11/2: Math.random, more methods About DrawLine.java modifications –allow user input –draw a curve Method definitions Math.random()

Allow User Input (1 of 2) Add in an init method for two input dialog boxes. double coeff;//note that I declared these double yInt; //two variables as global. public void init () { cText = JOptionPane.showInputDialog (“Coefficient, please.”); yText = JOptionPane.showInputDialog (“Y-intercept, please.”); coeff = Double.parseDouble ( cText ); yInt = Double.parseDouble ( yText ); }

Allow User Input (2 of 2) Use coeff & yInt in the drawDot method int drawDot ( int a ) { int b; //must cast to int below b = (int) (coeff * a + yInt) ; return b; }

Draw a Curve Change the formula in the drawDot method from: int drawDot ( int a ) { int b; b = 2 * a + 10 ; return b; } to: int drawDot ( int a ) { int b; b = 2 * ( a * a ) + 10 ; return b; }

Method Definitions in SquareInt.java (simple version below), we see a method called square: public void paint ( Graphics g ) { g.drawString(square(x) + “ “,25,25); } public int square ( int y ) { return y * y ; }

Method Definitions public void paint ( Graphics g ) { g.drawString(square(x) + “ “,25,25); } public int square ( int y ) { return y * y ; } the method square is called in this line. Because it is in parentheses, x is the value to use as the input.

Method Definitions public void paint ( Graphics g ) { g.drawString(square(x) + “ “,25,25); } public int square ( int y ) { return y * y ; } x fits the requirement that it must be an integer.

Method Definitions public void paint ( Graphics g ) { g.drawString(square(x) + “ “,25,25); } public int square ( int y ) { return y * y ; } using x as the input, the method calculates a return value.

Method Definitions public void paint ( Graphics g ) { g.drawString(square(x) + “ “,25,25); } public int square ( int y ) { return y * y ; } the returned value is what replaces the square(x). This value is of type int. x * x

Example: Maximum.java (p. 212) public void init () { (stuff left out…) double max = maximum ( num1, num2, num3 ); outputArea.setText ( “Max is “ + max ); } public double maximum ( double x, double y, double z ) { return Math.max ( x, Math.max ( y, z ) ); }

Important: Coercion of Arguments Java will automatically promote variables to larger types when necessary; EX: int to double. In Math.pow, the requested types of input are double. However, Java will create a double version of an int variable to use in that method automatically. go to Java class index for Math. pow Math. pow read p carefully

Math.random() generates a random double number in the range 0.0 <= x < 1.0 To utilize this function, we use scaling & shifting. scaling: multiplying the function by a number to get the range we want. shifting: adding a number to the function to get the beginning point of the range that we want.

Math.random() EX: Math.random () shift it by 1: Math.random() + 1 makes 1 to <2 scale it by 6: Math.random()* 6 makes 0 to <5 to simulate rolling a die, we want 6 possibilities (or a range of 6), from 1 to 6. They have to be integers. (int) ( Math.random() * );

1 st Program of the day pg. 220 RollDie.java Modify the program to allow the user to input how many times to roll the die.

About RollDie.java – pt. 1 //RollDie.java //Roll a six-sided die 6000 times import javax.swing.*; public class RollDie { public static void main ( String args[] ) { int freq1 = 0, freq2 = 0, freq3 = 0, freq4 = 0, freq5 = 0, freq6 = 0 ; int face;

About RollDie.java – pt. 2 for ( int roll = 1; roll <= 6000 ; roll++ ) { face = 1 + (int) ( Math.random() * 6 ); switch ( face ) { case 1: ++freq1; break; case 2: ++freq2; break; case 3: ++freq3; break; case 4: ++freq4; break; case 5: ++freq5; break; case 6: ++freq6; break; }

About RollDie.java – pt. 3 JTextArea outputArea = new JTextArea ( 7, 10 ); outputArea.setText ( "Face\tFrequency" + "\n1 \t" + freq1 + "\n2 \t" + freq2 + "\n3 \t" + freq3 + "\n4 \t" + freq4 + "\n5 \t" + freq5 + "\n6 \t" + freq6 ); JOptionPane.showMessageDialog ( null, outputArea, "Rolling a die 6000 times", JOptionPane.INFORMATION_MESSAGE); System.exit (0); } }

Modifying RollDie.java we need to replace the 6000 in the for loop with a user-defined value, so we add in int rolls; rolls = Integer.parseInt ( JOptionPane.showInputDialog ( “How many rolls?” ) ); and replace the 6000 with rolls. for ( int roll = 1; roll <= rolls ; roll++ ) { face = 1 + (int) ( Math.random() * 6 );

Automatic Variables: Duration Identifiers are used for variable names. Identifiers have a specific duration (or lifetime): –How long the identifier exists in memory (RAM). –Identifiers representing local variables exist ONLY while the program is inside that block. –A block is a set of compound statements in a program that contains declarations.

Automatic Variables: Duration Identifiers have a specific duration (or lifetime): –They are automatically created in memory upon entering the block and automatically destroyed in (deleted from) the computer’s memory when that block is exited. –They are said to have automatic duration. –They are called automatic duration variables, or simply automatic variables.

Instance Variables & Initialization Instance (class-wide) variables are initialized by default: –primitive data types to zero (0). –boolean types to false. –references (names given to objects) to null. Automatic (block-local) variables must be initialized manually. –you must initialize them or get a compiler error.

Scope Rules Identifiers have a specific scope: –where the identifier can be referenced in the program. –Class scope: accessible throughout the body of the class (between the class {}’s) EX: instance variables, methods –Block scope: accessible inside their block. EX: local variables, method parameters

Hidden Instance Variables Instance variables can be “hidden” if a local variable has the same name. While the program is in the local variable’s block, the instance variable is “hidden”. Scoping.java gives a taste of this. (pg. 192)

2 nd Program of the day Scoping.java After getting it to run correctly, analyze why the Container c is there.