputeFactorial Run
Example 4.7 Computing Factorial, cont.
factorial(4) = 4*factorial(3)
factorial(3) = 3*factorial(2)
factorial(2) = 2*factorial(1)
factorial(1) = 1*factorial(0)
Step 6: factorial(1) returns 1 (1*1)
main method: factorial(4)
Step 1: factorial(4) calls factorial(3)
factorial(4) is called in the main
Step 2: factorial(3) calls factorial(2)
Step 3: factorial(2) calls factorial(1)
factorial(0) = 1
Step 4: factorial(1) calls factorial(0)
Step 5: factorial(0) returns 1
Step 7: factorial(2) returns 2 (2*1)
Step 8: factorial(3) returns 6 (3*2)
Step 9: factorial(4) returns 24 (4*6)
Example 4.7 Computing Factorial, cont.
Space Requiredfor factorial(4)
1 Space Requiredfor factorial(4)
2 Space Requiredfor factorial(3)
Space Requiredfor factorial(4)
3
Space Requiredfor factorial(3)
Space Requiredfor factorial(2)
Space Requiredfor factorial(4)
4
Space Requiredfor factorial(3)
Space Requiredfor factorial(2)
Space Requiredfor factorial(1)
Space Requiredfor factorial(4)
5
Space Requiredfor factorial(3)
Space Requiredfor factorial(2)
Space Requiredfor factorial(1)
Space Requiredfor factorial(0)
Space Requiredfor factorial(4)
6
Space Requiredfor factorial(3)
Space Requiredfor factorial(2)
Space Requiredfor factorial(1)
Space Requiredfor factorial(4)
7
Space Requiredfor factorial(3)
Space Requiredfor factorial(2)
Space Requiredfor factorial(4)
8 Space Requiredfor factorial(3)
Space Requiredfor factorial(4)
9
Fibonacci Numbers
Example 4.8 Computing Finonacci Numbers0 1 1 2 3 5 8 13 21 34 55 89…f0 f1
fib(2) = fib(0) + fib(1);
fib(0) = 0;
fib(1) = 1;
fib(n) = fib(n-2) + fib(n-1); n>=2
Fibonacci Numbers, cont
ComputeFibonacciComputeFibonacci Run
Fibonnaci Numbers, cont.
fib(4)=
fib(3) + fib(2)
call fib(3)
1
fib(3)= fib(2) + fib(1)
2 return fib(3)
call fib(2)
fib(2)= fib(1) + fib(0)
3
return fib(2)
call fib(1)
fib(1)= 1
4
return fib(1)
fib(2)= fib(1) + fib(0)
7
fib(0)= 0
5
fib(1)= 1
6 fib(1)= 1
8
return fib(1)
fib(0)= 1
9
Towers of Hanoi
Example 4.9 Solving the Towers of Hanoi Problem
Solve the towers of Hanoi problem.
TowersOfHanoiTowersOfHanoi Run
Towers of Hanoi, cont.
A
A
B
C
Step 0: Starting status
C
B
Step 2: Move disk 2 from A to C
A B
Step 3: Move disk 1 from B to C
C
A B
Step 4: Move disk 3 from A to B
C
A B
Step 5: Move disk 1 from C to A
CA B
Step 1: Move disk 1 from A to B
C
A B
Step 7: Mve disk 1 from A to B
C
A B
Step 6: Move disk 2 from C to B
C