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JPanel doesn't get new values (anymore)

So, I'm trying to program a Game of Life simulation (Conway), and I want to show it in a JFrame.
For this purpose, I've created a JPanel, and it works perfectly, until I try to actually show a new generation. With prints, I've figured out, that the list is actually correct inside the newGeneration() method, but when paint(Graphics g) gets called (aka, when I try to repaint the JFrame), the list isn't updating.
I'm sure I've missed something obvious, and I'm not well versed in Java, but it's just getting so annoying. I'd really appreciate your help.
Here's my code;
import java.awt.*;
import javax.swing.*;
import java.awt.event.*;
public class Main {
public static void main(String[] args) {
new GameOfLife();
}
}
class GameOfLife {
// Initialising all class wide variables; sorted by type
JFrame frame = new JFrame("Game of Life");
JPanel panel;
Scanner gameSize = new Scanner(System.in);
String dimensions;
String splitHorizontal;
String splitVertical;
String confirmation;
Boolean accepted = false;
Integer split;
Integer horizontal;
Integer vertical;
Integer livingNeighbours;
int[][] cells;
int[][] newCells;
public GameOfLife() {
// Prompt for game Size
System.out.println("Please enter your game size in the following format; 'Horizontal,Vertical'");
// Run until viable game Size has been chosen
while (!accepted) {
dimensions = gameSize.nextLine();
// Check for correct format
if (dimensions.contains(",")) {
split = dimensions.indexOf(",");
splitHorizontal = dimensions.substring(0, split);
splitVertical = dimensions.substring(split + 1);
// Check for validity of inputs
if (splitHorizontal.matches("[0-9]+") && splitVertical.matches("[0-9]+")) {
horizontal = Integer.parseInt(dimensions.substring(0, split));
vertical = Integer.parseInt(dimensions.substring(split + 1));
// Check for game Size
if (horizontal > 1000 || vertical > 1000) {
System.out.println("A game of this Size may take too long to load.");
} else {
// Confirmation Prompt
System.out.println("Your game will contain " + horizontal + " columns, and " + vertical + " rows, please confirm (Y/N)");
confirmation = gameSize.nextLine();
// Check for confirmation, anything invalid is ignored
if (confirmation.matches("Y")) {
accepted = true;
System.out.println("Thank you for your confirmation. Please select live cells. Once your happy with your game, press Spacebar to start the Simulation.");
// Setting parameters depending on Size
frame.setSize(horizontal * 25 + 17, vertical * 25 + 40);
frame.setVisible(true);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
}
}
}
}
// Prompt asking for new dimensions in case of invalid dimensions or non confirmation
if (!accepted) {
System.out.println("Please enter different dimensions.");
}
}
// Creating list of cells
cells = new int[horizontal][vertical];
// Showing the empty panel for selection of live cells
panel = new PaintCells(horizontal, vertical, cells);
frame.add(panel);
// Select live cells
panel.addMouseListener(new MouseAdapter() {
