Transcript Slide 1

Programming Fundamentals I (COSC1336),
Lecture 5 (prepared after Chapter 5 of
Liang’s 2011 textbook)
Stefan Andrei
7/20/2015
COSC-1336, Lecture 5
1
Overview of Previous Lecture







7/20/2015
To write programs for executing statements repeatedly
using a while loop (§4.2).
To develop a program for GuessNumber and
SubtractionQuizLoop (§4.2.1).
To follow the loop design strategy to develop loops
(§4.2.2).
To develop a program for SubtractionQuizLoop
(§4.2.3).
To control a loop with a sentinel value (§4.2.3).
To obtain large input from a file using input redirection
rather than typing from the keyboard (§4.2.4).
To write loops using do-while statements (§4.3).
COSC-1336, Lecture 5
2
Overview of Previous Lecture (cont)

To write loops using for statements (§4.4).

To discover the similarities and differences of three
types of loop statements (§4.5).
To write nested loops (§4.6).
To learn the techniques for minimizing numerical errors
(§4.7).
To learn loops from a variety of examples (GCD,
FutureTuition, MonteCarloSimulation) (§4.8).
To implement program control with break and continue
(§4.9).
(GUI) To control a loop with a confirmation dialog
(§4.10).





7/20/2015
COSC-1336, Lecture 5
3
Motivation of the current lecture
Find the sum of integers from 1 to 10, from 20 to
30, and from 35 to 45, respectively.


7/20/2015
COSC-1336, Lecture 5
4
First Attempt
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);
7/20/2015
COSC-1336, Lecture 5
5
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);
7/20/2015
COSC-1336, Lecture 5
6
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));
}
7/20/2015
COSC-1336, Lecture 5
7
Overview of This Lecture




To define methods, invoke methods, and pass
arguments to a method (§5.2-5.5).
To develop reusable code that is modular, easy-toread, 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).
7/20/2015
COSC-1336, Lecture 5
8
Overview of This Lecture (cont.)




