Have a task to get N numbers from console, find the longest and the shortest ones and their length. The task is not difficult and works correctly, but I decided to make a check, if the console input corresponds the conditions of the task:
Are there only Integer numbers.
Are the exactly N numbers, not more/less.
I decided to write a boolean method isInputCorrect(), which would take the Scanner and check if the input is correct, but it doesn't work correctly.
public static void main(String[] args) {
int n = 5;
Scanner sc = new Scanner(System.in);
do {
System.out.println("Hello, please enter " + n + " integer numbers:");
while (!isInputCorrect(sc,n)){
System.out.println("Wrong! Try again:");
sc.next();
}
} while (!isInputCorrect(sc, 5));
String scLine = sc.nextLine();
String[] arr = scLine.split("\\s+");
String maxLengthNum = arr[0];
String minLengthNum = arr[0];
for (int i = 1; i < arr.length; i++){
if (maxLengthNum.length() < arr[i].length()){
maxLengthNum = arr[i];
}
if (minLengthNum.length() > arr[i].length()){
minLengthNum = arr[i];
}
}
String equalMaxNum = "";
String equalMinNum = "";
int countMax = 0;
int countMin = 0;
for (String s : arr){
if (maxLengthNum.length() == s.length()){
countMax += 1;
equalMaxNum += s + " ";
}
if (minLengthNum.length() == s.length()){
countMin += 1;
equalMinNum += s + " ";
}
}
if (countMax > 1){
System.out.println("The longest numbers are: " + equalMaxNum + " Their length is: " + maxLengthNum.length());
}
else {
System.out.println("The longest number is: " + maxLengthNum + " Its length is: " + maxLengthNum.length());
}
if (countMin > 1){
System.out.println("The shortest numbers are: " + equalMinNum + " Their length is: " + minLengthNum.length());
}
else {
System.out.println("The shortest number is: " + minLengthNum + " Its length is: " + minLengthNum.length());
}
}
public static boolean isInputCorrect(Scanner sc, int n){
boolean flag = true;
for (int i = 0; i < n; i++){
if (sc.hasNextInt()){
sc.next();
}else {
flag = false;
break;
}
}
return flag;
}
EDIT
This code still doesn't work. I realize, that the problem is in isDigit(). And exactly in regular conditions in last if statement. It is something like this:
public static boolean isDigit (String input, int n){
String[] arr = input.split("\\s+");
boolean flag = false;
if (arr.length != n){
flag = false;
}
else {
for (String s : arr) {
if (s.startsWith("-")) {
if (s.substring(1).matches("[0-9]*")) {
flag = true;
}
} else if (s.matches("[0-9]*")) {
flag = true;
} else {
flag = false;
}
}
}
return flag;
}
This method takes the console input as a string, then it checks, how many numbers(strings) does it contain, are there any negative numbers and so on. But it can be applied only for substrings(words without whitespace). In my case it can be applied to arr[i].
So I modified it to split String into array[] and tried to check every single element. I've got:
public static boolean isDigit (String input, int n){
String[] arr = input.split("\\s+");
boolean flag = false;
if (arr.length != n){
flag = false;
}
else {
for (String s : arr) {
if (s.startsWith("-")) {
if (s.substring(1).matches("[0-9]*")) {
flag = true;
}
} else if (s.matches("[0-9]*")) {
flag = true;
} else {
flag = false;
}
}
}
return flag;
}
but it returnes true even when input is:
1 3213 w 15 3
I can't understand, what is the problem? The full code is:
public static void main(String[] args) {
int n = 5;
boolean validInput = false;
String input;
do {
System.out.println("Please enter " + n + " integer numbers:");
Scanner sc = new Scanner(System.in);
input = sc.nextLine();
if (isDigit(input, n)) {
validInput = true;
} else {
System.out.println("Wrong input! Try again: ");
}
}
while (!validInput);
String[] arr = input.split("\\s+");
String maxLengthNum = arr[0];
String minLengthNum = arr[0];
for (int i = 1; i < arr.length; i++){
if (maxLengthNum.length() < arr[i].length()){
maxLengthNum = arr[i];
}
if (minLengthNum.length() > arr[i].length()){
minLengthNum = arr[i];
}
}
String equalMaxNum = "";
String equalMinNum = "";
int countMax = 0;
int countMin = 0;
for (String s : arr){
