使用x，y坐标点列表绘制曲线峰线

Question

I am trying to draw a curve line peaks using a list of x,y coordinates points. Will be useful for a mouse movement algorithm that I coded and would like a graph/analysis for it.

What I have tried:

import java.awt.Graphics2D;
import java.awt.geom.Path2D;
import javax.swing.JComponent;
import javax.swing.JFrame;

public class curve extends JFrame {

    public static void main(String[] args) {
        final JFrame f = new JFrame("My curve");
        f.setSize(600, 400);
        f.setVisible(true);

        f.add(new JComponent() {
            public void paintComponent(Graphics2D g) {

                Graphics2D graphics = g;// your graphics object
                double[] x = { 50, 100, 300, 500 }; // x coordinates of polyline
                double[] y = { 70, 120, 50, 280 }; // y coordinates of polyline
                Path2D polyline = new Path2D.Double();
                polyline.moveTo(x[0], y[0]);
                for (int i = 1; i < x.length; i++) {
                    polyline.lineTo(x[i], y[i]);
                }
                graphics.draw(polyline);
            }
        });

    }
}

Results I get: Empty JFrame

Expected results: Something like that Example

I have been trying to make it draw anything on the JFrame since the morning but with no luck. It will be really great if you could show me how to do it. Thanks!

Answer 1

Call setVisible after you call add . Swing's layouts are lazy, unless you tell them, they generally won't update themselves.

Next, add @Override to methods you "think" you're overriding...

        @Override
        public void paintComponent(Graphics2D g) {

This will provide you with compiler errors when you do something wrong (and spend a bunch of time scratching your head wondering why nothing works)

With this addition, you should then get an error message similar to this...

/.../Curve.java:20: error: method does not override or implement a method from a supertype
            @Override
1 error

A quick jump over to the JavaDocs would highlight the specific issue.

The expected method signature is paintComponent(Graphics) not paintComponent(Graphics2D)

And some minor tweaks later...

import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.geom.Path2D;
import javax.swing.JComponent;
import javax.swing.JFrame;

public class Curve extends JFrame {

    public static void main(String[] args) {
        final JFrame f = new JFrame("My curve");
        f.setSize(600, 400);

        f.add(new JComponent() {
            @Override
            protected void paintComponent(Graphics g) {
                Graphics2D graphics = (Graphics2D) g.create();// your graphics object
                double[] x = {50, 100, 300, 500}; // x coordinates of polyline
                double[] y = {70, 120, 50, 280}; // y coordinates of polyline
                Path2D polyline = new Path2D.Double();
                polyline.moveTo(x[0], y[0]);
                for (int i = 1; i < x.length; i++) {
                    polyline.lineTo(x[i], y[i]);
                }
                graphics.draw(polyline);
                graphics.dispose();
            }
        });
        f.setVisible(true);

    }
}

These are all very easy (and, regrettably, common) mistakes to make. The language provides tools for you to use to help identify and reduce the likely hood of these issues (ie @Override ).

When it comes to things like painting, the most common issues are;

• Not adding the component to a visible container (yes, this happens to everyone, more then we'd like to admit)
• Not actually overriding the method we thing we're overriding.

This when the documentation and available tutorials/examples are invaluable to helping solve the problems - Yes, I've been caught out many times with "But I typed it exactly" only to find I've made an assumption about a parameter, or worse, an import .