cop3502 programming fundamentals for cis majors 1
Post on 20-Feb-2016
45 Views
Preview:
DESCRIPTION
TRANSCRIPT
COP3502 Programming Fundamentals for CIS Majors 1
Instructor: Parisa Rashidi
Chapter 4 Loops
forwhiledo-while
Last Week
Chapter 5 Methods
Input argumentsOutputOverloading
Code reusability Scope of variables
Objectives
Methods
Suppose we want to write a program to find the sum of integers from 1 to 10 from 20 to 30 from 35 to 45
Motivation
Obvious solution
Naïve Solution
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);
What about some refactoring?
Refactor
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);
x
x
x
y
y
y
x y
x y
x y
A better approach is to use a method
Solution
public static int sum(int x, int y) { int sum = 0; for (int i = x; i <= y; i++) sum += i; return sum;}
modifier outputname
input
Methodbody
First, a method should be definedThen we can use the method
i.e. calling or invoking a method
Invoking a Method
public static void main(String[] args) { int total1 = sum(1, 10); int total2= sum(20, 30); int total3 = sum(35, 45); int total4 = sum(35,1000);}
When calling a method within the same class, we directly call the method
Invoking a Method
public class TestClass{ public static void main(String[] args) { int total1 = sum(1, 10);}//----------------------------------------------public static int sum(int x, int y) { int sum = 0; for (int i = x; i <= y; i++) sum += i; return sum;}
}
calling directly
When calling a method from another class, use class name if a static method
Invoking a Method
public class TestClass{ public static void main(String[] args) { int total1 = AnotherClass .sum(1, 10); }
}Class name
public class AnotherClass{ public static int sum(int x, int y) { int sum = 0; for (int i = x; i <= y; i++) sum += i; return sum;}
}
When calling a method from another class, use class name if a static method
Invoking a Method
public class TestClass{ public static void main(String[] args) { AnotherClass a = new AnotherClass(); int total1 = a.sum(1, 10); }
} Instance name
public class AnotherClass{ public int sum(int x, int y) { int sum = 0; for (int i = x; i <= y; i++) sum += i; return sum;}
}
Method is A collection of statements grouped together
to perform an operation To use a method
We invoke the method E.g. int result = sum(1,10);
So…
Method signature Combination of the method name and the
parameter list
Method Signature
public static int sum(int x, int y) { int sum = 0; for (int i = x; i <= y; i++) sum += i; return sum;}
signature
Method header
Parameters
Parameters
public static int sum(int x, int y) { int sum = 0; for (int i = x; i <= y; i++) sum += i; return sum;}
Formal parameter
public static void main(String[] args) { int total1 = sum(1, 10);} Actual parameter
Formal parameters: Variables defined in the method header
Actual parameters:When a method is invoked, you pass a
value to the parameter. This value is referred to as actual parameter or argument.
Parameters
If the method does not return a value, the “return type” is the keyword void. It means “nothing” will be returned
A method may return a value: Use return keyword to return the value…
E.g. return sum; “return” keyword is required if return type is
anything other than void
Output
A return statement is not required for a void method. return still can be used for terminating
the method This is not often done
Tip
Use “return” only once in a method! Easier to debug and trace
Programming Style
public int max(int x, int y) { if (x > y) return x; else return y; }
public int max(int x, int y) { int result= 0; if(x > y) result = x; else result = y; return result;} One exit point:
This is better
Two exit points
This program demonstrates calling a method max to return the largest value among a set of values.
Program
TestMax Run
Passing arguments
Program
public static int max(int x, int y) { int result= 0; if(x > y) result = x; else result = y; return result;}
public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j); }
Program Trace
public static int max(int x, int y) { int result= 0; if(x > y) result = x; else result = y; return result;}
public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j); }
i is now 5
Program Trace
Public static int max(int x, int y) { int result= 0; if(x > y) result = x; else result = y; return result;}
public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j); }
j is now 2
Program Trace
public static int max(int x, int y) { int result= 0; if(x > y) result = x; else result = y; return result;}
public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j); }
invoke method max(i,j)
Public static int max(int x, int y) { int result= 0; if(x > y) result = x; else result = y; return result;}
Program Trace
public static void main(String[] args){ int i = 5; int j = 2; int k = max(i, j); }
pass the value of i to xpass the value of j to y
Program Trace
Public static int max(int x, int y) { int result= 0; if(x > y) result = x; else result = y; return result;}
public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j); }
Declare variable result
Program Trace
Public static int max(int x, int y) { int result= 0; if(x > y) result = x; else result = y; return result;}
public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j); }
(x > y) is true because (5 > 2)
Program Trace
Public static int max(int x, int y) { int result= 0; if(x > y) result = x; else result = y; return result;}
public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j); }
result is now 5
Program Trace
Public static int max(int x, int y) { int result= 0; if(x > y) result = x; else result = y; return result;}
public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j); }
return result which is 5
Program Trace
Public static int max(int x, int y) { int result= 0; if(x > y) result = x; else result = y; return result;}
public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j); }
return max(i, j) and assign the return value to k
Program Trace
Public static int max(int x, int y) { int result= 0; if(x > y) result = x; else result = y; return result;}
public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j); }
finished
Methods reduce redundant coding and enable code reuse.
