public class Name {
            
    public static void main(String[] args) {
              Yuan yuan1=new Yuan();
        System.out.println("半径为"+yuan1.Banjing +"的圆的面积是："+yuan1.Mianji());
        }
}
         class Yuan{
             double Banjing;
            void banjing(){
                 Banjing=4.5;
             }
            void banjing(double newBanjing){
                Banjing=newBanjing;
            }
             double Mianji(){
                return Banjing*Banjing*Math.PI;
         }
}
执行结果显示如下：

运行不出与你期待的结果就是代码不正确

//1,在Yuan内定义了一个全局变量double Banjing，并没有直接赋值
//2,在banjing方法中赋值后，需在调用的时候调用一下这个方法，如systemout语句中的，如下代码：
public class Name {
     public static void main(String[] args) {
           Yuan yuan1=new Yuan();
           System.out.println("半径为"+yuan1.Banjing() +"的圆的面积是："+yuan1.Mianji());
         }
 }
 class Yuan{
           double Banjing;
           void banjing(){
                  Banjing=4.5;
           }
           void banjing(double newBanjing){
                 Banjing=newBanjing;
           }
           double Mianji(){
                 return Banjing*Banjing*Math.PI;
           }
 }

void banjing(){
                  Banjing=4.5;
           }
只是一个函数，你并没有调用给半径赋值，所以r一直是0，故输出0，你在main中调用下就好了，或者定义一个构造函数进行对r初始化

你的这个程序在 yuan1.Banjing()的地方提示有错误啊，是怎么回事？

//不好意思，yuan1.Banjing应写在system语句上面,system语句内写yuan1.Banjing 就ok了
public class Name {
      public static void main(String[] args) {
            Yuan yuan1=new Yuan();
            yuan1.banjing();
            System.out.println("半径为"+yuan1.Banjing +"的圆的面积是："+yuan1.Mianji());
          }
 }
 class Yuan{
            double Banjing;
            void banjing(){
                   Banjing=4.5;
            }
            void banjing(double newBanjing){
                  Banjing=newBanjing;
            }
            double Mianji(){
                  return Banjing*Banjing*Math.PI;
            }
 }

[ 本帖最后由 dzh151360980 于 2014-11-19 05:11 编辑 ]