Why collision algorithm in my applet game does not work properly ?

Question

This is my game project. It's very simple version of "Flappy Bird" and I have some serious problems with how the collisions algorithm works. I wrote 2 separate code fragments for collision, for wall1 and wall2. The problem begins when the ball is trying to go through a hole because somehow the program is detecting a collision with a wall. I'm almost positive that the collision algorithm was written correctly because I have been checking it all day.

import java.applet.*;
import java.awt.*;
import java.awt.event.*;
import java.util.Random;
import java.util.Timer;
import java.util.logging.Level;
import java.util.logging.Logger;

public class Game extends Applet implements KeyListener, Runnable {

    Image bground;
    Random generator = new Random();
    int r1;
    int wall1x;
    int wall1y;
    int wall1long;
    int wall2x;
    int wall2y;
    int wall2long;
    Timer timer;

    private Image i;
    private Graphics doubleG;
    int r2;
    int blok_x1 = 800;
    int blok_y1;
    int blok_x = 800;
    int blok_y = 0;
    int blok_x_v = 2;
    int ballX = 20;
    int ballY = 20;
    int dx = 0;
    int dyclimb = 1;
    int dyrise = 1;

    double gravity = 3;
    double jumptime = 0;
    int FPS = 100;
    public int tab[];
    public boolean grounded = true, up = false;
    boolean OVER = false;

    @Override
    public void init() {
        bground = getImage(getCodeBase(), "12.png");
        this.setSize(600, 400);
        tab = new int[100];
        for (int t = 0; t < 100; t++) {
            tab[t] = generator.nextInt(380) + 1;
        }
        addKeyListener(this);
    }

    @Override
    public void keyPressed(KeyEvent e) {
        if (e.getKeyCode() == KeyEvent.VK_LEFT) {
            ballX -= 10;

        }
        if (e.getKeyCode() == KeyEvent.VK_RIGHT) {
            ballX += 10;

        }
        if (e.getKeyCode() == KeyEvent.VK_UP) {
            ballY -= 10;

            up = true;
        }
        if (e.getKeyCode() == KeyEvent.VK_DOWN) {
            ballY += 10;

        }

    }

    @Override
    public void paint(Graphics g) {
        g.drawImage(bground, 0, 0, this);
        if (OVER == false) {

            for (int i = 0; i < 100; i++) {
                g.setColor(Color.green);
                wall1x = blok_x + i * 400;
                wall1y = blok_y;
                wall1long = tab[i];

                g.fillRect(wall1x, wall1y, 20, wall1long);
                g.setColor(Color.green);
                wall2x = blok_x1 + i * 400;
                wall2y = tab[i] + 60;
                wall2long = 400 - tab[i];
                g.fillRect(wall2x, wall2y, 20, wall2long);
                g.setColor(Color.green);

            }
            g.setColor(Color.red);
            g.fillOval(ballX, ballY, 20, 20);
        } else {
            g.drawString("GAME OVER", 300, 300);
        }
    }

    @Override
    public void update(Graphics g) {
        if (i == null) {
            g.setColor(Color.green);
            i = createImage(this.getSize().width, this.getSize().height);
            doubleG = i.getGraphics();
            g.setColor(Color.green);
        }
        doubleG.setColor(getBackground());
        g.setColor(Color.green);
        doubleG.fillRect(0, 0, this.getSize().width, this.getSize().height);
        doubleG.setColor(getForeground());
        g.setColor(Color.green);
        paint(doubleG);
        g.drawImage(i, 0, 0, this);

    }

    @Override
    public void run() {
        int time = 10;

        while (true) {
            if (up == true) {

                ballY -= dyclimb;
                time--;
            } else {
                ballY += dyrise;
            }
            if (time == 0) {
                time = 10;
                up = false;
            }

            blok_x--;
            blok_x1--;

            if (ballX > 600 || ballX < 0 || ballY > 400 || ballY < 0) {
                OVER = true;

            }

            for (int i = 0; i < 100; i++) {                // collision algorithm
                wall1x = blok_x + i * 400;
                wall1y = blok_y;
                wall1long = tab[i];

                if (ballX + 20 >= wall1x && ballX <= wall1x + 20 && ballY <= wall1y + wall1long && ballY >= wall1x - 20) {  //wall1
                    OVER = true;

                }

            }
            for (int i = 0; i < 100; i++) {
                wall2x = blok_x1 + i * 400;
                wall2y = tab[i] + 60;
                wall2long = 400 - tab[i];

                if (ballX + 20 >= wall2x && ballX <= wall2x + 20 && ballY <= wall2y + wall2long && ballY >= wall2x - 20) {  //wall2
                    OVER = true;

                }

            }
            repaint();
            try {
                Thread.sleep(17);
            } catch (InterruptedException ex) {
                Logger.getLogger(NewApplet.class.getName()).log(Level.SEVERE, null, ex);
            }
        }
    }

    @Override
    public void start() {
        Thread thread = new Thread(this);
        thread.start();

    }

    @Override
    public void keyTyped(KeyEvent e) {

    }

    @Override
    public void keyReleased(KeyEvent e) {

    }

}

Answer 1

Use Rectangle class. There's a method called Intersect or Intersection or something like that.

Say you have one object moving. Make a Rectangle to match the object in position(basically an invisible cover for the object).

Do the same things with another object.

When both are to collide, use the intersection method to check on the collision by using the rectangles.

These might help
http://docs.oracle.com/javase/7/docs/api/java/awt/Rectangle.html

Java method to find the rectangle that is the intersection of two rectangles using only left bottom point, width and height?

java.awt.Rectangle. intersection()