关于类和继承关系的运用

为什么代码是错的呀，该怎么去修改它才可以运行出结果？

public class TestApp38 {
        public static void main(String[] args) {
           Student38[] students=new Student38[4] ;
         students[0]=new Student38("1001","张三",60);
         students[1]=new Undergraduate38("1002","李四",90,20); 
         students[2]=new Undergraduate38("1003","王五",80,50); 
         students[3]=new Foreign38("1004","赵六",70,30,"Pakistan"); 
         for(Student38 student : students){ 
         System.out.println(student); 
         } 
         } 
        }


        public static class Student38 {
            private String clsNum;
            private String name;
            private int avgscore;
            private int xuefen;
            private String country;

            public Student38(){
            }

            public Student38(String clsNum,String name,int avgscore,int xuefen,String country){
                this.clsNum=clsNum;
                this.name=name;
                this.avgscore=avgscore;
                this.xuefen=xuefen;
                this.country=country;
            }
       
            public String getClsNum() {
                return clsNum;
            }
            public String getName() {
                return name;
            }
            public int getAvgscore() {
                return avgscore;
            }
            public int getXuefen() {
                return xuefen;
            }
            public String getCountry() {
                return country;
            }

            public String show(){
                return ("clsNum="+clsNum+" name="+name+"  avgscore="+avgscore+" xuefen"+xuefen+"  country="+country);
            }
        }

        public static class Undergraduate38 extends Student38 {
            private String Specialty;

            Undergraduate38(){
            }
            Undergraduate38(String clsNum,String name,int avgscore,int xuefen,String coutry,String Specialty){
                super(clsNum,name,avgscore,xuefen,country);
                this.Specialty=Specialty;
            }
            public String getSpecialty(){
                return Specialty;
            }
            public String show(){
                return(super.show()+"Specialty="+Specialty);
            }
       }
        public static class Foreign38 extends Student38 {
            private String direction;

            Foreign38(){
            }
            Foreign38(String clsNum,String name,int avgscore,int xuefen,String country,String direction){
                super(clsNum, name, avgscore, xuefen, country);
                this.direction=direction;
            }

            public String getDirection(){
                return direction;
            }

            public String show(){
                return (super.show()+"   direction="+direction);
            }
        }
    }

我按照题目要求和你的源码修改了一下，你参考着看一下吧

public class TestApp38 {
    public static void main(String[] args) {
        Student[] students=new Student[4] ;
        students[0]=new Student("1001","张三",60);
        students[1]=new Undergraduate("1002","李四",90,20); 
        students[2]=new Undergraduate("1003","王五",80,50); 
        students[3]=new Graduate("1004","赵六",70,30,"Pakistan"); 
        for(Student student : students){ 
            System.out.println(student); 
        } 
    } 

    public static class Student {
        private String clsNum;
        private String name;
        private int avgscore;

        public Student(){
        }

        public Student(String clsNum,String name,int avgscore){
            this.clsNum=clsNum;
            this.name=name;
            this.avgscore=avgscore;
        }
   
        public String getClsNum() {
            return clsNum;
        }
        public String getName() {
            return name;
        }
        public int getAvgscore() {
            return avgscore;
        }

        public String toString(){
            return ("clsNum="+clsNum+" name="+name+"  avgscore="+avgscore);
        }
    }

    public static class Undergraduate extends Student {
        private int xuefen;

        Undergraduate(String clsNum,String name,int avgscore,int xuefen){
            super(clsNum,name,avgscore);
            this.xuefen=xuefen;
        }
        public int getXuefen(){
            return xuefen;
        }
        public String toString(){
            return(super.toString()+" xuefen="+this.xuefen);
        }
    }
    public static class Graduate extends Student {
        private int xuefen;
        private String country;

        Graduate(){
        }
        Graduate(String clsNum,String name,int avgscore,int xuefen,String country){
            super(clsNum, name, avgscore);
            this.xuefen=xuefen;
            this.country=country;
        }

        public int getXuefen(){
            return xuefen;
        }
        public String getCountry(){
            return this.country;
        }

        public String toString(){
            return (super.toString()+" xuefen="+this.xuefen+" country="+this.country);
        }
    }
}

运行结果:

另外给个建议，做为新手，代码最好是规整一点，好找问题。

System.out.println(student); 改成 System.out.println(student.show());