为什么在使用Graphics g工作正常时会出现此异常，但Graphics2D g2 =（Graphics2D）g不会？错误是drawLine（）使用g2.draw（new Line2D）;我首先假设只是放置Graphics2D g2没有初始化对象，但错误仍然发生。显然，某些东西是空的，但我无法弄清楚这与g2有什么关系。请帮忙！
Exception in thread "Thread-0" java.lang.NullPointerException
at Triangles.Triangle_Game.drawLine(Triangle_Game.java:126)
at Triangles.Triangle_Game.drawTriangles(Triangle_Game.java:152)
at Triangles.Triangle_Game.render(Triangle_Game.java:78)
at Triangles.Triangle_Game.run(Triangle_Game.java:190)
at java.lang.Thread.run(Thread.java:745)
完整代码转储：
package Triangles;
import java.awt.Color;
import java.awt.Font;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.image.BufferStrategy;
import java.awt.Point;
import java.awt.geom.Line2D;
import java.util.ArrayList;
public class Triangle_Game implements Runnable {
// Vars
private static ArrayList<Point> trianglePoints = new ArrayList<>();
private static ArrayList<Point> triangleRotatedPoints = new ArrayList<>();
private Triangle_Display display;
private Triangles triangles;
private int width, height, numTriangles = 20, radius = 1, radiusCirc = 600;
private String title;
private Thread thread;
private boolean running = false;
private BufferStrategy bS;
private Graphics g;
private Graphics2D g2 = (Graphics2D) g;
private static int FPS = 1;
private static Font font20; // size 20
// Constructor
public Triangle_Game(String title, int width, int height) {
this.title = title;
this.width = width;
this.height = height;
}
private void init() {
//font10 = Triangle_FontLoader.loadFont("res/fonts/Fairlight.ttf", 10);
font20 = new Font("Arial", Font.PLAIN, 28);
display = new Triangle_Display(title, width, height);
triangles = new Triangles(numTriangles, radius);
triangles.runCalculation(triangles.numTriangles, triangles.radius);
ArrayList<Double> resArray = triangles.runLoop(); // resArray Length
triangles.printArray(resArray);
}
public void program() {
// update triangle points here
triangles.numTriangles += 1;
}
private void update() {
program();
}
private void render() {
bS = display.getCanvas().getBufferStrategy();
if (bS == null) {
display.getCanvas().createBufferStrategy(3);
return;
}
g = bS.getDrawGraphics();
g.clearRect(0, 0, width, height);// clear screen
// Start Draw
drawCircle(g, width/2 - radiusCirc, height/2 - radiusCirc, radiusCirc);
drawFillCircle(g, width/2 - 5, height/2 - 5, 5);
g.setColor(Color.RED);
drawTriangles(g, trianglePoints);
g.setColor(Color.BLACK);
Triangle_Text.drawString(g, "Radius:", 100, 100,
false, Color.BLACK, font20);
Triangle_Text.drawString(g, Integer.toString(radius), 200, 100,
false, Color.BLACK, font20);
Triangle_Text.drawString(g, "numTriangles:", 100, 150,
false, Color.BLACK, font20);
Triangle_Text.drawString(g, Integer.toString(triangles.numTriangles), 280, 150,
false, Color.BLACK, font20);
Triangle_Text.drawString(g, "Approx PI value:", 100, 200,
false, Color.BLACK, font20);
Triangle_Text.drawString(g, Double.toString(triangles.area), 320, 200,
false, Color.BLACK, font20);
// End Draw
bS.show();
g.dispose();
}
public static double translate(double var){
var += (double)100;
return var;
}
public static double stretch(double var){
var *= (double)2;
return var;
}
public void drawCircle(Graphics g, int posX, int posY, int radius) {
g.drawOval(posX, posY, radius * 2, radius * 2);
}
public void drawFillCircle(Graphics g, int posX, int posY, int radius) {
g.fillOval(posX, posY, radius * 2, radius * 2);
}
public void drawLine(Graphics2D g2, double x1, double y1, double x2, double y2) {
g2.draw(new Line2D.Double(x1 + (width/2), y1 + (height/2), x2 + (width/2), y2 + (height/2)));
}
public void rotationMatrix2D() {
Point origin = new Point(0, 0);
double x = triangles.calculateBase(triangles.theta)*radiusCirc;
double y = triangles.calculateHeight(triangles.theta)*radiusCirc;
trianglePoints.add(new Point((int) x, (int) y));
for (int i = 0; i < triangles.numTriangles*2; i++) {
Point p = trianglePoints.get(i);
double angle = Math.toRadians(triangles.theta);
double cos = Math.cos(angle);
double sin = Math.sin(angle);
double cx = origin.getX();
double cy = origin.getY();
x = cx + ((p.x - cx) * cos) - ((p.y - cy) * sin);
y = cy + ((p.x - cx) * sin) + ((p.y - cy) * cos);
triangleRotatedPoints.add(new Point((int) x, (int) y));
trianglePoints.add(i+1, triangleRotatedPoints.get(i));
}
}
public void drawTriangles(Graphics g, ArrayList<Point> array) {
rotationMatrix2D();
Point origin = new Point(0, 0);
for (int i = 0; i < triangles.numTriangles*2; i += 2) {
drawLine(g2, 20, 40, 60, 80);
/*drawLine(g2, origin.getX(), origin.getY(),
trianglePoints.get(i).getX(), trianglePoints.get(i).getY());
drawLine(g2, trianglePoints.get(i).getX(), trianglePoints.get(i).getY(),
triangleRotatedPoints.get(i+1).getX(), triangleRotatedPoints.get(i+1).getY());
drawLine(g2, triangleRotatedPoints.get(i+1).getX(), triangleRotatedPoints.get(i+1).getY(),
origin.getX(), origin.getY());*/
}
}
public static ArrayList<?> del(ArrayList<?> array) {// Return an empty array so that they don't grow enormous
if(array.size() > 0) {
array.clear();
}
return array;
}
public static void cleanup() {
del(trianglePoints);
}
public void run() {
init();
double timePerTick = 1000000000 / FPS;
double delta = 0;
long now;
long lastTime = System.nanoTime();
long timer = 0;
int ticks = 0;
while (running) {
now = System.nanoTime();
delta += (now - lastTime) / timePerTick;
timer += now - lastTime;
lastTime = now;
if (delta >= 1) {
update();
render();
cleanup();
ticks += 1;
delta--;
}
if (timer >= 1000000000) {
System.out.println("FPS: " + ticks);
ticks = 0;
timer = 0;
}
}
stop();
}
public synchronized void start() {
if (running) {
return;
}
running = true;
thread = new Thread(this);
thread.start();
}
public synchronized void stop() {
if (!running) {
return;
}
running = false;
try {
thread.join();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
public void printArray(ArrayList<?> array) {
for (Object item : array) {
System.out.println(item.toString());
}
}
// Getters and Setters
public int getWidth() {
return width;
}
public int getHeight() {
return height;
}
public String getTitle() {
return title;
}
public void setTitle(String title) {
this.title = title;
}
}
package Triangles;
import java.util.ArrayList;
public class Triangles {
double radius;
int numTriangles;
double twoTheta;
double theta;
double base;
double height;
double result;
double halfTriangleArea;
double triangleArea;
double area;
ArrayList<Double> arrayList = new ArrayList<>();
public Triangles(int numTriangles, double radius) {
this.numTriangles = numTriangles;
this.radius = radius;
init();
}
private void init() {
this.twoTheta = (double)360/numTriangles;
this.theta = (double)twoTheta/2;
}
// My methods
public double calculateBase(double theta) { // SOH
double thetaToRadians = Math.toRadians(theta);
double base = Math.sin(thetaToRadians) * radius;
return base;
}
public double calculateHeight(double theta) { // CAH
double thetaToRadians = Math.toRadians(theta);
double height = Math.cos(thetaToRadians) * radius;
return height;
}
public double checkPythag(double base, double height) {
double a = base;
double b = height;
double result = Math.sqrt(a*a + b*b);
return result;
}
public double calculateArea(double base, double height) {
double halfTriangleArea = (0.5) * base * height;
return halfTriangleArea;
}
public double runCalculation(int numTriangles, double theta) {
base = calculateBase(theta);
height = calculateHeight(theta);
result = checkPythag(base, height);
halfTriangleArea = calculateArea(base, height);
triangleArea = 2 * halfTriangleArea;
// C = Pi * D = Pi * 2 * r
// A = Pi * r.^2
area = numTriangles * triangleArea;
// Substitute Pi for X
// area = X * r.^2
// if r is 1
// area = X
return area;
}
public ArrayList<Double> runLoop() {
ArrayList<Double> resArray = new ArrayList<>();
for (int i = 3; i < numTriangles + 1; i++) {
// min 3 to current numTriangles amount plus one
double twoTheta = (double)360/i;
double theta = (double)twoTheta/2;
double area = runCalculation(i, theta);
double res = Math.PI - area;
resArray.add(res);
}
System.out.println("resArray length: " + resArray.size() + "\n");
return resArray;
}
public void printArray(ArrayList<Double> array) {
int triangleAmount = 3;
for (int i = 0; i < array.size(); i++) {
System.out.println("difference PI and " + triangleAmount + " triangles: " + array.get(i));
triangleAmount++;
if (i >= 1) {
double difference = Math.abs(array.get(i-1) - array.get(i));
arrayList.add(difference);
// absolute difference between value (i) and (i-1)
System.out.println(difference + " | " + arrayList.size());
}
if (i >= 2) {
double diffSign = arrayList.get(i-2) - arrayList.get(i-1);
System.out.println(diffSign);
if (diffSign < 0) {
System.out.println("Difference is not positive");
break;
}
}
System.out.println("\n");
}
}
// Runnable
public static void main(String[] args) { // create an instance of class to run in main
// min numTriangles is 3, otherwise Triangle Area does calculation not sensible
Triangles triangles = new Triangles(3, 1);
triangles.runCalculation(triangles.numTriangles, triangles.theta);
System.out.println("radius: " + triangles.radius);
System.out.println("numTriangles: " + triangles.numTriangles);
System.out.println("twoTheta " + triangles.twoTheta);
System.out.println("theta " + triangles.theta);
System.out.println("base: " + triangles.base);
System.out.println("height: " + triangles.height);
System.out.println("checkPythag " + triangles.result + " | " + triangles.radius);
System.out.println("halfTriangleArea: " + triangles.halfTriangleArea);
System.out.println("triangleArea: " + triangles.triangleArea);
System.out.println("Approximation of Pi by triangles: " + triangles.area);
ArrayList<Double> resArray = triangles.runLoop();
triangles.printArray(resArray);
}
}