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).
7/20/2015
COSC-1336, Lecture 5
9
Defining Methods
A method is a collection of statements that
are grouped together to perform an operation.
Define a method
modifier
method
header
return value
type
Invoke a method
method
name
formal
parameters
int z = max(x, y);
public static int max(int num1, int num2) {
actual parameters
(arguments)
int result;
method
body
if (num1 > num2)
result = num1;
else
result = num2;
return result;
parameter list
method
signature
return value
}
7/20/2015
COSC-1336, Lecture 5
10
Method Signature
Method signature is the combination of the method
name and the parameter list.
Define a method
modifier
method
header
return value
type
Invoke a method
method
name
formal
parameters
int z = max(x, y);
public static int max(int num1, int num2) {
actual parameters
(arguments)
int result;
method
body
if (num1 > num2)
result = num1;
else
result = num2;
return result;
parameter list
method
signature
return value
}
7/20/2015
COSC-1336, Lecture 5
11
Formal Parameters
The variables defined in the method header are
known as formal parameters.
Define a method
modifier
method
header
return value
type
Invoke a method
method
name
formal
parameters
int z = max(x, y);
public static int max(int num1, int num2) {
actual parameters
(arguments)
int result;
method
body
if (num1 > num2)
result = num1;
else
result = num2;
return result;
parameter list
method
signature
return value
}
7/20/2015
COSC-1336, Lecture 5
12
Actual Parameters
 When a method is invoked, you pass a value to the
parameter.
 This value is referred to as actual parameter or argument.
Define a method
modifier
method
header
return value
type
Invoke a method
method
name
formal
parameters
int z = max(x, y);
public static int max(int num1, int num2) {
actual parameters
(arguments)
int result;
method
body
if (num1 > num2)
result = num1;
else
result = num2;
return result;
parameter list
method
signature
return value
}
7/20/2015
COSC-1336, Lecture 5
13
Return Value Type
 A method may return a value.
 The return value type is the data type of the value the method returns.
 If the method does not return a value, the return value type is the
keyword void.
 For example, the return value type in the main() method is void.
Define a method
modifier
method
header
return value
type
Invoke a method
method
name
formal
parameters
int z = max(x, y);
public static int max(int num1, int num2) {
actual parameters
(arguments)
int result;
method
body
if (num1 > num2)
result = num1;
else
result = num2;
return result;
parameter list
method
signature
return value
}
7/20/2015
COSC-1336, Lecture 5
14
Calling Methods
 Testing the max() method;
 This program demonstrates calling a method
max to return the largest of the int values.
7/20/2015
COSC-1336, Lecture 5
15
animation
Calling Methods (cont.)
pass the value of i
pass the value of j
public static void main(String[] args) {
int i = 5;
int j = 2;
int k = max(i, j);
public static int max(int num1, int num2) {
int result;
if (num1 > num2)
result = num1;
else
result = num2;
System.out.println(
"The maximum between " + i +
" and " + j + " is " + k);
}
return result;
}
7/20/2015
COSC-1336, Lecture 5
16
animation
Trace Method Invocation
i is now 5
public static void main(String[] args) {
int i = 5;
int j = 2;
int k = max(i, j);
public static int max(int num1, int num2) {
int result;
if (num1 > num2)
result = num1;
else
result = num2;
System.out.println(
"The maximum between " + i +
" and " + j + " is " + k);
}
return result;
}
7/20/2015
COSC-1336, Lecture 5
17
animation
Trace Method Invocation
j is now 2
public static void main(String[] args) {
int i = 5;
int j = 2;
int k = max(i, j);
public static int max(int num1, int num2) {
