package javaapplication1;
class gouwu{
String name;
int danjia;
int shuliang;
int sum;
public void fff(int danjia,int shuliang, String name){
this.danjia=danjia;this.shuliang=shuliang;this.name=name;
}
public void gouwu(gouwu a[]){
for (int i=0;i<a.length;i++)
{
sum=sum+a[i].danjia*a[i].shuliang;
}
System.out.print(sum);
}
}

public class Main {
    public static void main(String[] args) {
  gouwu []a=new gouwu[3];
  
  a[0].fff(1, 1, "a");
  a[1].fff(2,2,"b");
  a[2].fff(3, 3, "c");
  gouwu b=new gouwu();
  b.gouwu(a);
}

}
 
 
a[0].fff(1, 1, "a");
  a[1].fff(2,2,"b");
说是这里有问题

你没有创建对象，空指针异常

package javaapplication1;

class Gouwu { //类名首字母要大写
    String name;
    int danjia;
    int shuliang;
    int sum;

    public void fff(int danjia, int shuliang, String name) {
        this.danjia = danjia;
        this.shuliang = shuliang;
        this.name = name;
    }

    public void gouwu(Gouwu a[]) {
        for (int i = 0; i < a.length; i++) {
            sum = sum + a[i].danjia * a[i].shuliang;
        }
        System.out.print(sum);
    }
}

public class Main {
    public static void main(String[] args) {
        Gouwu[] a = new Gouwu[3];
        a[0] = new Gouwu();//创建对象
        a[1] = new Gouwu();//创建对象
        a[2] = new Gouwu();//创建对象


        a[0].fff(1, 1, "a");
        a[1].fff(2, 2, "b");
        a[2].fff(3, 3, "c");
        Gouwu b = new Gouwu();
        b.gouwu(a);
    }

}

 

gouwu []a=new gouwu[3];
数组元素为空。相当于创建String[] s=new String[3]而没有赋值，所为应该这样写：
        gouwu [] a = new gouwu [3];
        a[0]=new gouwu();
        a[1]=new gouwu();
        a[2]=new gouwu();
        a[0].fff(1, 1, "a");
        a[1].fff(2, 2, "b");
        a[2].fff(3, 3, "c");
这样每个数组元素中就存放了一个对象，

谢谢

你的代码有很不规范，建议你参看本版置顶的《JAVA代码规范》

类名首字母要大写
构造函数名与类名一样首字母要大写
其他方法名首字母小写

学习，学习

变量名头单不用大写，以后的要大写