求助、、、、

[nuoop]

public class UseClass_1 {

/**
* @param args
*/
class Point{
int x,y;
public Point(int x,int y){
this.x=x;
this.y=y;
}
}
class Circle{
Point center=new Point(0,0);
double radius=0;
public Circle(int x,int y,double r){
center.x=x;
center.y=y;
radius=r;
}
public double area(){
return Math.PI*radius*radius;
}
}
public static void main(String[] args) {
// TODO Auto-generated method stub
Point p =new Point(7,8);
Circle c=new Circle(p.x,p.y,9);

System.out.println("圆中心点在"+"("+Point.x+","+Point.y+")");
System.out.println("半径为"+Circle.radius);
System.out.println("圆的面积为"+Circle.area());

}


}
这个程序出了什么问题、、、
？？

[宏Lee]

你有2个问题，内部类不能被new，类名.属性，是要加上static的属性才行

[宏Lee]

内部类就相当于你的属性，加上static才能new

[MiceRice]

除了1楼的问题外，你后面的输出也有问题，不能直接用类来试图访问成员变量，因该是：

System.out.println("圆中心点在" + "(" + c.center.x + "," + c.center.y + ")");
System.out.println("半径为" + c.radius);
System.out.println("圆的面积为" + c.area());

不过一楼解释的不完全准确，是因为main函数本身是static的，所以它引用的内部类也必须是static的；非static函数，可以直接new非static的内部类。

[zhaoct]

public static void main(String[] args) {
		UseClass_1 uc = new UseClass_1();
		UseClass_1.Point p = uc.new Point(7, 8);
		UseClass_1.Circle c = uc.new Circle(p.x, p.y, 9);

		System.out.println("圆中心点在" + "(" + p.x + "," + p.y + ")");
		System.out.println("半径为" + c.radius);
		System.out.println("圆的面积为" + c.area());

	}

[五哥]

/**
*把这个类放到类外面声明
*/
class Point{
int x,y;
public Point(int x,int y){
this.x=x;
this.y=y;
}
}
/**
*把这个类放到类外面声明
*/
class Circle{
Point center=new Point(0,0);
double radius=0;
public Circle(int x,int y,double r){
center.x=x;
center.y=y;
radius=r;
}
public double area(){
return Math.PI*radius*radius;
}
}
public class UseClass_1 {

/**
* @param args
*/

public static void main(String[] args) {
// TODO Auto-generated method stub
Point p =new Point(7,8);
Circle c=new Circle(p.x,p.y,9);

/**
*如果使用类名.方法(),那么你的方法需要定义成为static，即 public static double area();
*但是 area()中使用了radius这个属性也需要定义成为 static，因为静态方法不能访问类中非静态属性和方法
*/
System.out.println("圆中心点在"+"("+p.x+","+p.y+")");
System.out.println("半径为"+c.radius);
System.out.println("圆的面积为"+c.area());

}


}

[nuoop]

public class UseClass_1 {

/**
* @param args
*/
static class Point{
static int x,y;
public Point(int x,int y){
this.x=x;
this.y=y;
}
}
static class Circle{
public static String x;
public static String y;
Point center=new Point(0,0);
static double radius=0;
public Circle(int x,int y,double r){
center.x=x;
center.y=y;
radius=r;
}
public static double area(){
return Math.PI*radius*radius;
}
}
public static void main(String[] args) {
// TODO Auto-generated method stub

Circle c=new Circle(7,8,9);

System.out.println("圆中心点在"+"("+Point.x+","+Point.y+")");
System.out.println("半径为"+Circle.radius);
System.out.println("圆的面积为"+Circle.area());

}

[nuoop]

class Point{
static int x,y;
public Point(int x,int y){
this.x=x;
this.y=y;
}
}
class Circle{


Point center=new Point(0,0);
static double radius=0;
public Circle(int x,int y,double r){
center.x=x;
center.y=y;
radius=r;
}
public static double area(){
return Math.PI*radius*radius;
}
}
public class UseClass_1 {

/**
* @param args
*/

public static void main(String[] args) {
// TODO Auto-generated method stub
Point p=new Point(7,8);
Circle c=new Circle(p.x,p.y,9);

System.out.println("圆中心点在"+"("+Point.x+","+Point.y+")");
System.out.println("半径为"+Circle.radius);
System.out.println("圆的面积为"+Circle.area());

}


}

[sdojqy1122]

public class UseClass_1 {
	class Point{
		int x,y;
		public Point(int x,int y){
			this.x=x;
			this.y=y;
		}
	}
	class Circle{
		Point center=new Point(0,0);
		double radius=0;
		public Circle(int x,int y,double r){
			center.x=x;
			center.y=y;
			radius=r;
		}
		public double area(){
			return Math.PI*radius*radius;
		}
	}
	
	public void run(){
		Point p =new Point(7,8);
		Circle c=new Circle(p.x,p.y,9);
		System.out.println("圆中心点在"+"("+p.x+","+p.y+")");
		System.out.println("半径为"+c.radius);
		System.out.println("圆的面积为"+ c.area());
	}
	public Point newPoint(){
		
		return new Point(7,8);
	}
	public Circle newCircle(){
		Point p=newPoint();
		return new Circle(p.x,p.y,9);
	}
	public static void main(String[] args){
		UseClass_1 user = new UseClass_1();
		user.run();
		Point p = user.newPoint();
		Circle c = user.newCircle();
	}
}

我想纠正一下一楼的观点，内部类可以new出来，只是main方法是静态方法，而且类是UseClass_1对象的，所以必须先有UseClass_1的实例，才可以new出来。