我正在制作一个学生数据库应用程序，它有不同类型的学生来练习继承并体验其他OO原则。
我有一些课程继承自超级学生＆＃34;类。问题是我无法让子类重载构造函数。
Netbeans显然可以使用Student构造函数，但它在子类中告诉我，在Superclass中找不到合适的构造函数。仅此一点我就完全糊涂了。
到目前为止，这是代码：
class MyDB {
public abstract class Student {
//Variable declaration
//
public String firstName;
public String lastName;
public int studentID;
public float gpa;
public String status;
public String mentor;
//Constructor & overloaded constructors
//
public Student(String fn, String ln, int ID, float sgpa, String stat, String m) {
firstName = fn;
lastName = ln;
studentID = ID;
gpa = sgpa;
status = stat;
mentor = m;
}
public Student(String fn, String ln) {
this(fn, ln, 0, 0, "", "");
}
//Class Methods
//
public abstract float calculateTuition(String status, float numberOfCreditHours);
//Tuition is Calculated
/*
if(status == "resident") {
if(undergraduate) {
if(tuition) {
return numberOfCreditHours * 200;
} else {
return numberOfCreditHours * 250;
}
} else {
return numberOfCreditHours * 300;
}
} else {
if(undergraduate) {
if(tuition) {
return numberOfCreditHours * 400;
} else {
return numberOfCreditHours * 450;
}
} else {
return numberOfCreditHours * 350;
}
}
*/
public abstract void update();
//Database entry is updated
public abstract void add();
//Database entry for student is created
public abstract void delete();
//Database entry for student is deleted
public abstract void query();
//Query to DB is made, then info is returned to the screen
}
public class Undergraduate extends Student {
private String level;
private final String m;
public String getLevel() {
return level;
}
public void setLevel(String newLevel) {
level = newLevel;
}
//Overloaded Constructor
public Undergraduate(String newlevel, String fn, String ln, int ID, float sgpa, String stat, String m) {
}
@Override
public float calculateTuition(String status, float numberOfCreditHours) {
throw new UnsupportedOperationException("Not supported yet."); //To change body of generated methods, choose Tools | Templates.
}
@Override
public void update() {
throw new UnsupportedOperationException("Not supported yet."); //To change body of generated methods, choose Tools | Templates.
}
@Override
public void add() {
throw new UnsupportedOperationException("Not supported yet."); //To change body of generated methods, choose Tools | Templates.
}
@Override
public void delete() {
throw new UnsupportedOperationException("Not supported yet."); //To change body of generated methods, choose Tools | Templates.
}
@Override
public void query() {
throw new UnsupportedOperationException("Not supported yet."); //To change body of generated methods, choose Tools | Templates.
}
}
public class Graduate extends Student {
private String thesisTitle;
private String thesisAdvisor;
public String getThesisTitle() {
return thesisTitle;
}
public String getThesisAdvisor() {
return thesisAdvisor;
}
public void setThesisTitle(String newThesisTitle) {
thesisTitle = newThesisTitle;
}
public void setThesisAdvisor(String newThesisAdvisor) {
thesisAdvisor = newThesisAdvisor;
}
//Overloaded Constructor
@Override
public float calculateTuition(String status, float numberOfCreditHours) {
throw new UnsupportedOperationException("Not supported yet."); //To change body of generated methods, choose Tools | Templates.
}
@Override
public void update() {
throw new UnsupportedOperationException("Not supported yet."); //To change body of generated methods, choose Tools | Templates.
}
@Override
public void add() {
throw new UnsupportedOperationException("Not supported yet."); //To change body of generated methods, choose Tools | Templates.
}
@Override
public void delete() {
throw new UnsupportedOperationException("Not supported yet."); //To change body of generated methods, choose Tools | Templates.
}
@Override
public void query() {
throw new UnsupportedOperationException("Not supported yet."); //To change body of generated methods, choose Tools | Templates.
}
}
public class PartTime extends Student {
private String company;
//Name of sponsoring employer
public String getCompany() {
return company;
}
public void setCompany(String newCompany) {
company = newCompany;
}
//Overloaded Constructor
@Override
public float calculateTuition(String status, float numberOfCreditHours) {
throw new UnsupportedOperationException("Not supported yet."); //To change body of generated methods, choose Tools | Templates.
}
@Override
public void update() {
throw new UnsupportedOperationException("Not supported yet."); //To change body of generated methods, choose Tools | Templates.
}
@Override
public void add() {
throw new UnsupportedOperationException("Not supported yet."); //To change body of generated methods, choose Tools | Templates.
}
@Override
public void delete() {
throw new UnsupportedOperationException("Not supported yet."); //To change body of generated methods, choose Tools | Templates.
}
@Override
public void query() {
throw new UnsupportedOperationException("Not supported yet."); //To change body of generated methods, choose Tools | Templates.
}
}
public static void main(String[] args) {
}
}
请忽略覆盖语句。他们看起来很乱，因为我让netbeans auto做任何想要摆脱错误的事情。
应用程序还有很多工作要做，但目前问题在于构造函数。
我检查过的教程和问题通常涉及同一类的重写构造函数，或者不会覆盖构造函数的子类。
答案 0 :(得分：2)
您的子类必须调用其父构造函数之一。当类具有默认（无参数）构造函数时，这是隐式完成的。由于Student没有no-arg构造函数，因此从它扩展的任何内容必须在其构造函数中调用其构造函数之一，例如
public class Undergraduate extends Student {
public Undergraduate() {
super(null, null);
}
public Undergraduate(String fn, String ln) {
super(fn, ln);
}
//...
}
否则，只需向Student添加默认构造函数：
protected Student() {
this(null, null);
}