/**
     * Player tanks handle key release event.
     */
    public void keyReleased(int gameAction){
        keyPressed=false;
        switch (gameAction) {
            case GameCanvas.UP:
                if(direction == BattleField.NORTH){
                    direction = BattleField.NONE;
                    break;
                }
            case GameCanvas.DOWN:
                if(direction == BattleField.SOUTH){
                    direction = BattleField.NONE;
                    break;
                }
            case GameCanvas.LEFT:
                if(direction == BattleField.WEST){
                    direction = BattleField.NONE;
                    break;
                }
            case GameCanvas.RIGHT:
                if(direction == BattleField.EAST){
                    direction = BattleField.NONE;
                    break;
                }
            case GameCanvas.FIRE:
                shoot = false;
        }
    }
    
    ////////////////////////////////////////////////////////////////////////////
    //--------------------------------- REVISIONS ------------------------------
    // Date       Name                 Tracking #         Description
    // ---------  -------------------  -------------      ----------------------
    // 16JAN2008  James Shen                 	          Initial Creation
    ////////////////////////////////////////////////////////////////////////////
    /**
     * Upgrade player's tank
     */
    public void upgrade(Powerup powerup){
        switch(powerup.getType()){
            case Powerup.CLOCK:
                EnemyTank.immobilizedStartTime=System.currentTimeMillis();
                break;
            case Powerup.STAR:
                grade++;
                if(grade>MAX_GRADE ) grade=MAX_GRADE;
                switch(grade){
                    case GRADE_SPEED:
                    {
                        speed*=2;
                        int x=getX();
                        int y=getY();
                        x=(x / speed) *speed;
                        y=(y / speed) *speed;
                        setPosition(x,y);
                    }
                    break;
                    case GRADE_TWO_BULLETS:
                        avaiableBullets=2;
                        break;
                    case GRADE_THREE_BULLETS:
                        avaiableBullets=3;
                        break;
                }
        
                break;
            case Powerup.SHIELD:
                invulnerableTime=System.currentTimeMillis();
                invulnerabilityTicks=invulnerablePeriod;
                sheild.setPosition(getX(),getY());
                sheild.setVisible(true);
                break;
            case Powerup.BOMB:
                EnemyTank.explodeAllEmenies();
                break;
            case Powerup.TANK:
                avaiableLife++;
                break;
            case Powerup.SHOVEL:
                battleField.makeHomeConcreteWall();
                break;
                
        }
    }
    
    ////////////////////////////////////////////////////////////////////////////
    //--------------------------------- REVISIONS ------------------------------
    // Date       Name                 Tracking #         Description
    // ---------  -------------------  -------------      ----------------------
    // 16JAN2008  James Shen                 	          Initial Creation
    ////////////////////////////////////////////////////////////////////////////
    /**
     * set player avaiable lives.
     * @param live new life count.
     */
    public void setAvaiableLives(int live){
        avaiableLife=live;
    }
    
    ////////////////////////////////////////////////////////////////////////////
    //--------------------------------- REVISIONS ------------------------------
    // Date       Name                 Tracking #         Description
    // ---------  -------------------  -------------      ----------------------
    // 16JAN2008  James Shen                 	          Initial Creation
    ////////////////////////////////////////////////////////////////////////////
    /**
     * Get player avaiable lives.
     */
    public int getAvaiableLives(){
        return avaiableLife;
    }
    
    ////////////////////////////////////////////////////////////////////////////
    //--------------------------------- REVISIONS ------------------------------
    // Date       Name                 Tracking #         Description
    // ---------  -------------------  -------------      ----------------------
    // 16JAN2008  James Shen                 	          Initial Creation
    ////////////////////////////////////////////////////////////////////////////
    /**
     * Tank thinks before move.
     */
    public void think(){
        if(grade==NEW_BORN){
            newBornTimer++;
            if(newBornTimer>4){
                grade= MIN_GRADE;
                direction=BattleField.NONE;
                currentDirection=BattleField.NORTH;
                newBornTimer=0;
                setFrame(0);
                invulnerableTime=System.currentTimeMillis();
                invulnerabilityTicks=invulnerablePeriod/4;
                sheild.setPosition(getX(),getY());
                sheild.setVisible(true);
                