int result;
if (num1 > num2)
result = num1;
else
result = num2;
System.out.println(
"The maximum between " + i +
" and " + j + " is " + k);
}
return result;
}
7/20/2015
COSC-1336, Lecture 5
18
animation
Trace Method Invocation
invoke max(i, j)
public static void main(String[] args) {
int i = 5;
int j = 2;
int k = max(i, j);
public static int max(int num1, int num2) {
int result;
if (num1 > num2)
result = num1;
else
result = num2;
System.out.println(
"The maximum between " + i +
" and " + j + " is " + k);
}
return result;
}
7/20/2015
COSC-1336, Lecture 5
19
animation
Trace Method Invocation
invoke max(i, j)
Pass the value of i to num1
Pass the value of j to num2
public static void main(String[] args) {
int i = 5;
int j = 2;
int k = max(i, j);
public static int max(int num1, int num2) {
int result;
if (num1 > num2)
result = num1;
else
result = num2;
System.out.println(
"The maximum between " + i +
" and " + j + " is " + k);
}
return result;
}
7/20/2015
COSC-1336, Lecture 5
20
animation
Trace Method Invocation
declare variable result
public static void main(String[] args) {
int i = 5;
int j = 2;
int k = max(i, j);
public static int max(int num1, int num2) {
int result;
if (num1 > num2)
result = num1;
else
result = num2;
System.out.println(
"The maximum between " + i +
" and " + j + " is " + k);
}
return result;
}
7/20/2015
COSC-1336, Lecture 5
21
animation
Trace Method Invocation
(num1 > num2) is true since
num1 is 5 and num2 is 2
public static void main(String[] args) {
int i = 5;
int j = 2;
int k = max(i, j);
public static int max(int num1, int num2) {
int result;
if (num1 > num2)
result = num1;
else
result = num2;
System.out.println(
"The maximum between " + i +
" and " + j + " is " + k);
}
return result;
}
7/20/2015
COSC-1336, Lecture 5
22
animation
Trace Method Invocation
result is now 5
public static void main(String[] args) {
int i = 5;
int j = 2;
int k = max(i, j);
public static int max(int num1, int num2) {
int result;
if (num1 > num2)
result = num1;
else
result = num2;
System.out.println(
"The maximum between " + i +
" and " + j + " is " + k);
}
return result;
}
7/20/2015
COSC-1336, Lecture 5
23
animation
Trace Method Invocation
return result, which is 5
public static void main(String[] args) {
int i = 5;
int j = 2;
int k = max(i, j);
public static int max(int num1, int num2) {
int result;
if (num1 > num2)
result = num1;
else
result = num2;
System.out.println(
"The maximum between " + i +
" and " + j + " is " + k);
}
return result;
}
7/20/2015
COSC-1336, Lecture 5
24
animation
Trace Method Invocation
return max(i, j) and assign
the return value to k
public static void main(String[] args) {
int i = 5;
int j = 2;
int k = max(i, j);
public static int max(int num1, int num2) {
int result;
if (num1 > num2)
result = num1;
else
result = num2;
System.out.println(
"The maximum between " + i +
" and " + j + " is " + k);
}
return result;
}
7/20/2015
COSC-1336, Lecture 5
25
animation
Trace Method Invocation
Call the println() method
public static void main(String[] args) {
int i = 5;
int j = 2;
int k = max(i, j);
public static int max(int num1, int num2) {
int result;
if (num1 > num2)
result = num1;
else
result = num2;
System.out.println(
"The maximum between " + i +
" and " + j + " is " + k);
}
return result;
}
7/20/2015
COSC-1336, Lecture 5
26
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.
public static int sign(int n) {
if (n > 0)
return 1;
else if (n == 0)
return 0;
else if (n < 0)
return –1;
}
Should be
public static int sign(int n) {
if (n > 0)
return 1;
else if (n == 0)
return 0;
else
return –1;
}
(a)
(b)
 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.
7/20/2015
COSC-1336, Lecture 5
27
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).