if (maxLengthNum.length() == s.length()){
countMax += 1;
equalMaxNum += s + " ";
}
if (minLengthNum.length() == s.length()){
countMin += 1;
equalMinNum += s + " ";
}
}
if (countMax > 1){
System.out.println("The longest numbers are: " + equalMaxNum + " Their length is: " + maxLengthNum.length());
}
else {
System.out.println("The longest number is: " + maxLengthNum + " Its length is: " + maxLengthNum.length());
}
if (countMin > 1){
System.out.println("The shortest numbers are: " + equalMinNum + " Their length is: " + minLengthNum.length());
}
else {
System.out.println("The shortest number is: " + minLengthNum + " Its length is: " + minLengthNum.length());
}
}
public static boolean isDigit (String input, int n){
String[] arr = input.split("\\s+");
boolean flag = false;
if (arr.length != n){
flag = false;
}
else {
for (String s : arr) {
if (s.startsWith("-")) {
if (s.substring(1).matches("[0-9]*")) {
flag = true;
}
} else if (s.matches("[0-9]*")) {
flag = true;
} else {
flag = false;
}
}
}
return flag;
}
SOLVED
Thanks to everybody, your help was really usefull. I finally found the problem
It was in isDigit() method. It was checking out every element of array, and switched a flag according to the last result. I wrote "break" to stop the further checking if there was at least one false flag in loop.
public static boolean isDigit (String input, int n){
String[] arr = input.split("\\s+");
boolean flag = false;
if (arr.length != n){
flag = false;
}
else{
for (String s: arr){
if (s.startsWith("-")) {
if (s.substring(1).matches("[0-9]*")){
flag = true;
}
else {
flag = false;
break;
}
}
else {
if (s.matches("[0-9]*")){
flag = true;
}
else {
flag = false;
break;
}
}
}
}
return flag;
}
You may use regular expressions to verify that the input contains only integer numbers:
int n = 5;
// ... your current code
String scLine = sc.nextLine();
if (!scLine.matches("\\d+(?:\\s+\\d+)*")) {
throw new IllegalArgumentException("input contained non-integers");
}
String[] arr = scLine.split("\\s+");
if (arr.length != n) {
throw new IllegalArgumentException("found " + arr.length + " number inputs but expected " + n + ".");
}
you can check if input string has only numbers as below
public boolean isDigit(String input) {
if (input == null || input.length() < 0)
return false;
input = input.trim();
if ("".equals(input))
return false;
if (input.startsWith("-")) {
return input.substring(1).matches("[0-9]*");
} else {
return input.matches("[0-9]*");
}
}
EDIT:
allowing user to re-enter untill valid number is entered
boolean validInput = false;
do
{
System.out.println("Enter the number ");
// get user input
String input sc.nextLine();
if(isDigit(input))
validInput = true;
else
System.out.println("Enter valid Number");
}
while (!validInput );
Related
I just go the Symmetrical at makeuseof and it's currently in javascript and i converted it to Java, However there's an error on the line 38 which is the array. Please check the code below, Thank you.
import java.util.Scanner;
class check
{
public static void main(String args[])
{
String str, rev = "";
Scanner sc = new Scanner(System.in);
System.out.println("Enter a string:");
str = sc.nextLine();
int length = str.length();
for ( int i = length - 1; i >= 0; i-- )
rev = rev + str.charAt(i);
//Check if Symmetrical
if (isSymmetrical(str)) {
System.out.println(str +" is a symmetrical");
} else {
System.out.println(str +" is not a symmetrical");
}
//Check if palindrome
if (str.equals(rev))
System.out.println(str +" is a palindrome");
else
System.out.println(str +" is not a palindrome");
}
public static boolean isSymmetrical(String str){
double midIndex;
var length = str.length();
if (length % 2 == 0) {
midIndex = Math.floor(length/2);
}
else {
midIndex = Math.floor(length/2) + 1;
}
var pointer1 = 0;
var pointer2 = midIndex;
while(pointer1 < midIndex && pointer2 < length) {
if(str[pointer1] == str[pointer2]) {
pointer1 += 1;
pointer2 += 1;
}
else {
return false;
}
}
return true;
}
}
Error:
Thanks to for sharing the String::charAt this is very helpful
I used charAt and convert double to int.
