import java.util.*;
class Rational{
      double x,y;
      Rational(double x,double y){
              this.x=x;
              this.y=y;
      }
      double computerArea(){
             return (x/y);
      }     
}
public class Ex4_4{
       public static void main(String args[]){
              double area1=0,area2=0;
              Rational num1,num2;
              num1=new Rational();
              num2=new Rational();
              Scanner reader=new Scanner(System.in);
              System.out.println("输入分子：");
              double num1.x=reader.nextDouble();
              double num2.x=reader.nextDouble();
              System.out.println("输入分母：");
              double num1.y=reader.nextDouble();
              double num2.y=reader.nextDouble();
              area1=();
              area2=();
              System.out.println("分数的加法运算：area1+area2="+(area1+area2));
              System.out.println("分数的减法运算：area1-area2="+(area1-area2));
              System.out.println("分数的乘法运算：area1*area2="+(area1*area2));
              System.out.println("分数的除法运算：area1/area2="+(area1/area2));
       }
}
      

这个程序的：
              double num1.x=reader.nextDouble();
              double num2.x=reader.nextDouble();
              System.out.println("输入分母：");
              double num1.y=reader.nextDouble();
              double num2.y=reader.nextDouble();
有错误，但是不明白愿意。希望能说一下，谢谢！

根据你的第一种方法，我把程序改成了：
import java.util.*;
public class Ex4_4{
       public static void main(String args[]){
              double area1=0,area2=0;
            
              Scanner reader=new Scanner(System.in);
              System.out.println("输入第一个数的分子：");
              double x1=reader.nextDouble();
              
              System.out.println("输入第二个数的分子：");
              double x2=reader.nextDouble();

              System.out.println("输入第一个数分母：");
              double y1=reader.nextDouble();
              
              System.out.println("输入第二个数分母：");
              double y2=reader.nextDouble();
      
              area1=x1/y1;
              area2=x2/y2;
              System.out.println("分数的加法运算：area1+area2="+(area1+area2));
              System.out.println("分数的减法运算：area1-area2="+(area1-area2));
              System.out.println("分数的乘法运算：area1*area2="+(area1*area2));
              System.out.println("分数的除法运算：area1/area2="+(area1/area2));
       }
}