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How do you make sure the 2 balls are removed after the collision?

I am a beginner and I am making a space shooter game in Netbeans. I want if the spacecraft's bullet hits the enemy's bullet that the 2 bullets are removed. I have created an Arraylist of bullets and bulletsEnemy. Then I created a method hitByBullet that should ensure that I can search for the bullets that collided. Then in the method bulletsHit (), the bullets should be removed.
My enemies shoot in a loop and don't move. My problem is that the bullets go through each other. The radius of the bullets is 5.
These is the 2 methods that I have written
'''
public int hitByBullet(){
for(int i=0; i<bullets.size(); i++){
for(int j= 0;j< bulletsEnemy.size(); j++){
if( bullets.get(i).getY() - bulletsEnemy.get(j).getY()<= 5)
if(bullets.get(i).getX() -bulletsEnemy.get(j).getX()<= 5){
return i;
}
}
}
return -1;
}
'''
public boolean bulletsHit(){
if (hitByBullet()!= -1){
bullets.remove(this); //with 'this' I refer too 'i'
bulletsEnemy.remove(this);
return true;
}
return false;
}
'''
[This is how it looks like] (https://i.stack.imgur.com/4XsCR.png)

I will try to answer in a way to let you find your way to implement what you want. There are several frameworks out there which will help you with this and as you are new to programming there are a lot of things you could change/do better. But I will ignore these things.
I think your are trying to check if one 5x5 bullet intersects with another 5x5 bullet.
To make it easy let us asume your bullets are squares:
In your hitByBullet you should check your square centers (i asume that are x,y of your bullets) distance in both dimensions:
Sth. like this should check the collision:
if(Math.abs(bullets.get(i).getY() - bulletsEnemy.get(j).getY()) < 5 &&
Math.abs(bullets.get(i).getX() - bulletsEnemy.get(j).getX()) < 5) {
return i;
}
Depending of the size of your bullets you need to change the distance check ( <=5). If your bullets have a size of 5 px it should be < 10. If your bullets are 10x10 you should check against < 20 ...
Regarding a more realistic scenario, that your bullets are circles (e.g. resulting of a 5x5 square px or a radius r=5):
You need to check if the distance between your two bullet centers is smaller than the bullet radius * 2.
That will bring you to use the theorem of pythagoras:
https://en.wikipedia.org/wiki/Pythagorean_theorem
The distance between your bullet centers is c. a is the distance in dimensional x and b the distance in dimensional y. You don't need to take care about negative dimensional values cause you need pow them by 2 which will always give you a positive value.
To check for collision you should calculate the distance c as follows:
c = SquareRootOf(a^2+b^2)
Math.sqrt(Math.pow(bullets.get(i).getX() - bulletsEnemy.get(j).getX()),2) + Math.pow(bullets.get(i).getY() - bulletsEnemy.get(j).getY(),2)) < 10
I think you should read basics about collision detection and geometry. Just two possible points of information:
An easy to start example with code:
https://zetcode.com/javagames/collision/
Circle intersection:
https://www.geeksforgeeks.org/check-two-given-circles-touch-intersect/
Regarding the removal of the bullets I asume you have some kind of game loop in which you frequently draw the bullets and calculate the collision (call the bulletsHit method):
I will make an assumption to make things easier: The movement speed of your bullets is not faster than one pixel per loop. Otherwise I think you need to take care of your loop circle time and calculate the positions of all elements and also take care about bullets that won't intersect in one loop circle because they could have passed by from one loop circle to the next loop circle. I'm not familiar with game dev, so maybe there are more things to cover.
Regarding this you should change things:
The method hitByBullet() will only return one collision per call.
You could store your collidedBullets in two sets and after removing them from your bullets and bulletsEnemy list clear these temporary sets.
I don't understand what you are doing with "this" and your result of
hitByBullet.
I would do it like this
public void hitByBullet(){
for(int i=0; i<bullets.size(); i++){
for(int j= 0;j< bulletsEnemy.size(); j++){
if( Math.sqrt(Math.pow(bullets.get(i).getX() - bulletsEnemy.get(j).getX()),2) + Math.pow(bullets.get(i).getY() - bulletsEnemy.get(j).getY(),2)) < 10){
bulletsToDelete.add(bullets.get(i));
bulletsEnemyToDelete.add(bulletsEnemy.get(j));
}
}
}
}
public boolean bulletsHit(){
hitByBullet();
bulletsToDelete.forEach(bullet -> bullets.remove(bullet);
bulletsEnemyToDelete.forEach(bullet -> bulletsEnemy.remove(bullet);
boolean bulletsHit = !bulletsToDelete.isEmpty();
bulletsToDelete.clear();
bulletsEnemyToDelete.clear();
return bulletsHit;
}
As I mentioned before there are a lots of things you should improve:
general structure of your code
naming of your method (e.g. bulletsHit sounds like only checking if bullets are hit but you are also removing them)
double checking of bullets
Greetings