7/20/2015
COSC-1336, Lecture 5
28
Call Stacks
Space required for
the max method
num2: 2
num1: 5
Space required for
the max method
result: 5
num2: 2
num1: 5
Space required for
the main method
k:
2
j:
5
i:
Space required for
the main method
k:
2
j:
5
i:
Space required for
the main method
k:
2
j:
5
i:
Space required for
the main method
5
k:
2
j:
5
i:
(a) The main
method is invoked.
(b) The max
method is invoked.
(c) The max method
is being executed.
(d) The max method is
finished and the return
value is sent to k.
7/20/2015
COSC-1336, Lecture 5
Stack is empty
(e) The main
method is finished.
29
animation
Trace Call Stack
i is declared and initialized
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;
i: 5
The main method
is invoked.
return result;
}
7/20/2015
COSC-1336, Lecture 5
30
animation
Trace Call Stack
j is declared and initialized
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;
j: 2
i: 5
The main method
is invoked.
return result;
}
7/20/2015
COSC-1336, Lecture 5
31
animation
Trace Call Stack
Declare k
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;
Space required for the
main method
k:
j: 2
i: 5
The main method
is invoked.
return result;
}
7/20/2015
COSC-1336, Lecture 5
32
animation
Trace Call Stack
Invoke max(i, j)
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;
Space required for the
main method
k:
j: 2
i: 5
The main method
is invoked.
return result;
}
7/20/2015
COSC-1336, Lecture 5
33
animation
Trace Call Stack
pass the values of i and j to
num1 and num2
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);
num2: 2
num1: 5
}
public static int max(int num1, int num2) {
int result;
if (num1 > num2)
result = num1;
else
result = num2;
return result;
The max method is
invoked.
}
7/20/2015
Space required for the
main method
k:
j: 2
i: 5
COSC-1336, Lecture 5
34
animation
Trace Call Stack
declare variable result
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);
result:
num2: 2
num1: 5
}
public static int max(int num1, int num2) {
int result;
if (num1 > num2)
result = num1;
else
result = num2;
return result;
The max method is
invoked.
}
7/20/2015
Space required for the
main method
k:
j: 2
i: 5
COSC-1336, Lecture 5
35
animation
Trace Call Stack
(num1 > num2) is true
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);
result:
num2: 2
num1: 5
}
public static int max(int num1, int num2) {
int result;
if (num1 > num2)
result = num1;
else
result = num2;
return result;
The max method is
invoked.
}
7/20/2015
Space required for the
main method
k:
j: 2
i: 5
COSC-1336, Lecture 5
36
animation
Trace Call Stack
Assign num1 to result
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;
Space required for the
main method
k:
j: 2
i: 5
The max method is
invoked.
}
7/20/2015
Space required for the
max method
result: 5
num2: 2
num1: 5
COSC-1336, Lecture 5
37
animation
Trace Call Stack
Return result and assign it to k
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;
Space required for the
main method
k:5
j: 2
i: 5
The max method is
invoked.
}
7/20/2015
Space required for the
max method
result: 5
num2: 2
num1: 5
COSC-1336, Lecture 5
38
animation
Trace Call Stack
Call the println() 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);
}
public static int max(int num1, int num2) {
int result;
if (num1 > num2)
result = num1;
else
result = num2;
Space required for the
main method
k:5
j: 2
i: 5
The main method
is invoked.
return result;
}
7/20/2015
COSC-1336, Lecture 5
39
void
Method Example
 This type of method does not return a value.
 The method performs some actions.
public class TestVoidMethod {
public static void main(String[] args) {
System.out.print("The grade is ");
printGrade(78.5);
System.out.print("The grade is ");
printGrade(59.5);
}
// . . . public static void printGrade()
}
7/20/2015
COSC-1336, Lecture 5
40
Method public
static void
printGrade()
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'); }
}
7/20/2015
COSC-1336, Lecture 5
41
Compiling and running
TestVoidMethod.java
7/20/2015
COSC-1336, Lecture 5
42
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?
7/20/2015
COSC-1336, Lecture 5
43
Pass by Value
 This program demonstrates passing
values to the methods.
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);
}
}
7/20/2015
COSC-1336, Lecture 5
44
Compiling and running Increment.java
7/20/2015
COSC-1336, Lecture 5
45
Pass by Value
 Testing Pass by value.
 This program demonstrates passing values
to the methods.
public class TestPassByValue {
public static void main(String[] args) {
int num1 = 1, num2 = 2;
System.out.println("Before invoking the swap
method, num1 is " + num1 + " and num2 is " + num2);
swap(num1, num2);
System.out.println("After invoking the swap
method, num1 is " + num1 + " and num2 is " + num2);
}
// . . .
}
7/20/2015
COSC-1336, Lecture 5
46
public static void swap()
/** 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);
int temp = n1;
n1 = n2;
n2 = temp;
System.out.println("\t\tAfter swapping n1 is "
+ n1 + " n2 is " + n2);
}
7/20/2015
COSC-1336, Lecture 5
47
Compiling and running
TestPassByValue.java
7/20/2015
COSC-1336, Lecture 5
48
Pass by Value (cont.)
The values of num1 and num2 are
passed to n1 and n2. Executing swap
does not affect num1 and num2.
Space required for the
swap method
temp:
n2: 2
n1: 1
Space required for the
main method
num2: 2
num1: 1
The main method
is invoked
7/20/2015
Space required for the
main method
num2: 2
num1: 1
The swap method
is invoked
Space required for the
main method
num2: 2
num1: 1
The swap method
is finished
COSC-1336, Lecture 5
Stack is empty
The main method
is finished
49
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.

