3/25: Scope Rules, More Methods

• about RollDie.java & modifications

• Scope rules

• More methods

• Program of the day

About RollDie.java – pt. 1//RollDie.java//Roll a six-sided die 6000 timesimport 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.javawe 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. 276)

Program of the day

• Scoping.java

• After getting it to run correctly, try to analyze why the Container c is there.