            }else{
                try{
                    setFrame(newBornTimer*9-1);
                }catch(Exception e){
                    //System.out.println("Playertank");
                }
            }
            
        } else{
            long tickTime = System.currentTimeMillis();
            if(tickTime-invulnerableTime>invulnerabilityTicks){
                sheild.setVisible(false);
            }else{
                sheild.setPosition(getX(),getY());
                sheild.setVisible(true);
            }
            changeDirection(direction);
            if(currentDirection!=BattleField.NONE){
                switchImage=!switchImage;
                int offset=switchImage? 0:1;
                try{
                    setFrame(currentDirection*9+((int)(grade-1)/2)*2 +offset);
                }catch(Exception e){
                    //System.out.println("Playertank1");
                }
            }
        }
        
    }
    
    
    ////////////////////////////////////////////////////////////////////////////
    //--------------------------------- REVISIONS ------------------------------
    // Date       Name                 Tracking #         Description
    // ---------  -------------------  -------------      ----------------------
    // 16JAN200 8  James Shen                 	          Initial Creation
    ////////////////////////////////////////////////////////////////////////////
    /**
     * Check if the player's tank is invulnerable
     * @return true,the tank is invulnerable.
     */
    public boolean isInvulnerable(){
        return sheild.isVisible();
    }
    
    ////////////////////////////////////////////////////////////////////////////
    //--------------------------------- REVISIONS ------------------------------
    // Date       Name                 Tracking #         Description
    // ---------  -------------------  -------------      ----------------------
    // 16JAN200 8  James Shen                 	          Initial Creation
    ////////////////////////////////////////////////////////////////////////////
    /**
     * Tank shoots.
     * @return the bullet the tank shoots.
     */
    public Bullet shoot(){
        Bullet bullet=null;
        int bulletCount=Bullet.getPlayerBulletCount();
        if(shoot && bulletCount<avaiableBullets){
            int step=ResourceManager.TILE_WIDTH;
            bullet=Bullet.getFreeBullet();
            if(bullet!=null){
                if(ResourceManager.isPlayingSound){
                    ResourceManager.playSound(ResourceManager.SHOOT_SOUND);
                }
                int x = getRefPixelX();
                int y = getRefPixelY();
                
                switch (currentDirection) {
                    case BattleField.NORTH:
                        y -= step / 2;
                        break;
                    case BattleField.EAST:
                        x += step / 2;
                        break;
                    case BattleField.SOUTH:
                        y += step / 2;
                        break;
                    case BattleField.WEST:
                        x -= step / 2;
                        break;
                }
                bullet.setSpeed(ResourceManager.TILE_WIDTH/2);
                bullet.setDirection(currentDirection);
                if(grade>=GRADE_BREAK_WATER){
                    bullet.setStrength(Bullet.GRADE_BREAK_WATER);
                }else if(grade>=GRADE_BREAK_CONCRETE_WALL){
                    bullet.setStrength(Bullet.GRADE_BREAK_CONCRETE_WALL);
                }else{
                    bullet.setStrength(Bullet.GRADE_DEFAULT);
                }
                
                bullet.setRefPixelPosition(x-1, y-1);
                bullet.setFriendly(true);
                bullet.setVisible(true);
            }
        }
        return bullet;
    }

    ////////////////////////////////////////////////////////////////////////////
    //--------------------------------- REVISIONS ------------------------------
    // Date       Name                 Tracking #         Description
    // ---------  -------------------  -------------      ----------------------
    // 16JAN2008  James Shen                 	          Initial Creation
    ////////////////////////////////////////////////////////////////////////////
    /**
     * Stop the player tank
     */
    public void stop(){
        direction=BattleField.NONE;
        shoot=false;
    }
    ////////////////////////////////////////////////////////////////////////////
    //--------------------------------- REVISIONS ------------------------------
    // Date       Name                 Tracking #         Description
    // ---------  -------------------  -------------      ----------------------
    // 16JAN2008  James Shen                 	          Initial Creation
    ////////////////////////////////////////////////////////////////////////////
    /**
     * Explode a tank.
     */
    protected void explode() {
        if(!isInvulnerable()){
            if(grade>=GRADE_BREAK_WATER){
                grade--;
            }else{
                avaiableLife--;
                super.explode();
            }
        }
    }
}