7/20/2015
COSC-1336, Lecture 5
50
Method Overloading
Method overloading is the process of giving a
single method name multiple definitions.
 If a method is overloaded, the method name is not
sufficient to determine which method is being called.
 The signature of each overloaded method must
be unique.
 The signature includes the number, type, and
order of the parameters.
 The return type of the method is not part of the
signature.
 That is, overloaded methods cannot differ only by
their return type.

7/20/2015
COSC-1336, Lecture 5
51
Method Overloading

The compiler determines which method is being
invoked by analyzing the parameters (the first
signature is (int) and the second one is (int,
double):
float method1(int x)
{
return x + .375;
}
Invocation
result = method1(25, 4.32)
float method1(int x, double y)
{
return x * y;
}
7/20/2015
COSC-1336, Lecture 5
52
Method Overloading

The println() method is overloaded:
println(String s)
println(int i)
println(double d)
and so on...

The following lines invoke different versions of
the println() method:
System.out.println("The total is:");
System.out.println(total);
7/20/2015
COSC-1336, Lecture 5
53
Overloading Methods

Overloading the max() method:
public static double max(double num1,
double num2) {
if (num1 > num2)
return num1;
else
return num2;
}
7/20/2015
COSC-1336, Lecture 5
54
TestMethodOverloading.java
public class TestMethodOverloading {
public static void main(String[] args) {
System.out.println("The maximum between 3 and
4 is " + max(3, 4));
System.out.println("The maximum between 3.0
and 5.4 is " + max(3.0, 5.4));
System.out.println("The maximum between 3.0,
5.4, and 10.14 is " + max(3.0, 5.4, 10.14));
}
// . . . The max() methods come next
}
7/20/2015
COSC-1336, Lecture 5
55
TestMethodOverloading.java
(cont.)
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);
}
7/20/2015
COSC-1336, Lecture 5
56
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.

7/20/2015
COSC-1336, Lecture 5
57
Ambiguous Invocation
public 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;
}
}
7/20/2015
COSC-1336, Lecture 5
58
Type error during compilation
7/20/2015
COSC-1336, Lecture 5
59
Scope of Local Variables




A 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.
7/20/2015
COSC-1336, Lecture 5
60
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.

7/20/2015
COSC-1336, Lecture 5
61
Scope of Local Variables (cont.)
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
and to the end of the block that contains the variable.

The scope of i
The scope of j
7/20/2015
public static void method1() {
.
.
for (int i = 1; i < 10; i++) {
.
.
int j;
.
.
.
}
}
COSC-1336, Lecture 5
62
Scope of Local Variables (cont.)
It is fine to declare i in two
non-nesting blocks
It is wrong to declare i in
two nesting blocks
public static void method1() {
int x = 1;
int y = 1;
public static void method2() {
int i = 1;
int sum = 0;
for (int i = 1; i < 10; i++) {
x += i;
}
for (int i = 1; i < 10; i++) {
y += i;
}
for (int i = 1; i < 10; i++) {
sum += i;
}
}
}
7/20/2015
COSC-1336, Lecture 5
63
Scope of Local Variables (cont.)
// Fine with no errors
public 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;
}
}
7/20/2015
COSC-1336, Lecture 5
64
Scope of Local Variables (cont.)
// With no errors
public static void incorrectMethod() {
int x = 1;
int y = 1;
for (int i = 1; i < 10; i++) {
int x = 0;
x += i;
}
}
7/20/2015
COSC-1336, Lecture 5
65
Method Abstraction
You can think of the method body as a black
box that contains the detailed implementation
for the method.

Optional arguments
for Input
Optional return
value
Method Header
Black Box
Method body
7/20/2015
COSC-1336, Lecture 5
66
Benefits of Methods

Write a method once and reuse it
anywhere.
 Information hiding: hide the implementation
from the user.
 Reduce complexity.
7/20/2015
COSC-1336, Lecture 5
67
The Math Class

Class constants:



PI
E
Class methods:




7/20/2015
Trigonometric Methods
Exponent Methods
Rounding Methods
min(), max(), abs(), and random() Methods
COSC-1336, Lecture 5
68
Trigonometric Methods

sin(double a)
 Examples:

cos(double a)
Math.sin(0) returns 0.0

tan(double a)
Math.sin(Math.PI / 6) returns
0.5

asin(double a)

acos(double a)

atan(double a)
Radians
Math.sin(Math.PI / 2) returns
1.0
Math.cos(0) returns 1.0
Math.cos(Math.PI / 6) returns
0.866
Math.cos(Math.PI / 2) returns 0
toRadians(90)
7/20/2015
COSC-1336, Lecture 5
69
Exponent Methods

exp(double a)
Returns e raised to the power of a.

log(double a)
Returns the natural logarithm of a.