import java.util.Scanner;
class check
{
public static void main(String args[])
{
String str, rev = "";
Scanner sc = new Scanner(System.in);
System.out.println("Enter a string:");
str = sc.nextLine();
int length = str.length();
for ( int i = length - 1; i >= 0; i-- )
rev = rev + str.charAt(i);
//Check if Symmetrical
if (isSymmetrical(str)) {
System.out.println(str +" is a symmetrical");
} else {
System.out.println(str +" is not a symmetrical");
}
//Check if palindrome
if (str.equals(rev))
System.out.println(str +" is a palindrome");
else
System.out.println(str +" is not a palindrome");
}
public static boolean isSymmetrical(String str){
double midIndex;
int length = str.length();
if (length % 2 == 0) {
midIndex = Math.floor(length/2);
}
else {
midIndex = Math.floor(length/2) + 1;
}
int pointer1 = 0;
double pointer2 = midIndex;
while(pointer1 < midIndex && pointer2 < length) {
if(str.charAt(pointer1) == str.charAt((int)pointer2)) {
pointer1 += 1;
pointer2 += 1;
}
else {
return false;
}
}
return true;
}
}
Output:
So I have been given a project in where I must validate ISBN-10 and ISBN-13 numbers. My issue is that I want to use an ArrayList based on what the user inputs(the user adds as many numbers as they want to the ArrayList). Here is my code (without an ArrayList). How can I modify this so that the user can put as many ISBN number as they want?
public static void main(String args[]) {
Scanner input = new Scanner(System.in);
String isbn;
//Get the ISBN
System.out.print("Enter an ISBN number ");
isbn = input.nextLine();
input.close();
//Strip out the spaces/System.out.println("Press 1 to enter a list of ISBN numbers to verify. ");System.out.println("Press 1 to enter a list of ISBN numbers to verify. ");dashes by replacing with empty character.
isbn = isbn.replaceAll("( |-)", "");
//Check depending on length.
boolean isValid = false;
if(isbn.length()== 10){
isValid = CheckISBN10(isbn);
}else if (isbn.length()== 13){
isValid = CheckISBN13(isbn);
}else{
isValid = false;
}
//Print check Status
if(isValid){
System.out.println(isbn + " IS a valid ISBN");
}else{
System.out.println(isbn + " IS NOT a valid ISBN");
}
}
//Checking ISBN-10 numbers are valid
//
private static boolean CheckISBN10(String isbn){
int sum = 0;
String dStr;
for (int d = 0; d < 10; d++){
dStr = isbn.substring(d, d + 1);
if (d < 9 || dStr != "X"){
sum += Integer.parseInt(dStr) * (10-d);
}else {
sum += 10;
}
}
return (sum %11 == 10);
}
private static boolean CheckISBN13(String isbn){
int sum = 0;
int dVal;
for (int d = 0; d < 13; d++){
dVal = Integer.parseInt(isbn.substring(d, d + 1));
if (d % 2 == 0){
sum += dVal * 1;
}else {
sum += dVal * 3;
}
}
return (sum % 10 == 0);
}
}
public static List<String> scanNumberToListUntilAppears(String end) {
if(end == null || end.isEmpty())
end = "end";
List<String> numbers = new ArrayList<>();
String message = "Enter an ISBN number: ";
try (Scanner input = new Scanner(System.in)) {
System.out.print(message);
while(input.hasNext()) {
String isbn = input.nextLine();
if(isbn.equalsIgnoreCase(end)) {
if(!numbers.isEmpty())
break;
} else {
numbers.add(isbn);
if(numbers.size() == 1)
message = "Enter the next ISBN number or '" + end + "': ";
}
System.out.print(message);
}
}
return numbers;
}
public static void main(String args[]) {
Scanner input = new Scanner(System.in);
String isbn;
String ans;
ArrayList<String> isbns = new ArrayList<String>();
// user will enter at least 1 ISBN
do{
//Get the ISBN
System.out.println("Enter an ISBN number ");
isbns.add(input.nextLine());
//loops till answer is yes or no
while(true){
System.out.println("Would you like to add another ISBN?");
ans = input.nextLine();
if(ans.equalsIgnoreCase("no"))
break;
else if (!(ans.equalsIgnoreCase("yes"))
System.out.println("Please say Yes or No");
}
}while(!(ans.equalsIgnoreCase("yes"));
input.close();
//Strip out the spaces/System.out.println("Press 1 to enter a list of ISBN numbers to verify. ");System.out.println("Press 1 to enter a list of ISBN numbers to verify. ");dashes by replacing with empty character.
