Sprite position changes lead to crash

Question

Hullo folks,
I'm having some trouble with my current code. I'm trying to have a book item spawn in my minigame and I want it to move from it's original position (on the bottom side of the screen) to the top side of the screen. The code I wrote for this is simple enough and can be found below;
public void fireWeapon() throws IOException
          bookWeapon.setPosition(player.getX(), player.getY()-4); // Spawns the book near the player
          for(int i=0; i<160; i++)
               int currentX = bookWeapon.getX( );
               int newY = bookWeapon.getY( ) - WEAPON_SPEED;
               if (newY <= 1)
                    bookWeapon.setVisible(false);
               else
                    bookWeapon.setPosition(currentX, newY);
     }So what is this bookWeapon, you might wonder? Well it's instantiated earlier in the code;
Image imgBook = Image.createImage("/book.png");
Sprite bookWeapon = new Sprite(imgBook);
bookWeapon.setPosition(80, 150);
mLayerManager = new LayerManager();
mLayerManager.append(bookWeapon);So what I'm basically trying to do is as follows; Have the game create the book and add it to the layermanager at a position somewhere in the gamefield. (Or GameCanvas, if you will.) Then, when the player presses the fire button, the fireWeapon(); method is called and that sets the position of the book from whereever the player is standing. After this, the book is moved upwards using a negative Y value (WEAPON_SPEED). Then the if/else statement checks if the book has gone off the screen. If so, then it's removed, if not, then its new position is set.
Simple fact is; it doesn't work. Whenever I try to either get the position of the book in the form of getX() or getY() or try to set its position using setPosition() the game hangs with a nullPointerException.
I've tried all sorts of things but it doesn't work so, ideas, anyone?
One last note; the fireWeapon() method is started after the player presses the fire button in the input method. However, I assume most here know what an input method looks like, so I didn't deem it important enough to warrant inclusion in this code example.

Answer 1

Sprite bookWeapon = new Sprite(imgBook);is obviously inside a method and the variable reference bookWeapon goes out of scope when the method returns. You need to declare the variable as an instance fieldprivate Sprite bookWeapon;at the class level and initialize it before calling any method on it withbookWeapon = new Sprite(imgBook);Read more on scope of variables at
{color:#0000ff}http://home.cogeco.ca/~ve3ll/jatutor2.htm#st{color}
Problem solved?
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