#Override
public void mousePressed(MouseEvent e) {
if (cells[(int) Math.ceil(e.getX() / 25)][(int) Math.ceil(e.getY() / 25)] == 1) {
cells[(int) Math.ceil(e.getX() / 25)][(int) Math.ceil(e.getY() / 25)] = 0;
} else {
cells[(int) Math.ceil(e.getX() / 25)][(int) Math.ceil(e.getY() / 25)] = 1;
}
frame.repaint();
}
});
// Simulation start
frame.addKeyListener(new KeyListener() {
#Override
public void keyTyped(KeyEvent e) {
if (e.getKeyChar() == ' ') {
newGeneration();
}
}
#Override
public void keyPressed(KeyEvent e) {
}
#Override
public void keyReleased(KeyEvent e) {
}
});
}
// Generating new generations
void newGeneration() {
newCells = new int[horizontal][vertical];
// Pause inbetween generations
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
/*
* Way of Life Rules:
* Living cells with 2 or 3 living neighbours live on to the next generation.
* Dead cells with exactly 3 living neighbours become living cells in the next generation.
* Every other living cell dies.
*/
// iterate through every cell
for (int l = 0; l < vertical; l++) {
for (int k = 0; k < horizontal; k++) {
livingNeighbours = 0;
// check amount of neighbours
if (k - 1 > -1) {
if (l - 1 > -1) {
if (cells[k - 1][l - 1] == 1) {
livingNeighbours++;
}
}
if (l + 1 < vertical) {
if (cells[k - 1][l + 1] == 1) {
livingNeighbours++;
}
}
if (cells[k - 1][l] == 1) {
livingNeighbours++;
}
}
if (k + 1 < horizontal) {
if (l - 1 >= 0) {
if (cells[k + 1][l - 1] == 1) {
livingNeighbours++;
}
}
if (l + 1 < vertical) {
if (cells[k + 1][l + 1] == 1) {
livingNeighbours++;
}
}
if (cells[k + 1][l] == 1) {
livingNeighbours++;
}
}
if (l - 1 >= 0) {
if (cells[k][l - 1] == 1) {
livingNeighbours++;
}
}
if (l + 1 < vertical) {
if (cells[k][l + 1] == 1) {
livingNeighbours++;
}
}
// change cell value depending on amount of neighbours
if (cells[k][l] == 1) {
if (livingNeighbours < 2 || livingNeighbours > 3) {
newCells[k][l] = 0;
} else {
newCells[k][l] = 1;
}
} else {
if (livingNeighbours == 3) {
newCells[k][l] = 1;
}
}
}
}
cells = newCells;
frame.validate();
frame.paint(frame.getGraphics());
newGeneration();
}
}
// Our canvas
class PaintCells extends JPanel {
private Integer horizontal;
private Integer vertical;
private int[][] newOriginalCells;
// Get our X and Y from the original prompts
public PaintCells(Integer originalHorizontal, Integer originalVertical, int[][] originalCells) {
this.horizontal = originalHorizontal;
this.vertical = originalVertical;
this.newOriginalCells = originalCells;
}
#Override
public void paint(Graphics g) {
for (int i = 0; i < vertical; i++) {
for (int j = 0; j < horizontal; j++) {
// Check cell value
if (newOriginalCells[j][i] == 1) {
g.setColor(Color.black);
} else {
g.setColor(Color.white);
}
// paint according to value
g.fillRect(j * 25, i * 25, 25, 25);
if (newOriginalCells[j][i] == 1) {
g.setColor(Color.white);
} else {
g.setColor(Color.black);
} // maybe change style?
g.drawRect(j * 25, i * 25, 25, 25);
}
}
}
}
I'm guessing, the problem is somewhere in newGeneration(), but other than that, I really have no idea anymore.