for(int i = 0; i<isbns.size(); i++)
isbns.set(i, isbns.get(i).replaceAll("( |-)", ""));
isbn = isbn.replaceAll("( |-)", "");
//Check depending on length.
boolean isValid = false;
for(String isbn : isbns){
if(isbn.length()== 10){
isValid = CheckISBN10(isbn);
print(isbn, isValid);
}else if (isbn.length()== 13){
isValid = CheckISBN13(isbn);
print(isbn, isValid);
}else{
isValid = false;
print(isbn, isValid);
}
}
public static void print(String isbn, boolean isValid){
if(isValid){
System.out.println(isbn + " IS a valid ISBN");
}else{ System.out.println(isbn + " IS NOT a valid ISBN");
}
}
//Checking ISBN-10 numbers are valid
private static boolean CheckISBN10(String isbn){
int sum = 0;
String dStr;
for (int d = 0; d < 10; d++){
dStr = isbn.substring(d, d + 1);
if (d < 9 || dStr != "X"){
sum += Integer.parseInt(dStr) * (10-d);
}else {
sum += 10;
}
}
return (sum %11 == 10);
}
private static boolean CheckISBN13(String isbn){
int sum = 0;
int dVal;
for (int d = 0; d < 13; d++){
dVal = Integer.parseInt(isbn.substring(d, d + 1));
if (d % 2 == 0){
sum += dVal * 1;
}else {
sum += dVal * 3;
}
}
return (sum % 10 == 0);
}
i have a problem writing guess the word game java code .
i'll show you my current code and till you the problem .
import java.util.Scanner;
class Q
{
public static void main(String argss[])
{
String strs[] = { "kareem", "java", "izz", "tamtam", "anas" };
Scanner input = new Scanner(System.in);
int index = ((int) (Math.random() * 5));
int points = 50;
String c;
boolean result = false;
boolean finalResult = false;
boolean tempResult = false;
String word = strs[index];
System.out.println(
"\t *** Enter a character and guess the world*** \n\t ***You will loose if your points become 0 *** \n ");
System.out.println(stars(word));
System.out.println("Your points: " + points);
System.out.print("Enter and guess a character! ");
String temp = stars(word);
String oldTemp = temp;
c = input.next();
while (points > 0)
{
for (int i = 0; i < word.length(); i++)
{
result = (word.charAt(i) + "").equals(c);
if (result == true)
{
temp = toChar(i, temp, c);
}
}
finalResult = temp.equals(word);
tempResult = temp.equals(oldTemp);
System.out.println(temp);
if (finalResult == true)
{
System.out.println("\n \n YOU HAVE GUESSED THE WORLD,YOU WON ! ");
break;
}
if (tempResult == true)
{
points = points - 5;
System.out.println("False and now your points are " + points);
}
else if (tempResult == false)
{
System.out.println("True and now your points are " + points);
}
if (points <= 0)
{
System.out.println("\n\n*********************\n* Sorry , you loose *\n********************* ");
break;
}
System.out.print("Guess another time,enter a character: ");
c = input.next();
}
}
public static String stars(String word)
{
String temp = "";
for (int i = 0; i < word.length(); i++)
temp = temp + "*";
return temp;
}
public static String toChar(int index, String temp, String c)
{
char[] tempChar = temp.toCharArray();
char s = c.charAt(0);
tempChar[index] = s;
temp = String.valueOf(tempChar);
return temp;
}
}
now as you can see in line number 39 , i have a little problem here because when its false it'll be no longer right .
do you know another way to compare if the entry is right or not ?
Doesn't look like you are changing oldTemp inside the while loop. Try setting it to equal to temp after all of the if statements.
if (points <= 0)
{
System.out.println("\n\n*********************\n* Sorry , you loose *\n********************* ");
break;
}
oldTemp = temp; // add this here
System.out.print("Guess another time,enter a character: ");
c = input.next();
I want my program to test two BigIntegers at the same time. As of now my code is testing one at a time. Since I want the code to be the same for both runs, is there a simple synchronized statement that I would be able to use to implement this?
public static void main(String[] args) {
String input = JOptionPane.showInputDialog("Enter number ");
int number = Integer.parseInt(input);
BigInteger num = new BigInteger(input);
String output = num + " is" + (IsPrime(num) ? " " : " not ")
+ "a prime number.";
JOptionPane.showMessageDialog(null, output);
}
public static boolean IsPrime(BigInteger num) {
if (num.mod(new BigInteger("2")).compareTo(BigInteger.ZERO) == 0) {
return false;
for (BigInteger i = new BigInteger("3"); i.multiply(i).compareTo(num) <= 0; i =
i.add(new BigInteger("2"))) {
if (num.mod(i).compareTo(BigInteger.ZERO) == 0) {
return false;
}
}
return true;
}
}
I have reached a solution to my question.