Examples:
Math.exp(1) returns 2.71
Math.log(2.71) returns 1.0
Math.pow(2, 3) returns 8.0
log10(double a)
Math.pow(3, 2) returns 9.0
Returns the 10-based logarithm of a.
Math.pow(3.5, 2.5) returns
22.91765
pow(double a, double b)
Math.sqrt(4) returns 2.0
Returns a raised to the power of b.
Math.sqrt(10.5) returns 3.24
sqrt(double a)
Returns the square root of a.
7/20/2015
COSC-1336, Lecture 5
70
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).
7/20/2015
COSC-1336, Lecture 5
71
Rounding Methods Examples
Math.ceil(2.1) returns 3.0
Math.rint(2.0) returns 2.0
Math.ceil(2.0) returns 2.0
Math.rint(-2.0) returns –2.0
Math.ceil(-2.0) returns –2.0
Math.rint(-2.1) returns -2.0
Math.ceil(-2.1) returns -2.0
Math.rint(2.5) returns 2.0
Math.floor(2.1) returns 2.0
Math.rint(-2.5) returns -2.0
Math.floor(2.0) returns 2.0
Math.round(2.6f) returns 3
Math.floor(-2.0) returns –2.0
Math.round(2.0) returns 2
Math.floor(-2.1) returns -3.0
Math.round(-2.0f) returns -2
Math.rint(2.1) returns 2.0
Math.round(-2.6) returns -3
7/20/2015
COSC-1336, Lecture 5
72
min(), max(),

and abs()
max(a, b)and min(a, b) Examples:
Returns the maximum or
minimum of two parameters.


Math.max(2, 3) returns 3
abs(a)
Math.max(2.5, 3) returns 3.0
Returns the absolute value of
the parameter.
Math.min(2.5, 3.6) returns 2.5
random()
Returns a random double
value in the range [0.0, 1.0).
7/20/2015
Math.abs(-2) returns 2
Math.abs(-2.1) returns 2.1
COSC-1336, Lecture 5
73
The random() Method
Generates a random double value greater than or equal to
0.0 and less than 1.0 (0 <= Math.random() < 1.0).

 Examples:
(int)(Math.random() * 10)
Returns a random integer
between 0 and 9.
50 + (int)(Math.random() * 50)
Returns a random integer
between 50 and 99.
 In general,
a + Math.random() * b
7/20/2015
Returns a random number between
a and a + b, excluding a + b.
COSC-1336, Lecture 5
74
Case Study: Generating Random
Characters
Computer programs process numerical data and
characters.
 You have seen many examples that involve
numerical data.
 It is also important to understand characters and
how to process them.
 As introduced in Section 2.9, each character has a
unique Unicode between 0 and FFFF in hexadecimal
(65535 in decimal).

7/20/2015
COSC-1336, Lecture 5
75
Case Study: Generating Random
Characters (cont.)
To generate a random character is to generate a
random integer between 0 and 65535 using the
following expression:

note that since 0 <= Math.random() < 1.0, you have to
add 1 to 65535.
 (int)(Math.random() * (65535 + 1))

7/20/2015
COSC-1336, Lecture 5
76
Case Study: Generating Random
Characters (cont.)
Now let us consider how to generate a random
lowercase letter.
 The Unicode for lowercase letters are
consecutive integers starting from the Unicode for
'a', then for 'b', 'c', ..., and 'z'.
 The Unicode for 'a' is (int)'a'.
 So, a random integer between (int)'a' and
(int)'z' is:


(int)((int)'a' + Math.random() *
((int)'z' – (int)'a' + 1)
7/20/2015
COSC-1336, Lecture 5
77
Case Study: Generating Random
Characters (cont.)
As discussed in Chapter 2, all numeric operators
can be applied to the char operands.
 The char operand is cast into a number if the
other operand is a number or a character.
 So, the preceding expression can be simplified
as follows:

'a' + Math.random() * ('z' - 'a' + 1)

So a random lowercase letter is
(char)('a' + Math.random() * ('z' - 'a' + 1))
7/20/2015
COSC-1336, Lecture 5
78
Case Study: Generating Random
Characters (cont.)
To generalize the foregoing discussion, a random
character between any two characters ch1 and ch2
with ch1 < ch2 can be generated as follows:

