我试图用List和ArrayList创建不同的圆:
List<Circle> views = new ArrayList<Circle>();
for(int i = 1; i < 5; i++) {
views.add(new Circle());
}
但是当我使用for循环获取圆圈时:
Random rand=new Random();
int a,b;
for(int k=1;k<5;k++){
a=rand.nextInt(400)+20;
b=rand.nextInt(400)+20;
views.get(k).setCenterX(a);
views.get(k).setCenterY(b);
views.get(k).setRadius(10);
views.get(k).setFill(Color.DARKRED);
}
它显示了一个错误.
最佳答案 将圆圈添加到列表后,您可以通过索引获取它们.编程的惯例是索引从0开始,而不是从1开始.
要了解原因,请参阅相关问题:
> Why are zero-based arrays the norm?
假设你运行你的程序:
import javafx.application.Application;
import javafx.collections.FXCollections;
import javafx.scene.*;
import javafx.scene.paint.Color;
import javafx.scene.shape.Circle;
import javafx.stage.Stage;
import java.util.ArrayList;
import java.util.List;
import java.util.Random;
public class IndexError extends Application {
@Override
public void start(Stage stage) throws Exception {
List<Circle> views = new ArrayList<Circle>();
for(int i = 1; i < 5; i++) {
System.out.println("Adding circle at index " + (i -1));
views.add(new Circle());
}
Random rand=new Random();
int a,b;
for(int k=1;k<5;k++){
System.out.println("Getting circle at index " + k);
a=rand.nextInt(400)+20;
b=rand.nextInt(400)+20;
views.get(k).setCenterX(a);
views.get(k).setCenterY(b);
views.get(k).setRadius(10);
views.get(k).setFill(Color.DARKRED);
}
stage.setScene(new Scene(new Group(FXCollections.observableArrayList(views))));
stage.show();
}
public static void main(String[] args) {
launch(args);
}
}
输出将是:
Adding circle at index 0
Adding circle at index 1
Adding circle at index 2
Adding circle at index 3
Getting circle at index 1
Getting circle at index 2
Getting circle at index 3
Getting circle at index 4
Exception in Application start method
Exception in thread "main" java.lang.RuntimeException: Exception in Application start method
at com.sun.javafx.application.LauncherImpl.launchApplication1(LauncherImpl.java:917)
at com.sun.javafx.application.LauncherImpl.lambda$launchApplication$155(LauncherImpl.java:182)
at java.lang.Thread.run(Thread.java:745)
Caused by: java.lang.IndexOutOfBoundsException: Index: 4, Size: 4
at java.util.ArrayList.rangeCheck(ArrayList.java:653)
at java.util.ArrayList.get(ArrayList.java:429)
at gui.IndexError.start(IndexError.java:33)
at com.sun.javafx.application.LauncherImpl.lambda$launchApplication1$162(LauncherImpl.java:863)
at com.sun.javafx.application.PlatformImpl.lambda$runAndWait$175(PlatformImpl.java:326)
at com.sun.javafx.application.PlatformImpl.lambda$null$173(PlatformImpl.java:295)
at java.security.AccessController.doPrivileged(Native Method)
at com.sun.javafx.application.PlatformImpl.lambda$runLater$174(PlatformImpl.java:294)
at com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:95)
这是因为您试图在不存在的索引处获得一个圆圈.您只添加了4个圈子,索引范围为0,1,2,3;所以可用的最高指数是3).
而不是从1开始循环,尝试从0开始循环.可能然后应用程序将开始按预期运行.
例如,使用以下方法替换start方法:
public void start(Stage stage) throws Exception {
List<Circle> views = new ArrayList<Circle>();
for(int i = 0; i < 5; i++) {
System.out.println("Adding circle at index " + i);
views.add(new Circle());
}
Random rand=new Random();
int a,b;
for(int k=0;k<5;k++){
System.out.println("Getting circle at index " + k);
a=rand.nextInt(400)+20;
b=rand.nextInt(400)+20;
views.get(k).setCenterX(a);
views.get(k).setCenterY(b);
views.get(k).setRadius(10);
views.get(k).setFill(Color.DARKRED);
}
stage.setScene(new Scene(new Group(FXCollections.observableArrayList(views))));
stage.show();
}
输出是: