public void populate(){ //Generates particles in the world.
        ball bl;
        for(int i=0;i<(NumberOfParticleSlider.particle);i++) //Adds
          addObject(new ball(),10+Greenfoot.getRandomNumber(1000-1),10+Greenfoot.getRandomNumber(900-1));
    }

import greenfoot.*;

public class TemperatureSlider extends Actor
{
    private GreenfootImage background, foreground; //The background and foreground images.
    
    private int height, fgWidth = 9, fgHeight = 10; //The height and width of the foreground
    
    private int maxValue, actualWidth, valueWidth, valueLabelWidth, padding, valueX, valueRight; //The maximum value, width in pixels, width of the selectable area, width of the value label,
                                                                                                 //the padding either side of the selectable area, the x-coordinate of the selectable area
                                                                                                 //and the x-coordinate of the right-hand side of the selectable area
    
    private int majorSections, minorSections; //The number of major and minor sections.
    
    private boolean showValue = false,imagesInvalid = true; //Display the value, re-draw the images.
    
    private float pixelsPerValue; //How wide each number is in the selectable value.
    
    public static int temp; //The current value of temperature of the system.
    
    public TemperatureSlider(){ //Creates a new slider with the size of 150 pixels wide by 20 pixels high.
        this(150, 20);
    }
    
    public TemperatureSlider(int width, int height){ //Creates a new slider.
        majorSections = 2;
        minorSections = 2;
        
        maxValue = 273;
        actualWidth = width;
        this.height = height;
        fgHeight = height/2;
        fgWidth = fgHeight;
        if(fgWidth % 2 == 0){
            fgWidth--;
        }
        
        setImage( new GreenfootImage(actualWidth, height) );
    }
    
    public void addedToWorld(World world){ //Method is called by the World class when the slider is added to the world.
        createImages();
        updateImage();
        imagesInvalid = false;
        valueX = getX()*getWorld().getCellSize() - actualWidth/2 + padding;
        valueRight = valueX + valueWidth;
    }
    
    public void act(){ //This method is called by Greenfoot which updates the slider if the user has moved the slider.
        if(Greenfoot.mouseDragged(this) || Greenfoot.mousePressed(this)){
            MouseInfo mouseInfo = Greenfoot.getMouseInfo();
            int x = Math.max(valueX, mouseInfo.getX()*getWorld().getCellSize());
            x -= valueX;
            x = Math.min(x, valueRight);
            setValue(posToValue(x));
            imagesInvalid = true;
        }
        if(imagesInvalid){
            createImages();
            updateImage();
            imagesInvalid = false;
        }
    }
    
    public void setValue(int value){ //Sets the current seleted value of the slider and the dusplay will change to represent this new value.
        if(value < 0){
            value = 0;
        }if(value > maxValue){
            value = maxValue;
        }
        this.temp = value;
        imagesInvalid = true;
    }
    
    public int getValue(){ //Get the current value.
        return temp;
    }
    
    public void showValue(boolean show){ //set whether the value should be displayed next to the slider.
        if(showValue != show){
            imagesInvalid = true;
        }
        showValue = show;
    }
    
    public boolean isShowingValue(){ //Check whether the slider is set to display the value.
        return showValue;
    }
    
    public void setMajorSections(int sections){ //Set the number of major sections to be marked.
        majorSections = sections;
        imagesInvalid = true;
    }
    
    public int getMajorSections(){ //Get the number of major sections that are marked.
        return majorSections;
    }
    

    public void setMinorSections(int sections){ //Set the number of minor sections to be marked per major section.
        minorSections = sections;
        imagesInvalid = true;
    }
    
    public int getMinorSections(){ //Get the minor sections that are marked.
        return minorSections;
    }
    
    public void setMaximumValue(int value){ //Get the maximum value that the user can select.
        if(value <= 0) return;
        maxValue = value;
        pixelsPerValue = valueWidth/(float)maxValue;
        setValue(value);
    }

    public int getMaximumValue(){ //Get the current maximum value which the user can select
        return maxValue;
    }

    private int posToValue(int x){ //Convert a pixel position to a selected value.
        int v = Math.round(x / pixelsPerValue);
        return v;
    }
    
    private int valueToPos(int v){ //Converts the selected value to its equivalent pixel position.
        int p = Math.round(v * pixelsPerValue);
        return p;
    }
    
    private void createImages(){ //Creates the backgroun and the foreground images.
        valueLabelWidth = (int)(Math.floor(Math.log10(maxValue))+1)*10;
        valueWidth = actualWidth - fgWidth - valueLabelWidth;
        valueWidth -= valueWidth%2;
        padding = fgWidth/2;
        pixelsPerValue = valueWidth/(float)maxValue;
        assert padding >= 0;
        
        background = new GreenfootImage(actualWidth, height);
        foreground = new GreenfootImage(fgWidth, height);
        background.drawLine(padding, height/2, padding+valueWidth, height/2);
        if(majorSections > 0){
            float pixelsPerMarker = valueWidth/(float)majorSections;
            float pixelsPerMinor = 0;
            if(minorSections > 0) pixelsPerMinor = pixelsPerMarker/(float)minorSections;
            for(int i=0; i<=majorSections; i++){
                int x = Math.round(i*pixelsPerMarker + padding);
                background.drawLine(x, height, x, 2*height/3);
                for(int m=1; m<minorSections && i<majorSections; m++){
                    int mx = Math.round(x + m*pixelsPerMinor);
                    background.drawLine(mx, 5*height/6, mx, 4*height/6);
                }
            }
        }
        foreground.fillRect(0, 0, fgWidth, fgHeight);
        int pointHeight = Math.min(fgWidth/2, height-fgHeight);
        int[] pointerXPoints = {0, fgWidth/2, fgWidth-1};
        int[] pointerYPoints = {fgHeight, pointHeight+fgHeight, fgHeight};
        foreground.fillPolygon(pointerXPoints, pointerYPoints, 3);
        if(foreground.getWidth()%2 == 0){
            foreground.scale(foreground.getWidth()+1, foreground.getHeight());
        }
    }

    private void updateImage(){ //Updates the position of the foreground and the text
        GreenfootImage image = new GreenfootImage(background);
        image.drawImage(foreground, valueToPos(temp), 0);
        image.drawString(String.valueOf(temp), valueWidth+padding*2, image.getHeight());
        setImage(image);
    }
}