(char)(ch1 + Math.random() * (ch2 – ch1 + 1))
7/20/2015
COSC-1336, Lecture 5
79
The RandomCharacter Class
// RandomCharacter.java: Generate random characters
public class RandomCharacter {
/** Generate a random character between ch1 and ch2 */
public static char getRandomCharacter(char ch1, char ch2) {
return (char)(ch1 + Math.random() * (ch2 - ch1 + 1));
}
/** Generate a random lowercase letter */
public static char getRandomLowerCaseLetter() {
return getRandomCharacter('a', 'z');
}
/** Generate a random uppercase letter */
public static char getRandomUpperCaseLetter() {
return getRandomCharacter('A', 'Z');
}
/** Generate a random digit character */
public static char getRandomDigitCharacter() {
return getRandomCharacter('0', '9');
}
/** Generate a random character */
public static char getRandomCharacter() {
return getRandomCharacter('\u0000', '\uFFFF');
}
}
7/20/2015
COSC-1336, Lecture 5
80
Stepwise Refinement (Optional)
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.

7/20/2015
COSC-1336, Lecture 5
81
PrintCalendar
Case Study
Let us use the PrintCalendar example to
demonstrate the stepwise refinement approach.

7/20/2015
COSC-1336, Lecture 5
82
Design Diagram
printCalendar
(main)
printMonth
readInput
printMonthTitle
getMonthName
printMonthBody
getStartDay
getTotalNumOfDays
getNumOfDaysInMonth
isLeapYear
7/20/2015
COSC-1336, Lecture 5
83
Design Diagram
printCalendar
(main)
printMonth
readInput
printMonthTitle
getMonthName
printMonthBody
getStartDay
getTotalNumOfDays
getNumOfDaysInMonth
isLeapYear
7/20/2015
COSC-1336, Lecture 5
84
Design Diagram
printCalendar
(main)
printMonth
readInput
printMonthTitle
getMonthName
printMonthBody
getStartDay
getTotalNumOfDays
getNumOfDaysInMonth
isLeapYear
7/20/2015
COSC-1336, Lecture 5
85
Design Diagram
printCalendar
(main)
printMonth
readInput
printMonthTitle
getMonthName
printMonthBody
getStartDay
getTotalNumOfDays
getNumOfDaysInMonth
isLeapYear
7/20/2015
COSC-1336, Lecture 5
86
Design Diagram
printCalendar
(main)
printMonth
readInput
printMonthTitle
getMonthName
printMonthBody
getStartDay
getTotalNumOfDays
getNumOfDaysInMonth
isLeapYear
7/20/2015
COSC-1336, Lecture 5
87
Design Diagram
printCalendar
(main)
printMonth
readInput
printMonthTitle
getMonthName
printMonthBody
getStartDay
getTotalNumOfDays
getNumOfDaysInMonth
isLeapYear
7/20/2015
COSC-1336, Lecture 5
88
Implementation: Top-Down
 Top-down approach is to implement one method in
the structure chart at a time from the top to the bottom.
 Stubs can be used for the methods waiting to be
implemented.
 A stub is a simple but incomplete version of a method.
 The use of stubs enables you to test invoking the
method from a caller.
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.
7/20/2015
COSC-1336, Lecture 5
89
Implementation: Bottom-Up
 Bottom-up approach is to implement one method
in the structure chart at a time from the bottom to the
top.
 For each method implemented, write a test
program to test it.
 Both top-down and bottom-up methods are fine.
 Both approaches implement the methods
incrementally and help to isolate programming errors
and makes debugging easy.
 Sometimes, they can be used together.
7/20/2015
COSC-1336, Lecture 5
90
Summary




To define methods, invoke methods, and pass
arguments to a method (§5.2-5.5).
To develop reusable code that is modular, easy-toread, 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).
7/20/2015
COSC-1336, Lecture 5
91
Summary (cont.)




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).
7/20/2015
COSC-1336, Lecture 5
92
Reading suggestions

From [Liang: Introduction to Java programming:
Eight Edition, 2011 Pearson Education,
0132130807]
 Chapter 5 (Methods)
7/20/2015
COSC-1336, Lecture 5
93
Coming up next

From [Liang: Introduction to Java
programming: Eight Edition, 2011 Pearson
Education, 0132130807]

7/20/2015
Chapter 9 (Strings)
COSC-1336, Lecture 5
94
Thank you for your attention!
Questions?
7/20/2015
COSC-1336, Lecture 5
95