You have a common problem which I had myself a few months ago.
Java Swing GUI system works in thread called Event Dispatch Thread (EDT). This thread handle events like mouse clicks, typing etc. and paint the components to the screen. You should use this thread not as your main thread, but as sub-thread which working only once a certain time/when event happens, and not let him run continuously. In your code, since the user choose the cell to live, this thread run non-stop (because you started the program inside a listener, which is part of the EDT), and your GUI stuck, because it's updating only at the end of the thread.
You can solve this by using javax.swing.Timer. Timer is an object that allows you do tasks once a while, and it is perfect to this problem.
Use code like this:
ActionListener actionListaner = new ActionListener(){
public void actionPerformed(ActionEvent e){
//Put here you ne genration repeating code
}
};
int delay = 1000;//You delay between generations in millis
Timer timer = new timer(delay, actionListener);
The code in the actionPerformed method will repeat every second (or any other time you want it to repeat), and every operation of the timer will recall EDT instead of let it run non-stop.

How to make buttons glow while iterating through array of buttons?

I would like to display how linear search works visually.
I have created and ADT class of just integers. I also have a frame with buttons on it, when I hit the fillButton, it generates an array of random integers which are displayed on an array of buttons.
When i hit the findButton it will look for the specific number entered. As i am iterating through the array, i would like to make corresponding button change color.
I had created a similar program that iterated through an array of buttons, and changed the color as it went through. I had used Thread.sleep(), and it was just the main class. This time i have two classes and i am not sure how to go about it. I dont't know how to go about making a connection between the ADT class and the GUI class. I've used EventObjects and custom EventListeners before, but that was merely to store objects. Any help pointing in the right direction is appreciated. Thank you.
This is part of the ADT class
public class ADT {
private int[] a;
private int nElems;
private int SIZE = 60;
public ADT(){
a = new int[SIZE];
nElems=0;
}
public void initialPlacement(int index, int value,int initialCount){
a[index] = value;
nElems = initialCount;
}
public int linearSearch(int searchKey){
int index = 0;
for(int i = 0; i < nElems; i++){
if(getVal(i) == searchKey){
index = i;
break;
}
else{
index = -1;
}
}
return index;
}
And here is part of the GUI class
public NumberFrame(){///CONSTRUCTOR===========================
arr = new ADT();
//CREATE COMPONENTS
for(int i = 0; i < 60; i++){
listButtons[i] = new JButton(String.valueOf("_"));
}
for(int i = 0; i < 60; i++){
listLabels[i] = new JLabel(String.valueOf("["+i+"]"));
}
for(int i = 0; i < 60; i++){
listMiniPanels[i] = new JPanel();
listMiniPanels[i].add(listLabels[i]);
listMiniPanels[i].add(listButtons[i]);
}
fillButton.addActionListener(new ActionListener(){
#Override
public void actionPerformed(ActionEvent e) {
boolean sort = true;
if(linearRadio.isSelected()){
System.out.println("linear is checked");
fill(!sort);//fills half the array and array of buttons with random numbers, unsorted
}else if(binaryRadio.isSelected()){
System.out.println("binary is checked");
fill(sort);//fills half the array with random numbers and sorts it
}else{
JOptionPane.showMessageDialog(null, "Please check a sorting method");
}
}
})
findButton.addActionListener(new ActionListener(){
#Override
public void actionPerformed(ActionEvent arg0) {
int index= 0;
if(numberField.getText().equals("")){
System.out.print("Arr size = " + arr.size());
JOptionPane.showMessageDialog(null, "You did not enter a number");
}else {
try {
int searchKey = Integer.parseInt(numberField.getText());
if(linearRadio.isSelected()){
index = arr.linearSearch(searchKey);
listButtons[index].setBackground(Color.GREEN);
if(index > -1)
JOptionPane.showMessageDialog(null, "Found number " +searchKey + " # index [" + index + "]");
else
JOptionPane.showMessageDialog(null, "No such number");
}else{
index = arr.binarySearch(searchKey);
if(index > -1)
JOptionPane.showMessageDialog(null, "Found number " + searchKey+ " # index [" + index + "]");
else
JOptionPane.showMessageDialog(null, "No such number!");
}
} catch (NumberFormatException nfe) {
System.out.println("Arr size = " + arr.size());
JOptionPane.showMessageDialog(null, "Not an integer, pleas try again!");
}
}
}
});
}//=======CONSTRUCTOR END=============================

Why am i getting the error java.lang.NullPointerException in my battleship game? [duplicate]

