如何創建Graphics對象以在JFrame上繪制形狀

Question

我想制作一個使彈丸動畫化的程序（一個以2D形式在彈丸中飛行的球）。 在主程序中，我調用了一個Shoot（）函數，其參數為Graphics對象，但是我不知道如何創建該對象，以便它可以在我的JFrame對象上繪制。 請幫我。

import java.lang.Math;
import java.applet.*;
import java.awt.Color;
import java.awt.Font;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Rectangle;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;
import java.util.ArrayList;
import java.util.Random;

import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;
import javax.swing.Timer;

public class Projectile extends JPanel{
    public static int ScreenH=1500;
    public static int ScreenW=3000;
    public static int Xcor=0;
    public static int Ycor=0;
    public static int ballRadius=20;
    public static int prevXcor = 0;
    public static int prevYcor = 0;
    public static int newXcor = 0;
    public static int newYcor = 0;
    public static int Time = 0;
    public static int Angle = 45;
    public static int Velocity = 10;
    public static double Acceleration = 9.8;

    public static void InitGraphics(){
        JFrame jframe = new JFrame();
        jframe.setTitle("Projectile");
        jframe.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        jframe.setSize(ScreenW, ScreenH);
        jframe.setResizable(false);
        jframe.setVisible(true);
        jframe.add(new Projectile());
    }

    public static void drawCenteredCircle(Graphics g, int x, int y) {
          int a = x;
          int b = 1000 - y;
          g.fillOval(a,b, ballRadius, ballRadius);
        }

    public static void move() throws InterruptedException {
        Thread.sleep(20);
        Time += 20;
        double XVelocity = Math.sin(Velocity);
        double YVelocity = Math.cos(Velocity);
        int X = (int) (XVelocity*Time);
        int Y = (int) ((YVelocity*Time) + (0.5*Acceleration*Time*Time));
        newXcor = X;
        newYcor = Y;
    }

    public static void repaint(Graphics g) {
        g.setColor(Color.white);
        drawCenteredCircle(g, prevXcor, prevYcor);

        g.setColor(Color.red);
        drawCenteredCircle(g, newXcor, newYcor);

        prevXcor = newXcor;
        prevYcor = newYcor;
    }

    public static void Shoot(Graphics g) throws InterruptedException {
        while ( (newXcor < (ScreenW - (4*ballRadius))) && (newYcor < (ScreenH - (4*ballRadius)))) {
            move();
            repaint(g);
        }
    }


    public static void main(String[]args) {
        InitGraphics();
        Shoot();
    }
}

Answer 1

JFrame只是窗口，您需要向其添加一個JComponent。 JComponents包含一個protected void paintComponent(Graphics g)方法，您需要像這樣重寫它：

    JFrame frame = new JFrame();
    JComponent canvas = new JComponent() {
        protected void paintComponent(Graphics g) {
            //call repaint(g) here instead of this
            g.setColor(Color.RED);
            g.fillRect(0, 0, getWidth(), getHeight());
        };
    };
    frame.add(canvas);

您可能需要清除重塗背景