Methods modularize code and improve the quality of the program.
Modularizing Code
PrimeNumberMethod
Run
Chapter 5 Methods
Input arguments Output
Previously
Methods Overload …
Memory management
Today
Write a method that converts a decimal integer to a hexadecimal.
Program
Decimal2HexConversion Run
Converting a decimal number x (e.g. 74) into a hexadecimal number (e.g. 4A) 1. Divide x by 162. Save remainder and quotient3. Repeat step 1 and 2 until quotient is 04. Form the hexadecimal number from remainders
(the most recent remainder will be the leftmost digit)
Program Explained
Memory & Methods
A data structure Last in, first out (LIFO) Two basic operation
Pop Push
Stack
xYZ
Push Pop
How memory is arranged1. Registers
Inside the processor, very limited, you have no direct access
2. RAM Stack memory
Inside RAM, very fast, lifetime of objects should be known, all primitive variables placed here
Heap memory Inside RAM, reference values placed here
3. Constant values Will be directly replaced in code
Memory
Revisiting Program
Public static int max(int x, int y) { int result= 0; if(x > y) result = x; else result = y; return result;}
public static void main(String[] args) { int i = 5; int j = 2; int k = max(i, j); }
Each time a method is called, the system stores parameters and variables in stack memory.
How memory is managed?
Memory
Space required for
main method:
k:i:5j:2
Space required for
main method:
k:i:5j:2
Space required for
main method:
k:i:5j:2
Space required for
main method:
k: 5i:5j:2
Stackis empty
Space required for
maxmethod:
x:5y:2
Space required for
maxmethod:
Result: 5x:5y:2
What about reference types? E.g. Random r = new Random();
Pass by Reference
Space required for main method:
r(reference)
Stack Memory Heap Memory
…
Actual Object
What about reference types? E.g. Random r = new Random();
Pass by Reference
Space required for main method:
r(reference)
Stack Memory Heap Memory
…
Actual Object
Space required for
testmethod:
x
If primitive types are passed Value is passed Changes inside method will not affect the
variable outside the method If a reference type is passed
Reference is passed (address of memory location containing actual object)
Changes inside the method will affect the variable
Passing Arguments
Method Overload
Different versions of the same method accepting different arguments
Method Overload
TestMethodOverloading Run
Method overload is only based on input arguments
Method overload can not be based on different output values
Method overload cannot be based on different modifiers
Tip
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.
Method Overload
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) { double result = 0; if (num1 > num2) result = num1; else result = num2; return result; } public static double max(double num1, int num2) { double result = 0; if (num1 > num2) result = num1; else result = num2; return result; }}
Variable Scope
Scope: Part of the program where the variable
can be referenced.A local variable:
A variable defined inside a method. The scope of a local variable starts from
its declaration and continues to the end of the block that contains the variable.
Variable Scope
A variable declared in the initial action part of a for loop has its scope in the entire loop.
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.
Variable Scope
public static void method1() { . . for (int i = 1; i < 10; i++) { . . int j; . . . } }
The scope of j
The scope of i
You can declare a local variable with the same name multiple times in different non-nesting blocks in a method
You cannot declare a local variable twice in nested blocks.
Variable Scope
A variable declared in a method
Variable Scope
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
Method Abstraction
You can think of the method body as a black box that contains the detailed implementation for the method.
Abstraction
Method Header
Method body Black Box
Optional arguments for Input
Optional return value
Write a method once and reuse it anywhere.
Hide the implementation from the user. Change it without affecting the user
Reduce complexity.