This question already has answers here:
What is a NullPointerException, and how do I fix it?
(12 answers)
Closed 8 years ago.
I'm trying to code a battleship game and im getting a weird error when i run it: Exception in thread "main" java.lang.NullPointerException
at Caculations.findShip(Caculations.java:29)
at Board.main(Board.java:60)
Please help im stuck and i dont know how to continue! Here is my code: (Note, its in 2 class files in my eclipse work enviorment)
public class Board {
public static void main(String[] args) {
boolean continuePlay = true;
int[][] board = new int[10][10]; // creating 2d array 'board'
char[][] boardGraphical = new char[10][10]; // creating 2d array 'board
// this time the visual
for (int x = 0; x < 10; x++) { // for within for //initializing elements
// in both boards using double for
// method
for (int y = 0; y < 10; y++) {
board[x][y] = 0;
boardGraphical[x][y] = 'o';
System.out.println("Board element " + x + " " + y // printing
// initialized
// elements
// here
+ " initialized");
}
}
/*
* 1) Make user ships 1 and computer ships 2 (all numbers other than 0 =
* true)
*
* 2) Make it where if the computer gets a hit on a '1' than it sets
* that value to like a 3 or something so it knows when the ship is
* sunk. So in a if it does if([x][y] && [x][y])
* System.out.println("You sunk my ship!");
*
* 3) REMEMBER YOU CAN DO MULTIPLE IFS INSIDE IFS FOR MULTIPLE
* CONDITIONS. 4) declare ships here!
*
* 5) PUT STUFF IN A WHILE LOOP SO COMP CAN KEEP GOING
*/
board[3][3] = 1; // declaring a battleship. Very important.
board[3][4] = 1;
board[3][5] = 1;
boardGraphical[3][3] = 's';
boardGraphical[3][4] = 's';
boardGraphical[3][5] = 's';
while (continuePlay == true) { // while loop so that computer keeps
// guessing
// WITHIN THIS LOOP KEEP REPRINTING THE BOARD
double computerChoiceXd = Math.floor(Math.random() * 10); // using
// Math.random
// functions
// for
// computers
// first
// guess
// to be
// a
// random
// num
double computerChoiceYd = Math.floor(Math.random() * 10);
int computerChoiceX = (int) computerChoiceXd;
int computerChoiceY = (int) computerChoiceYd;
if (board[computerChoiceX][computerChoiceY] == 1) { // checking if
// math.random
// landed on a
// ship point
System.out.println("Computer got a hit at " + computerChoiceX
+ " " + computerChoiceY);
board[computerChoiceX][computerChoiceY] = 2; // setting the
// point as 2 or
// 'hit'
boardGraphical[computerChoiceX][computerChoiceY] = 'H';
for (int row = 0; row < 10; row++) { // printing out graphical
// board using the same
// method as when
// intializing
for (int col = 0; col < 10; col++) {
System.out.print(boardGraphical[row][col]);
}
System.out.println(" "); // spacer for printing
}
Caculations test = new Caculations(computerChoiceX, // Creating
// a new
// object of
// calculation
computerChoiceY, board);
test.findShip();
// break;
if (board[3][3] == 2) { // checking to see if ship is sunk using
// a triple if statement
if (board[3][4] == 2) {
if (board[3][5] == 2) {
System.out.println("Battleship sunk!");
continuePlay = false; // if so than it breaks out of
// loop to end the game
break;
}
}
}
} else if (board[computerChoiceX][computerChoiceY] == 0) { // otherwise
// if
// the
// area
// is a
// 0 or
// 'unmarked'
System.out.println("Computer missed at " + computerChoiceX
+ " " + computerChoiceY);
boardGraphical[computerChoiceX][computerChoiceY] = 'x'; // mark
// area
// as a
// miss
for (int row = 0; row < 10; row++) { // print out board
for (int col = 0; col < 10; col++) {
System.out.print(boardGraphical[row][col]);
}
System.out.println(" ");
}
}
}
}
}
public class Caculations {
int xValue;
int yValue;
int[][] myArray;
int[][] storage = new int[10][10];
boolean xAxisChangeP;
boolean yAxisChangeP;
boolean xAxisChangeN;
boolean yAxisChangeN;
boolean notSunk;
Caculations(int x, int y, int[][] myArray) {
xValue = x;
yValue = y;
xAxisChangeP = true;
yAxisChangeP = true;
xAxisChangeN = true;
yAxisChangeN = true;
notSunk = true;
}
void findShip() {
while (notSunk == true) {
// 1
while (xAxisChangeP == true) {
if (myArray[xValue + 1][yValue] == 1) {
myArray[xValue + 1][yValue] = 2;
if (myArray[3][3] == 2) {
if (myArray[3][4] == 2) {
if (myArray[3][5] == 2) {
System.out.println("Battleship sunk!");
notSunk = false;
}
}
}
continue;
}
else {
xAxisChangeP = false;
}
}
while (xAxisChangeN == true) {
if (myArray[xValue - 1][yValue] == 1) {
myArray[xValue - 1][yValue] = 2;
if (myArray[3][3] == 2) {
if (myArray[3][4] == 2) {
if (myArray[3][5] == 2) {
System.out.println("Battleship sunk!");
notSunk = false;
}
}
}
continue;
}
else {
xAxisChangeN = false;
}
}
// 1
while (yAxisChangeP == true) {
if (myArray[xValue][yValue + 1] == 1) {
myArray[xValue][yValue + 1] = 2;
if (myArray[3][3] == 2) {
if (myArray[3][4] == 2) {
if (myArray[3][5] == 2) {
System.out.println("Battleship sunk!");
notSunk = false;
}
}
}
continue;
}
else {
yAxisChangeP = false;
}
}
while (yAxisChangeN == true) {
if (myArray[xValue][yValue - 1] == 1) {
myArray[xValue][yValue - 1] = 2;
if (myArray[3][3] == 2) {
if (myArray[3][4] == 2) {
if (myArray[3][5] == 2) {
System.out.println("Battleship sunk!");
notSunk = false;
}
}
}
continue;
}
else {
yAxisChangeN = false;
}
}
}
}
}