How to get old position of snake body parts in Java

I will try my best to explain the issue. So basically I have come to the point in my snake game which I feared the most - the array. So I what I have done is an ArrayList full of Rectangles.
I then add a Rectangle each time I eat the food and I am now at the "looping" part where I have to loop the different rectangles.
I succeded with adding one rectangle to the snake - I just took the old head.x location and the head.y location and but it into the snakeParts.get(0).setLocation.
The problem I am having is drawing the rest of the array (which would be index nr 1 to infinity).
I can't seem to get the rest of the ArrayLists's old positions. For example: I want snakeParts.get(1) to get snakeParts.get(0)'s old position but I can't seem to figure out how to do that logic. I wonder if any of you could give me a hand?
Here is the part of the code that is affected:
repaint();
//Test
for(int z = 0; z < snakeParts.size(); z++) {
System.out.println(z); //Test printing
if(z == 0) {
snakeParts.get(z).setLocation(head.x, head.y); //Printing index 0
}
else {
snakeParts.get(z).setLocation(snakeParts.get(z - 1).getLocation());
//Takes all the indexes and puts them where the snakeParts.get(0) is. I want them to get longer - like the Snake game
}
//Loop different might solve the issue?
}
head.x += speedx;
head.y += speedy;
I am doing this in JPanel and my ArrayList is an array of the Rectangle class which can be found here: Rectangle Class
If you want the entire code - please ask! I thought it would be easier to just show you guys this code sample because it's the only part that affects what I am trying to achieve.
Thanks in advance!

Start from the end and go towards the head, get last part, set its position to where last-1 is.
Then go, for last-1, set its position to where last-2 is.
Repeat that until you reach second part, once you set its position to head, then move your head.
Edit.
Here is how does forward vs backward loop look like:
for(int z = 0; z < snakeParts.size(); z++) {}
for(int z = snakeParts.size() - 1; z >= 0; z--){}
First one starts at 0, goes to size of snake parts by increases of 1.
Second one starts at size of snakeparts - 1 (we decrease by 1 because snakeParts are counted from 0, not from 1), works as long as z is greater-equal to 0, and at each step it reduces z by 1.
Hope that helps :)
In your code you should make sure you are not trying to access indices smaller or larger than the array.

How do I count the number of bombs in my Minesweeper code?

So well I tried creating a simpler Minesweeper game and I encountered one main problem..
I am not able to count the number of bombs and print it in a JTextField
Any ideas as to how to count these as I'm setting a random value to check whether they are a bomb
Tried counting it in the ActionListener but the bombs were counted only after the button was clicked.
if(e.getSource()==b[i][j])
{
b[i][j].setVisible(false);
tf[i][j].setVisible(true);
int r1 = rand.nextInt(6);
if(r1>1)
{
tf[i][j].setText("Safe");
tf[i][j].setBackground(Color.green);
}
else
{ count++;
tf[i][j].setText("Bomb");
tf[i][j].setBackground(Color.red);
f.setVisible(false);
restart.setVisible(true);
}
}

As I understand you decide if the the tile will be a bomb in the run-time using a random generator. Doing this you can't really know how many mines are in your game. I think you should decide the number of mines at the beginning of the game and randomly place them to your game board (you can choose the number according to a difficulty level).
EDIT
You can create a list with some random points that contain the mines
int numOfMines = 10;
int rows=5,columns=5;
ArrayList listWithMines = new ArrayList();
while(listWithMines.size()<numOfMines) {
int randRow = random.nextInt(rows);
int randCol = random.nextInt(columns);
Point point = new Point(randRow, randCol);
if(listWithMines.contains(point))
continue;
else
listWithMines.add(point);
}
This list now contains the Points that have the mines.
You can check if Point(x,y) has a mine like this:
if(listWithMines.contains(new Point(1, 2))) {...}
Instead of a list you can use a 2D array, store a boolean value (or int if you store more states) and make a loop until you place 10 mines. You should keep a counter(placedMines like the list.size()) of the mines you placed and make sure you don't add a mine to a tile that has already a mine and you increase the counter(placedMines) until it reaches the numOfMines.

