Java Methods Making Subprograms Learning Objectives u Be

Java Methods Making Subprograms

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

Program with static method public class Fresh. Prince { public static void main(String[] args) { rap(); System. out. println(); rap(); } 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

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) <modifier>[static] <return-type><subroutine-name>( parameter-list ) { Start with a lowercase letter, <statements> and describes what the method is return something doing. } return: Jumps out of the method and returns the answer. A method can have more than one return statement Method Syntax void: Nothing. Like a procedure int, double, char, String, int [], String [], Class name (Later): Defines the thing being returned.

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 number 1, number 2, sum; System. out. print( "Enter first integer: " ); // prompt number 1 = input. next. Int(); // read first number from user System. out. print( "Enter second integer: " ); // prompt number 2 = input. next. Int(); // read second number from user sum = total(number 1, number 2); // add numbers Example Can be called from System. out. printf( "Sum is %dn", sum ); // display sum outside the class. } // end method main public static int total(int first, int second) { return first+second; } } // end class Addition Does not need to be instantiated, made into an object, to use. Returns an integer

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) { Name of the method Parameters/ messages. Everything is a value parameter! return first+second; } Used to return a value from the method and kick out of the method

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 Methods. Example { public static void main(String[] args) void { message 1() { public static System. out. println("This is message 1. "); message 1(); } message 2(); public static void message 2() { System. out. println("This is message 2. "); message 1(); System. out. println("Done with message 2. "); System. out. println("Done with main. "); }. . . } } public static void message 1() { System. out. println("This is message 1. "); } 8

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); }

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; }

What does the following method Order of ops do? 1) () public static int process(double amt) { return (int) (amt * 100 + 0. 5) % 100; } 2) [], x++, x--, !x, (type casting) 3) *, /, % 4) +, 5) <. <=, >= 6) ==, != 7) && 8) || 9) =, +=, -=, 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

Dry run 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 % 3 + 10 * change(value/3); }

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

First Method Program • Song: – 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.

Dry Run

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

Third Method Program: For one of the following, write a class with the associated methods • Electricity Class – Resistance: given Voltage and Current Voltage = current *Resistance – Current: Given resistance and voltage Power = current *Voltage – Voltage: Given Resistance and current – Power given: current and resistance • Movement Class – Distance: Sent the initial distance (d 0), velocity (V 0) , acceleration (a) and time (t) • • Return the distance covered Distance = d 0 + Vo*t + (1/2) a t^2 – Velocity: Sent Initial velocity (V 0) , acceleration (a) and time (t) • Calculates the velocity at the given time. • Velocity = V 0 + a*t – Acceleration: Sent initial velocity V 0, ending velocity V 1 and time (t) • • Acceleration = (v 1 – v 0)/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)!)