Have you initialized myarray? Best is debug your code to see, which statement throws the exception. In eclipse you can add NullPointerExeption as your breakpoint and debug.

You use myArray, but you never initialize it.
public class Caculations {
int xValue;
int yValue;
int[][] myArray; // array declared but never initialized
// ....
void findShip() {
while (notSunk == true) {
// 1
while (xAxisChangeP == true) {
if (myArray[xValue + 1][yValue] == 1) // then you use it here
Solution: initialize variables before using.
More importantly, you need to learn the general concepts of how to debug a NPE (NullPointerException). You should critically read your exception's stacktrace to find the line of code at fault, the line that throws the exception, and then inspect that line carefully, find out which variable is null, and then trace back into your code to see why. You will run into these again and again, trust me.

In your constructor for Calculations, you never initialized myArray:
Caculations(int x, int y, int[][] myArray) {
xValue = x;
yValue = y;
xAxisChangeP = true;
yAxisChangeP = true;
xAxisChangeN = true;
yAxisChangeN = true;
notSunk = true;
this.myArray = myArray; //Add this line
}
This is a direct answer to your problem, but all in all, you should do some research regarding the meaning behind the exception that was thrown so you understand what it means.
Why this problem happened
In Java, all objects and primitives, if not initialized manually, are given a default value.
For default values of primitives, check this: Primitive Data Types
In case of non-primitive types - such as Object, String, Thread, etc, as well as any user-defined class (i.e. Calculations) and also arrays (i.e. myArray) - the default value is null.
With that in mind, inside your constructor, as exemplified above, you have not initialized myArray, which means that when this variable was accessed for the first time, the value returned was null.
So, what's the problem with null?
Well, by itself, it does no harm. It's there. It doesn't bother you. Until you decide to use a variable that doesn't have an object assigned to it, but somehow you forget that and treat it as if it held something like a String or an array.
That's when Java will tell you: "Hey! There's no object here. I can't work like this. Let's throw an exception!".

How do I check if a class' return of a method equals null?

