Question

I am trying to show a histogram for a image in jlabel, but its not working.

//hist : array containing histogram values
//wid: width of image
//ht: height of image
mylabel.add(new showData(hist,wid,ht));

The code that I am using to display histogram is:

class showData extends JLabel{
int w,h;
int hist[] = new int[256];
int max_hist=0;
public showData(int[] histValue,int w, int h) {
    System.arraycopy(histValue, 0, hist, 0, 256);
    this.w = w;
    this.h = h;
    for (int i = 0; i < 256; i++) {
        if(hist[i]>max_hist)
            max_hist=hist[i];
    }
}


@Override
protected void paintComponent(Graphics g) {
super.paintComponent(g);

int x = (w - 256) / 2;
int lasty = h - h * hist[0] / max_hist;
for (int i=0; i<256; i++, x++) {
  int y = h - h * hist[i] / max_hist;
  g.setColor(new Color(i, i, i));
  g.fillRect(x, y, 1, h);
  g.setColor(Color.red);
  g.drawLine(x-1,lasty,x,y);
  lasty = y;
}
}
}

When debugged, I found that showData() method was getting invoked, but paintComponent() doesn’t. Why is it so? The Jlabel ‘mylabel’ doesn’t show anything?

Answer 1

The following code works for me. Pay attention to the setting of the preferredSize in the constructor:

import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;

import javax.swing.JFrame;
import javax.swing.JLabel;

class ShowData extends JLabel {
    int w, h;
    int hist[];
    int max_hist = 0;

    public ShowData(int[] histValue, int w, int h) {
        hist = new int[histValue.length];
        System.arraycopy(histValue, 0, hist, 0, histValue.length);
        this.w = w;
        this.h = h;
        setPreferredSize(new Dimension(w, h * 2));
        for (int i = 0; i < hist.length; i++) {
            if (hist[i] > max_hist) {
                max_hist = hist[i];
            }
        }
    }

    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);

        int x = (w - hist.length) / 2;
        int lasty = h - h * hist[0] / max_hist;
        for (int i = 0; i < hist.length; i++, x++) {
            int y = h - h * hist[i] / max_hist;
            g.setColor(new Color(i, i, i));
            g.fillRect(x, y, 1, h);
            g.setColor(Color.red);
            g.drawLine(x - 1, lasty, x, y);
            lasty = y;
        }
    }

    public static void main(String[] args) {
        JFrame frame = new JFrame();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        ShowData data = new ShowData(new int[] { 1, 2, 3, 4, 5, 6, 7 }, 100,
             100);
        frame.add(data);
        frame.pack();
        frame.setVisible(true);    
    }
}