How Can I Exit Anywhere Within A Java Program

Programming 101 question: Why won't it run? (Java)?

This is my code:

import java.awt.Color;
import java.awt.Graphics;
import java.util.*;

public class Spiral {


public static void main(String[] args) {
Scanner console = new Scanner (System.in);
int myColor = console.nextInt();
DrawingPanel panel = new DrawingPanel (170,170); // Here I declare the drawing panel, our "canvas"'
Graphics g = panel.getGraphics(); // Here is our "paintbrush" (Building Java Programs, p. 197)


public static void BackgroundColor (int myColor) {
if (myColor == 1) {
panel.setBackground(Color.BLUE);
} else if (myColor == 2) {
panel.setBackground(Color.LIGHT_GRAY...
}
else if (myColor == 3) {
panel.setBackground(Color.MAGENTA);
}



It says "panel cannot be resolved" I don't understand what that means.

How to re-run the main method in java?

The main() method is public and static and can be called from anywhere else in your code. If you want to run the additional instance of your application under the same process as the original instance, you can just call this method, passing in any necessary String parameters. However, it would be advisable to do this from another thread, particularly if you are initiating the new instance from the Event-Dispatching Thread (e.g. as a result of a user clicking a button or somesuch).

Thread t = new Thread()
{
public void run()
{
String[] args = { };
MyClass.main(args);
}
};
t.start();

If, however, you want to re-launch your application in a new process (i.e. under a separate instance of the JVM), you can use Runtime.getRuntime().exec(...); to launch the java virtual machine executable passing in your application's entry-point class.

Java Problem: Counting all instances of repeating fractions from a text file?

Here's your code, with modifications so it works properly. I've add a boolean array counted so that it knows if a fraction has been counted already; I've also modified the final print statement. Seems to work okay now. Good luck!

import java.util.Scanner;
import java.io.FileNotFoundException;
import java.lang.String;
import java.io.FileInputStream;

public class Fractions{

public static void main(String [] args){
//Reads file containing fractions
Scanner inputFile = null;
try {
inputFile = new Scanner(new FileInputStream("fractions.txt"));
}
catch (FileNotFoundException e) {
System.out.println("File not found.");
System.exit(0);
}

//variables
String[] fractions = new String[100]; //will take in the fractions
String[] split = new String[2]; //used to split the fractions
int[] numerator = new int[100]; //store numerators
int[] denominator = new int[100]; //store denominators
int count = 0; //number of lines in file
int duplicate = 0; //number of fractions that are the duplicate
boolean[] counted = new boolean[100];

//count the number of lines in the file, put each line into the string[] fractions
for (int i = 0; inputFile.hasNextLine(); i++){
fractions[i]=inputFile.nextLine();
count++;
}

//split the fractions[] into two arrays: numerator and denominator
for(int i = 0; i < count; i++){
split = fractions[i].split("/");
numerator[i] = Integer.parseInt(split[0]);
denominator[i] = Integer.parseInt(split[1]);
}

//used to compare specific numerator and denominator to the rest of the numbers
int num;
int den;

//start off by comparing denominator, and then compares the numerator
//of like denominators
for(int i = 0; i < count; i++){
den = denominator[i];
num = numerator[i];

for(int a = 0; a < count; a++){
if (!counted[a]) {
if(den == denominator[a]){ //compare denominators
if(num == numerator[a]){ //compare numerators
duplicate++;
counted[a] = true;
}
}
}
}

if (duplicate>0) {
System.out.println(num + "/" + den + " has a count of "+ duplicate);
}
duplicate = 0;
}
}
}

// end of program

Formatting in Java?

I am having trouble formatting the outputs at the end to one decimal place.

import java.util.Scanner;

public class OrganizingGrades
{
public static void main(String[] args)
{
int gradeA, gradeB, gradeC, gradeD, gradeF, gradeTotal, gradeInput;
double percentA, percentB, percentC, percentD, percentF;
Scanner keyboard;

gradeA = 0;
gradeB = 0;
gradeC = 0;
gradeD = 0;
gradeF = 0;
gradeTotal = 0;

...
System.out.println("ERROR: Not a valid grade.");
}

else if (gradeTotal == 0)
{
System.out.println("No grades entered.");
}
else
{
percentA = ((double) gradeA / (double) gradeTotal) * 100;
percentB = ((double) gradeB / (double) gradeTotal) * 100;
percentC = ((double) gradeC / (double) gradeTotal) * 100;
percentD = ((double) gradeD / (double) gradeTotal) * 100;
percentF = ((double) gradeF / (double) gradeTotal) * 100;

System.out.println("Total number of grades = " + gradeTotal);
System.out.printf("Number or A's = " + gradeA + " which is %.1f.%\n", percentA);
System.out.printf("Number or B's = " + gradeB + " which is %.1f.%\n", percentB);
System.out.printf("Number or C's = " + gradeC + " which is %.1fn%\n", percentC);
System.out.printf("Number or D's = " + gradeD + " which is %.1f.%\n", percentD);
System.out.printf("Number or F's = " + gradeF + " which is %.1f.%\n", percentF);
}
}
}