In my program, I have a while loop that will display a list of shops and asks for an input, which corresponds with the shop ID. If the user enters an integer outside the array of shops, created with a Shop class, it will exit the loop and continue. Inside this loop is another while loop which calls the sellItem method of my Shop class below:
public Item sellItem()
{
displayItems();
int indexID = Shop.getInput();
if (indexID <= -1 || indexID >= wares.length)
{
System.out.println("Null"); // Testing purposes
return null;
}
else
{
return wares[indexID];
}
}
private void displayItems()
{
System.out.println("Name\t\t\t\tWeight\t\t\t\tPrice");
System.out.println("0. Return to Shops");
for(int i = 0; i < wares.length; i++)
{
System.out.print(i + 1 + ". ");
System.out.println(wares[i].getName() + "\t\t\t\t" + wares[i].getWeight() + "\t\t\t\t" + wares[i].getPrice());
}
}
private static int getInput()
{
Scanner scanInput = new Scanner(System.in);
int itemID = scanInput.nextInt();
int indexID = itemID - 1;
return indexID;
}
The while loop in my main class method is as follows:
boolean exitAllShops = true;
while(exitAllShops)
{
System.out.println("Where would you like to go?\nEnter the number which corresponds with the shop.\n1. Pete's Produce\n2. Moore's Meats\n3. Howards Hunting\n4. Foster's Farming\n5. Leighton's Liquor\n6. Carter's Clothing\n7. Hill's Household Products\n8. Lewis' Livery, Animals, and Wagon supplies\n9. Dr. Miller's Medicine\n10. Leave Shops (YOU WILL NOT BE ABLE TO RETURN)");
int shopInput = scan.nextInt();
if(shopInput >= 1 && shopInput <= allShops.length)
{
boolean leaveShop = true;
while(leaveShop)
{
allShops[shopInput - 1].sellItem();
if(allShops == null)
{
System.out.println("still null"); // Testing purposes
leaveShop = false;
}
}
}
else
{
System.out.println("Are you sure you want to leave?\n1. Yes\n2. No");
int confirm = scan.nextInt();
if(confirm == 1)
{
exitAllShops = false;
}
}
The problem is here:
boolean leaveShop = true;
while(leaveShop)
{
allShops[shopInput - 1].sellItem();
if(allShops == null)
{
System.out.println("still null"); // Testing purposes
leaveShop = false;
}
}
No matter what I do, I can't get "still null" to print to confirm that I'm correctly calling the return statement of the method sellItem of the class Shop. What am I doing wrong?

After calling allShops[...].sellItem(), allShops is still a valid array reference -- there's no way it could be null! You probably want to test the return value from sellItem:
if(allShops[shopInput-1].sellItem() == null)