Because two integers were being tested I needed to create a textOne and a textTwo to test each individually for primeness.
public static boolean IsPrime(BigInteger textOne) {
// check if number is a multiple of 2
if (textOne.mod(new BigInteger("2")).compareTo(BigInteger.ZERO) == 0) {
return false;
}// if not, then just check the odds
for (BigInteger i = new BigInteger("3"); i.multiply(i).compareTo(textOne) <= 0; i =
i.add(new BigInteger("2"))) {
if (textOne.mod(i).compareTo(BigInteger.ZERO) == 0) {
return false;
}
}
return true;
}
public static boolean IsPrime2(BigInteger textTwo) {
// check if number is a multiple of 2
if (textTwo.mod(new BigInteger("2")).compareTo(BigInteger.ZERO) == 0) {
return false;
}// if not, then just check the odds
for (BigInteger i = new BigInteger("3"); i.multiply(i).compareTo(textTwo) <= 0; i =
i.add(new BigInteger("2"))) {
if (textTwo.mod(i).compareTo(BigInteger.ZERO) == 0) {
return false;
}
}
return true;
}
I designated that num and num2 be designated for the BigIntegers so that output would reflect the following..
public void run() {
String output = num + " is" + (IsPrime(num) ? " " : " not ")
+ "a prime number.";
lblResults.setText(output);
String output2 = num2 + " is" + (IsPrime(num2) ? " " : " not ")
+ "a prime number.";
lblResults2.setText(output2);
}}
Below is my code, and there are no errors in the actual code, but it won't run and I am not sure how to fix it. I have been working on this assignment for hours and keep receiving the same error in the output box over and over.
import java.util.Scanner;
public class whatTheGerbils
{
public static void main(String[] args)
{
whatTheGerbils toRun = new whatTheGerbils();
toRun.gerbLab();
}
Gerbil[] gerbilArray;
Food[] foodArray;
int numGerbils;
int foodnum;
public whatTheGerbils() {}
public void gerbLab()
{
boolean gerbLab = true;
while(gerbLab)
{
foodnum = 0;
numGerbils = 0;
String nameOfFood = "";
String colorOfFood;
int maxAmount = 0;
String ID;
String name;
String onebite;
String oneescape;
boolean bite = true;
boolean escape = true;
Scanner keyboard = new Scanner(System.in);
System.out.println("How many types of food do the gerbils eat?");
int foodnum = Integer.parseInt(keyboard.nextLine());
foodArray = new Food[foodnum];
for (int i = 0; i < foodnum; i++)
{
System.out.println("The name of food item " + (i+1) + ":"); //this is different
nameOfFood = keyboard.nextLine();
System.out.println("The color of food item " + (i+1) + ":");
colorOfFood = keyboard.nextLine();
System.out.println("Maximum amount consumed per gerbil:");
maxAmount = keyboard.nextInt();
keyboard.nextLine();
foodArray[i] = new Food(nameOfFood, colorOfFood, maxAmount);
}
System.out.println("How many gerbils are in the lab?");
int numGerbils = Integer.parseInt(keyboard.nextLine()); // this is different
gerbilArray = new Gerbil[numGerbils];
for (int i = 0; i < numGerbils; i++)
{
System.out.println("Gerbil " + (i+1) + "'s lab ID:");
ID = keyboard.nextLine();
System.out.println("What name did the undergrads give to "
+ ID + "?");
name = keyboard.nextLine();
Food[] gerbsBetterThanMice = new Food[foodnum];
int foodType = 0;
for(int k = 0; k < foodnum; k++)
{
boolean unsound = true;
while(unsound)
{
System.out.println("How much " + foodArray[k].getnameOfFood() + "does" + name + " eat?");
foodType = keyboard.nextInt();
keyboard.nextLine();
if(foodType <= foodArray[k].getamtOfFood())
{
unsound = false;
}
else
{
System.out.println("try again");
}
}
gerbsBetterThanMice[k] = new Food(foodArray[k].getnameOfFood(), foodArray[k].getcolorOfFood(), foodType);
}
boolean runIt = true;
while (runIt)
{
