1. Java Methods
Making Subprograms

2. Learning Objectives Be able to read a program that uses methods.
Be able to write a write a program that uses methods.

3. Program with static method
public class FreshPrince {
public static void main(String[] args)
rap();
System.out.println(); }
public static void rap()
System.out.println("Now this is the story all about how");
System.out.println("My life got flipped turned upside-down");
Output:
Now this is the story all about how
My life got flipped turned upside-down

4. public: It can be called from outside the class
public: It can be called from outside the class. For now we will make them all public.
private: Can only be called from within the class. (Later)
Default (No modifier) Can be accessed within the class and the package (folder).
static
You do not need to make an instance of the object to use it.
For now our methods will be static which makes them Class Methods.
If not defined as static, then it is not static.
Everything is a value parameter!
(type var, type var, type var)
<modifier>[static] <return-type><subroutine-name>( parameter-list ) {
<statements>
return something }
Start with a lowercase letter, and describes what the method is doing.
return: Jumps out of the method and returns the answer.
A method can have more than one return statement
void: Nothing. Like a procedure
int, double, char, String, int [], String [],Class name (Later): Defines the thing being returned.
Method Syntax

5. Example Can be called from outside the class.
import java.util.Scanner; // program uses class Scanner
public class Addition {
// main method begins execution of Java application
public static void main( String args[] )
// create Scanner to obtain input from command window
Scanner input = new Scanner( System.in );
int number1, number2, sum;
System.out.print( "Enter first integer: " ); // prompt
number1 = input.nextInt(); // read first number from user
System.out.print( "Enter second integer: " ); // prompt
number2 = input.nextInt(); // read second number from user
sum = total(number1, number2); // add numbers
System.out.printf( "Sum is %d\n", sum ); // display sum
} // end method main
public static int total(int first, int second)
return first+second; }
} // end class Addition
Example
Can be called from outside the class.
Does not need to be instantiated, made into an object, to use.
Returns an integer

6. Breaking down the method
Can be called from outside the class
Can be called without first making an object
Returns an integer value
public static int total(int first, int second) {
return first+second; }
Name of the method
Parameters/ messages. Everything is a value parameter!
Used to return a value from the method and kick out of the method

8. Control flow When a method is called, the program's execution...
"jumps" into that method, executing its statements, then
"jumps" back to the point where the method was called.
public class MethodsExample {
public static void main(String[] args) {
message1();
message2();
System.out.println("Done with main."); }
...
public static void message1() {
System.out.println("This is message1."); }
public static void message2() {
System.out.println("This is message2.");
message1();
System.out.println("Done with message2."); }
public static void message1() {
System.out.println("This is message1."); }

9. Dry run public static void main(String [] args) { int x = 3, y = 5;
fun(x, y);
System.out.println(x + “ “ + y); }
public static void fun(int a, int b)
a+=b;
b+=a;
System.out.println(a + “ “ + b);

10. What are the values of a and b after the following?
public static void main(String [] args) {
int a = 3, b = 7;
b = fun(a, b);
a = fun(b, a); }
public static int fun(int x, int y)
y -= x;
return y;

11. What does the following method do?
Order of ops ()
[], x++, x--, (type casting)
*, /, %
+, -
<. <=, >=
==, !=
&& ||
=, +=, -=,
public static int process(double amt) {
return (int) (amt * ) % 100; }
A) Returns the cent portion of amt.
B) Returns the number of whole dollars in amt.
C) Returns amt converted to the nearest cent
D) Returns amt rounded to the nearest integer
E) Returns amt truncated to the nearest integer

12. Dry run Challenge public static void main(String [] args) {
int x = change(45);
System.out.println(x); }
public static int change(int value)
if(value < 3)
return value % 3;
else
return value % * change(value/3);

13. Style Declare the main method as the first method.
Declare other methods later.

14. First Method Program Song: Average Method.
Write a program that has methods for chorus, first verse, second verse, … for a song of your choice.
The main body will call each of the methods for the song
Average Method.
Write a program that has a method. In the method the user will input an unknown number of scores, then the average of the scores will be calculated and returned to the main body.
The main body will then shows the average.

16. Dry Run

18. Second Method Programs
F to C
Sent a temperature in Fahrenheit and returns the temperature in Celsius.
C = 5/9(F-32)
Just the Factorials
Sent a positive integer
Returns its’ factorial.
Hypotenuse
The lengths of the sides are entered in the main body.
Sent the lengths of two sides of a right triangle
Returns the length of the hypotenuse

19. Voltage = current *Resistance Power = current *Voltage
Third Method Program: For one of the following, write a class with the associated methods. Include enough of a main body to test the methods
Electricity Class
Resistance: given Voltage and Current
Current: Given resistance and voltage
Voltage: Given Resistance and current
Power given: current and resistance
Movement Class
Distance: Sent the initial distance (d0), velocity (V0) , acceleration (a) and time (t)
Return the distance covered
Distance = d0 + Vo*t + (1/2) a t^2
Velocity: Sent Initial velocity (V0) , acceleration (a) and time (t)
Calculates the velocity at the given time.
Velocity = V0 + a*t
Acceleration: Sent initial velocity V0, ending velocity V1 and time (t)
Acceleration = (v1 – v0)/t
Statistics Class
Factorial
Permutations
Write a method that will find the number of permutations of n items taken r at a time.
Permutations of n items taken r at a time= n!/(n-r)!
Push add methods for the following.
Combinations
Write a method that will find the number of combinations of n items taken r at a time.
Combinations of n items take r at a time= n!/((r!)(n-r)!)
Voltage = current *Resistance
Power = current *Voltage