Discovering a region with no mines on minesweeper

I am new in this page, it hope get to some help, basically I am doing a minesweeper game on Java but it have a problem with a function: discover the region with no mines and no numbers like in the game on windows, when you click in one place and all withe cells appear. I tried make recursive but I can't, some help?
Sorry for the code, the original is in spanish but i tried make a pseudocode:
Matriz = multidimensional Array (the minesweeper)
min and max returns the index min and max to iterate (8 sorroud cells)
private void discoverWitheCell(int x, int y) {
if(matriz[x][y].getdiscovered() == false){
matriz[x][y].setDiscovered(true);
}
else{
if(matriz[x][y].getNumberOfMinesArround() == 0){
for(int i=min(x);i<max(x);i++)
for(int j=min(y);j<max(y);j++)
discoverWhiteCell(i,j);
}
}
}

There's not a lot of code here but I feel like you're coming at it backwards.
Sorry, I'm not a Java speaker so I'm guessing at some of the syntax. Note that this can go out of bounds--personally, I would add a layer of empty cells around my map so I never need to concern myself with bounds checking.
private void ClickSquare(int x, int y)
{
// Did the user click an already exposed square? If so, ignore
if (matriz[x][y].getDiscovered()) return;
matriz[x][y].SetDiscovered(true);
if (matriz[x][y].getNumberOfMinesAround != 0) return;
// If empty, click all the neighbors
for (int xloop = x - 1; xloop <= x + 1; xloop++)
for (int yloop = y - 1; yloop <= y + 1; yloop++)
ClickSquare(xloop, yloop);
}
I believe you have the discovered test messed up and your version appears to be able to go into infinite (until the stack overflows) recursion as if the neighbor is also zero it will come back to the original cell. My version stops this recursion by only processing a cell if it hasn't already been processed.

Making a player move on 2D array game grid

I am creating a game using a 10x10 2D array. The player starts at the top left hand corner indicated as "P" and the objective is to get the player to avoid obstacles to get to the treasure indicated as "T" located in the lower right corner.
How would I go about making the player move about the grid using commands Up/Down/Left/Right?
Would I use a for loop to count through the elements in the array to designate the move?
Here is what I have so far:
import java.util.Scanner;
import java.util.Random;
public class Adventure {
public static void main(String[] args) {
char grid[][]= new char[10][10];
Scanner move = new Scanner(System.in);
System.out.println("Here is the current game board:");
System.out.println("-------------------------------");
for(int i=0; i<grid.length; i++) {
for(int j=0; j<grid.length; j++) {
double random = Math.random();
if(random <=.05) {
grid[i][j]='*';
}
else if(random > .06 && random <= .15) {
grid[i][j]='X';
}
else {
grid[i][j]='.';
}
grid[0][0]='P';
grid[9][9]='T';
System.out.print(grid[i][j]);
}
System.out.println("");
}
System.out.print("Enter your move (U/D/L/R)>");
}
}

you should keep track of the current position of the player and just update those variables.
initial values would be (0,0) as you said.
int px = 0;
int py = 0;
when a move is made, update the variables accordingly:
grid[px][py] = <empty cell>;
switch (move) {
case 'u': py += 1; break;
case 'r': px += 1; break;
...
}
grid[px][py] = 'P';
of course you shouldn't just updated the values "blindly", you should insert some validation logic to follow the rules of the game:
if (grid[px][py] != <obstacle> )
// update player coordinates...

Looks like you're using row-major ordering, judging from the way your board prints out. Based on that, here's what you'll need to do:
First, you need to store the player's position somewhere. Right now it's hardcoded to 0,0.
Second, you need to read in the player's move. That will have to happen in a loop, where you get a move, check if the move is allowed, perform the move, and display the results.
Third, you need to be able to calculate the new position based on the move. Up means row -= 1. Right means column += 1. Etc.
Given the new coordinates, you need to make sure the move is valid. At the very least, you have to stop them from walking off the board, but you may also prevent them from entering a square with an obstacle, etc.
Once you know that the move is valid, you have to update the variables you're storing the current coordinates in.
At the end of the loop, you'll need to redraw the board.
That's the basic gist of it. Right now you are doing everything in main(), and that's okay, but if it were me I would start to split things out into separate methods, like InitializeBoard(), GetNextMove(), CheckIfMoveIsValid(int r, int c), and so on. That way, main() becomes a high-level view of your game loop, and the guts of the different operations are compartmentalized and more easy to deal with. This will require storing off things like your game board into class variables rather than local variables, which should actually make things like obstacle detection easier than it would be currently.

All of the above answers are great. Here are a few suggestions I would make:
Instead of a char two-dimensional array, I would make a custom object, such as Space, and define a two-dimensional array of Spaces (eg, Space[][]). There are a few reasons for this:
You can define a space in a variety of ways (rather than just 1 character). For example, Space[i][j].hasTreasure() can return a boolean to let you know whether or not you found the treasure.
If you want to add functionality later, its as easy as adding an attribute to your Space class. Again, you are not limited to one character here.
More to your question of movement, I would also recommend a few things. Similar to redneckjedi's suggestion of a CheckIfMoveIsValid() method, I would pass the grid and move direction as parameters and return a boolean. To ensure that you do not end up with ArrayIndexOutOfBounds issues, I would also suggest adding a row/column of walls on each side. I would widen the grid out to 12x12 and put a strip of obstacle-type blocks around the outside. This will ensure that you cannot step outside of the grid as hitting a wall will always return 'false' on a valid move.