System.out.println("Does " + ID + "bite? (Type in True or False)");
onebite = keyboard.nextLine();
if(onebite.equalsIgnoreCase("True"))
{
bite = true;
runIt = false;
}
else if (onebite.equalsIgnoreCase("False"))
{
bite = false;
runIt = false;
}
else {
System.out.println("try again");
}
}
runIt = true;
while(runIt)
{
System.out.println("Does " + ID + "try to escape? (Type in True or False)");
oneescape = keyboard.nextLine();
if(oneescape.equalsIgnoreCase("True"))
{
escape = true;
runIt = false;
}
else if(oneescape.equalsIgnoreCase("False"))
{
escape = false;
runIt = false;
}
else
{
System.out.println("try again");
}
}
gerbilArray[i] = new Gerbil(ID, name, bite, escape, gerbsBetterThanMice);
}
for(int i = 0; i < numGerbils; i++)
{
for(int k = 0; k < numGerbils; k++)
{
if(gerbilArray[i].getID().compareTo(gerbilArray[k].getID()) >
0)
{
Gerbil tar = gerbilArray[i];
gerbilArray[i] = gerbilArray[k];
gerbilArray[k] = tar;
}
}
}
boolean stop = false;
String prompt = "";
while(!stop)
{
System.out.println("Which function would you like to carry out: average, search, restart, or quit?");
prompt = keyboard.nextLine();
if (prompt.equalsIgnoreCase("average"))
{
System.out.println(averageFood());
}
else if (prompt.equalsIgnoreCase("search"))
{
String searchForID = "";
System.out.println("What lab ID do you want to search for?");
searchForID = keyboard.nextLine();
Gerbil findGerbil = findThatRat(searchForID);
if(findGerbil != null)
{
int integer1 = 0;
int integer2 = 0;
for (int i = 0; i < numGerbils; i++)
{
integer1 = integer1 + foodArray[i].getamtOfFood();
integer2 = integer2 + findGerbil.getGerbFood()[i].getamtOfFood();
}
System.out.println("Gerbil name: " +
findGerbil.getName() + " (" + findGerbil.getBite() + ", " +
findGerbil.getEscape() + ") " +
Integer.toString(integer2) + "/" + Integer.toString(integer1));
}
else
{
System.out.println("error");
}
}
else if (prompt.equalsIgnoreCase("restart"))
{
stop = true;
break;
}
else if(prompt.equalsIgnoreCase("quit"))
{
System.out.println("program is going to quit");
gerbLab = false;
stop = true;
keyboard.close();
}
else
{
System.out.println("error, you did not type average, search, restart or quit");
}
}
}
}
public String averageFood()
{
String averageStuff = "";
double avg = 0;
double num1 = 0;
double num2 = 0;
for (int i = 0; i < numGerbils; i++)
{
averageStuff = averageStuff + gerbilArray[i].getID();
averageStuff = averageStuff + " (";
averageStuff = averageStuff + gerbilArray[i].getName();
averageStuff = averageStuff + ") ";
for (int k = 0; k < foodnum; k++)
{
num1 = num1 + foodArray[k].getamtOfFood();
num2 = num2 + gerbilArray[i].getGerbFood()[k].getamtOfFood();
}
avg = 100*(num2/num1);
avg = Math.round(avg);
averageStuff = averageStuff + Double.toString(avg);
averageStuff = averageStuff + "%\n";
}
return averageStuff;
}
public Gerbil findThatRat (String ID)
{
for(int i = 0; i < numGerbils; i++)
{
if(ID.contentEquals(gerbilArray[i].getID()))
{
return gerbilArray[i];
}
}
return null;
}
}
Whenever a Java program runs, it starts with a method called main. You are getting the error because you don't have such a method. If you write such a method, then what it needs to do is this.
Create an object of the whatTheGerbils class.
Run the gerbLab() method for the object that was created.
The simplest way to do this would be to add the following code inside the whatTheGerbils class.
public static void main(String[] args){
whatTheGerbils toRun = new whatTheGerbils();
toRun.gerbLab();
}
You need a main method in your code for it to run, add in something like this
public static void main(String [] args){
gerLab();
}