错误：超级必须是构造函数中的第一条语句

Question

无法弄清楚为什么我收到此错误。 在记事本中工作，通过cmd运行Javac（显然对文件而言更多）。 如果您需要我链接更多，我可以。 我的错误发生在“ setCruiseShip”，我试图将当前链接链接到超类。 错误：对super的调用必须是构造函数中的第一条语句。

class Ship
{ protected String name;
  protected float speed;
  protected int year;

  public Ship() {name=""; speed=year=0;}
  public Ship(String n, float sp, int y) {name=n; speed=sp; year=y;}
  public String getName()  {return name;}
  public float  getSpeed() {return speed;}
  public int    getYear()  {return year;}
  public void   setShip(String n, float sp, int y) {name=n; speed=sp; year=y;}
  public String toString() {return "Ship named "+name+ " with max speed of " 
                           +speed+ " was built in " +year;}
}

class CruiseShip extends Ship
{ protected int maxpass;

  public CruiseShip() {super(); maxpass=0;}
  public CruiseShip(String n, float sp, int y, int mp) {super(n, sp, y);
                                                        maxpass=mp;}
  public int  setMaxpass(int mp) {maxpass=mp;}
  public void setCruiseShip(String n, float sp, int y, int mp){super(n,sp,y);
                                                               maxxpass=mp;}
  public String toString() { return super.toString() +" carries "
                            +maxpass+ "  passengers";}
} 

Answer 1

class Ship没有名为setCruiseShip方法。 您可能想要的是调用setShip方法。 更换

super(n,sp,y);

与

setShip(n,sp,y);

Answer 2

超级构造函数只能从子类的构造函数调用（不能从任何方法调用）。 下面给出了经过修改和编译的程序

class Ship {
    protected String name;
    protected float speed;
    protected int year;

    public Ship() {
        name = "";
        speed = year = 0;
    }

    public Ship(String n, float sp, int y) {
        name = n;
        speed = sp;
        year = y;
    }

    public String getName() {
        return name;
    }

    public float getSpeed() {
        return speed;
    }

    public int getYear() {
        return year;
    }

    public void setShip(String n, float sp, int y) {
        name = n;
        speed = sp;
        year = y;
    }

    public String toString() {
        return "Ship named " + name + " with max speed of " + speed
                + " was built in " + year;
    }
}

class CruiseShip extends Ship {
    protected int maxpass;

    public CruiseShip() {
        super();
        maxpass = 0;
    }

    public CruiseShip(String n, float sp, int y, int mp) {
        super(n, sp, y);
        maxpass = mp;
    }

    public int setMaxpass(int mp) {
        maxpass = mp;
    }

    public void setCruiseShip(String n, float sp, int y, int mp) {
        setShip(n, sp, y);
        maxpass = mp;
    }

    public String toString() {
        return super.toString() + " carries " + maxpass + "  passengers";
    }
}

Answer 3

可以从子类的构造函数中调用super(somevalues...) 。 Java不允许从方法中调用super(somevalues...) 。 同样，当您在子类中定义构造函数时，其第一条语句应该是super()/super(somevalues)构造函数或this(somevalues...) 。 如果不这样做，则编译器会隐式添加默认的no-arg超级构造函数。

this()表示构造函数重载。
给定的事实是

public void setCruiseShip(String n, float sp, int y, int mp){super(n,sp,y);

不是子类的构造函数。

构造函数definition是，构造函数名称必须与没有返回类型的类名称相同（我不是说“应该”），并且允许使用任何访问修饰符（例如private，default，protected，public）。

所以setCruiseShip与CruiseShip