getHeight（）和getWidth（）对于绘制半圆返回0

Question

我正在尝试制作一个gui应用程序，该应用程序在由用户运行时显示彩虹。 无论出于何种原因，半圆都不会出现在窗口中。 我做了一个调试方法，它显示x和y均为0。有人可以告诉我这是怎么回事吗？

import java.awt.Color;
import java.awt.Graphics;
import java.awt.Container;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class Rainbow extends JPanel
{
   // Declare skyColor:
  static Color skyColor = Color.CYAN;

public Rainbow()
{
setBackground(skyColor);
}

  // Draws the rainbow.

public void paintComponent(Graphics g)
{
super.paintComponent(g);
int width = getWidth();    
int height = getHeight();

// Declare and initialize local int variables xCenter, yCenter
// that represent the center of the rainbow rings:
int xCenter = (1/2)*width;
int yCenter = (3/4)*height;
// Declare and initialize the radius of the large semicircle:
int smallRadius = height/4;
int largeRadius = width/4;
int mediumRadius = (int) Math.sqrt(smallRadius * largeRadius);
g.setColor(Color.RED);
g.fillArc(xCenter - largeRadius/2,yCenter - largeRadius/2 + largeRadius/4 -height/4,largeRadius,largeRadius,0,180);
// Draw the large semicircle:

// Declare and initialize the radii of the small and medium
// semicircles and draw them:
g.setColor(Color.GREEN);
g.fillArc(largeRadius+mediumRadius/2, yCenter-(largeRadius+mediumRadius)/2, mediumRadius, mediumRadius, 0, 180);    
g.setColor(Color.MAGENTA);
g.fillArc(largeRadius+smallRadius/2, yCenter-(largeRadius+smallRadius)/2, smallRadius, smallRadius, 0, 180);      // Calculate the radius of the innermost (sky-color) semicircle
debug(xCenter,yCenter,smallRadius,largeRadius,mediumRadius);
// Draw the sky-color semicircle:
// ________________________________________________
}
public static void debug(int x,int y,int r1,int r2,int r3)
{
System.out.println("xCenter is " + x + ".");
System.out.println("yCenter is " + y + ".");
System.out.println("smallRadius is " + r1 + ".");       
System.out.println("largeRadius is " + r2 + ".");  
System.out.println("mediumRadius is " + r3 + ".");  
}
public static void main(String[] args)
{
  JFrame w = new JFrame("Rainbow");
  w.setBounds(300, 300, 300, 200);
  w.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
  Container c = w.getContentPane();
  c.add(new Rainbow());
  w.setVisible(true);
}
}

Answer 1

问题在这里：

int xCenter = (1/2)*width;   // this division truncates decimals, returns 0
int yCenter = (3/4)*height;

尝试这个：

int xCenter = (int)((1.0/2)*width);  // this division preserves decimals
int yCenter = (int)((3.0/4)*height);

甚至更简单：

int xCenter = (int)(0.5*width);
int yCenter = (int)(0.75*height);

您正在执行整数除法，该表达式： 1/2返回0 ，任何数字乘以0就是0 。

Answer 2

您正在做int除法。 1/2和3/4均为0。取而代之的是进行双除法，并将其强制转换为int，或者对您的示例更好，在除法之前先将分子乘以。

例如，代替：

int xCenter = (1/2)*width;
int yCenter = (3/4)*height;

做：

// No need to cast!
int xCenter = width / 2;
int yCenter = (3 * height) / 4;