Java JLabel array remove element after use

So I have a JFrame in Netbeans that holds 20 labels of math equations.
mathLabel1 is "2 + 2"
mathLabel2 is "4 * 4"
etc...
and if mathLabel1 is shown and the user guesses the right answer (4) then I want to setVisible(false) and remove that element from my array, so it doesn't come up as a question again.
Basically, no repeats.
Here is a short version of my code:
//declare variables
String strUserAnswer;
int i;
Random r = new Random();
int randvalue = r.nextInt(19);
JLabel[] math = {mathLabel1, mathLabel2, mathLabel3, mathLabel4, mathLabel5, mathLabel6, mathLabel7, mathLabel8, mathLabel9, mathLabel10,
mathLabel11, mathLabel12, mathLabel13, mathLabel14, mathLabel15, mathLabel16, mathLabel17, mathLabel18, mathLabel19, mathLabel20};
JLabel test;
//method that chooses random math equation
public void random(JLabel test) {
r = new Random();
randvalue = r.nextInt(19);
test = math[randvalue];
if (test == math[0]) {
mathLabel1.setVisible(true);
}
else if (test == math[1]){
mathLabel2.setVisible (true);
}
else if (test == math[2]){
mathLabel3.setVisible (true);
}
else if (test == math[3]){
mathLabel4.setVisible (true);
}
else if (test == math[4]){
mathLabel5.setVisible (true);
}
else if (test == math[5]){
mathLabel6.setVisible (true);
}
else if (test == math[6]){
mathLabel7.setVisible (true);
}
else if (test == math[7]){
mathLabel8.setVisible (true);
}
else if (test == math[8]){
mathLabel9.setVisible (true);
}
else if (test == math[9]){
mathLabel10.setVisible (true);
}
else if (test == math[10]){
mathLabel11.setVisible (true);
}
else if (test == math[11]){
mathLabel12.setVisible (true);
}
else if (test == math[12]){
mathLabel13.setVisible (true);
}
else if (test == math[13]){
mathLabel14.setVisible (true);
}
else if (test == math[14]){
mathLabel15.setVisible (true);
}
else if (test == math[15]){
mathLabel16.setVisible (true);
}
else if (test == math[16]){
mathLabel17.setVisible (true);
}
else if (test == math[17]){
mathLabel18.setVisible (true);
}
else if (test == math[18]){
mathLabel19.setVisible (true);
}
else if (test == math[19]){
mathLabel20.setVisible (true);
}
}
private void guessButtonActionPerformed(java.awt.event.ActionEvent evt) {
// User clicks guess to enter answer, if correct part of puzzle appears
strUserAnswer = answerText.getText();
test = math[randvalue];
//if the math equation chosen is 2+2...
if (test == math[0]) {
//show math equation
mathLabel1.setVisible(true);
//if answer is right...
if (strUserAnswer.equals("4")) {
JOptionPane.showMessageDialog(null, "Yay!! That is right!");
//show puzzle piece, hide equation, and choose a new one
label1.setVisible(true);
mathLabel1.setVisible(false);
//test.remove(math[0]);
test = math[randvalue];
answerText.setText(null);
random(test);
//if answer is wrong...
} else {
JOptionPane.showMessageDialog(null, " Sorry, try again!");
answerText.setRequestFocusEnabled(true);
}
}
and that's repeated for math[1], math[2], math[3], etc...
So how would I do this? I tried the remove() method but that was a shot in the dark...

ok, so this might make your day or break your heart, but you are doing way more than you need to with your random() method. first off, it doesn't seem like you need to take in a parameter
because it looks like you are manually changing the value before you ever use it. also, because each value in the array is in fact a JLabel, you can just say math[randValue].setVisible(true) instead of going through the whole if statement thing. and to solve your problem of removing stuff, there is a quick and dirty way you can do it that i will show you, but you are better off using an ArrayList instead of an Array.
public void random() {
Random r = new Random();
randValue = r.nextInt(math.length); //make sure the index is always within the array
JLabel[] temp = new JLabel[math.length - 1]; //this will do the trick
math[randValue].setVisible(true);
for (int i = 0; i < randvalue; i++) {
temp[i] = math[i]; //fill the new array up to the chosen label
}
for (int i = randValue; i < temp.length; i++) {
temp[i] = math[i + 1]; //fill the rest, omitting the chosen label
}
math = new JLabel[temp.length]; //math is now shorter
math = temp; //put everything back in the original array
}
this should work as a solution using arrays.
hope it helps.

If your data structure will constantly be changing, try using a List instead of an Array:
List<JLabel> labels = new ArrayList<JLabel>();
int numLabels = 20;
for (int i = 0; i < numLabels; i++) {
labels.add(new JLabel(i + " " + i));
}
From there you could always call:
labels.get(4).setVisible(false);
or
labels.remove(4);
And then revalidate your JPanel.
EDIT 2:
I may have misunderstood your question - it seems you want to remove a number and never create a label for it again. This is the correct way to do it:
int numIntegers = 20;
Set<Integer> possibleNumbers = new HashSet<Integer>();
for (int i = 0; i < numIntegers; i++) {
possibleNumbers.add(i);
}
When you want to remove an item, use:
possibleNumbers.remove(14);
Then when you want to present this data, you can use:
panel.clear();
for (Integer number : possibleNumbers) {
panel.add(new JLabel(number + " " + number));
}
(Please note I was incorrect calling JLabels data - they are part of the presentation.)