Method Benefits
Example Math class provides many methods Trigonometric Methods Exponent Methods Rounding Methods min, max, abs, and random Methods
Method Benefits
HW4 due todayHW5 postedMonday: homework review session
Agenda
Chapter 5 Code reusability & program design
Objectives
When writing a large program, you can use the “divide and conquer” strategy, also known as stepwise refinement, to decompose it into sub-problems.
The sub-problems can be further decomposed into smaller, more manageable problems.
Stepwise Refinement
Main Task
Task 1
Task 1-1
Task 1-2
Task 2
Task 3
An example to demonstrate the stepwise refinement approach:
Stepwise Refinement
PrintCalendar Run
Stepwise Refinement
printCalendar (main)
readInput printMonth
getStartDay
printMonthTitle printMonthBody
getTotalNumOfDays
getNumOfDaysInMonth
getMonthName
isLeapYear
Stepwise Refinement
printCalendar (main)
readInput printMonth
getStartDay
printMonthTitle printMonthBody
getTotalNumOfDays
getNumOfDaysInMonth
getMonthName
isLeapYear
Stepwise Refinement
printCalendar (main)
readInput printMonth
getStartDay
printMonthTitle printMonthBody
getTotalNumOfDays
getNumOfDaysInMonth
getMonthName
isLeapYear
Stepwise Refinement
printCalendar (main)
readInput printMonth
getStartDay
printMonthTitle printMonthBody
getTotalNumOfDays
getNumOfDaysInMonth
getMonthName
isLeapYear
Stepwise Refinement
printCalendar (main)
readInput printMonth
getStartDay
printMonthTitle printMonthBody
getTotalNumOfDays
getNumOfDaysInMonth
getMonthName
isLeapYear
Stepwise Refinement
printCalendar (main)
readInput printMonth
getStartDay
printMonthTitle printMonthBody
getTotalNumOfDays
getNumOfDaysInMonth
getMonthName
isLeapYear
Top-down approach is to implement one method in the structure chart at a time from the “top to the bottom”.
Implement the main method first and then use a stub for each one of the methods.
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.
Top Down Design
A Skeleton for printCalendar
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.
Bottom Up Design
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.
Comparison
An example to demonstrate the stepwise refinement approach:
Program Revisited
PrintCalendar Run
As introduced before, each character has a unique Unicode between 0 and FFFF in hexadecimal (65535 in decimal). To generate a random character is to generate a random integer between 0 and 65535. The Unicode for lowercase letters are consecutive integers starting from the Unicode for 'a', then for 'b', 'c', ..., and 'z'. A random character between any two characters ch1 and ch2 with ch1 < ch2 can be generated as follows:
Random r = new Random();char x = ch1 + r.nextInt(ch2 – ch1 + 1)
Program
Random character generator
Program
TestRandomCharacter
Run
RandomCharacter
Math Class
Class constants: PI E
Class methods: Trigonometric Methods Exponent Methods Rounding Methods min, max, abs, and random Methods
Math Class
Trigonometric methods sin(double a) cos(double a) tan(double a) acos(double a) asin(double a) atan(double a)
Math Class
Examples:
Math.sin(0) returns 0.0 Math.sin(Math.PI / 6) returns
0.5 Math.sin(Math.PI / 2) returns
1.0Math.cos(0) returns 1.0Math.cos(Math.PI / 6)
returns 0.866 Math.cos(Math.PI / 2)
returns 0
Exponent methods exp(double a)
Returns e raised to the power of a. log(double a)
Returns the natural logarithm of a. log10(double a)
Returns the 10-based 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.
Math Class
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).
Math Class
Rounding examplesMath.ceil(2.1) returns 3.0 Math.ceil(2.0) returns 2.0Math.ceil(-2.0) returns –2.0Math.ceil(-2.1) returns -2.0Math.floor(2.1) returns 2.0Math.floor(2.0) returns 2.0Math.floor(-2.0) returns –2.0Math.floor(-2.1) returns -3.0Math.rint(2.1) returns 2.0Math.rint(2.0) returns 2.0Math.rint(-2.0) returns –2.0Math.rint(-2.1) returns -2.0Math.rint(2.5) returns 2.0Math.rint(-2.5) returns -2.0Math.round(2.6f) returns 3 Math.round(2.0) returns 2 Math.round(-2.0f) returns -2
Math.round(-2.6) returns -3
Math Class
Min, max, etc 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).
Math Class
random method Generates a random double value greater than
or equal to 0.0 and less than 1.0 (0 <= Math.random() < 1.0).
Math Class
(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.
a + Math.random() * b Returns a random number between
a and a + b, excluding a + b.
top related