方法不会从父类型@override覆盖或实现方法编译错误

Question

I'm getting an error method does not override or implement a method from a supertype @Override . I want to print "cannot change capacity of a car" after it prints one car. I need to override the setCapacity to print this other part. I believe the code is mostly correct, just not sure why it's not correctly override the setCapacity method. The final output is:

New capacity = 1600
Vehicle Info:
capacity = 1600cc
make = Mazda
Cannot change capacity of a car
Vehicle Info:
capacity = 1200cc
make = Holden
type = sedan
model = Barina

My code is:

class Vehicle {  // base class

   public void setCapacity(int setCapacity) {
     this.capacity = setCapacity;
      System.out.println("New Capacity = " + setCapacity);
   }

   int capacity;
   String make;

   Vehicle(int theCapacity, String theMake) {
      capacity = theCapacity;
      make = theMake;
   }

   void print() {
      System.out.println("Vehicle Info:");
      System.out.println("  capacity = " + capacity + "cc" );
      System.out.println("  make = " + make );
   }
}

class Car extends Vehicle {
   public String type;
   public String model;

   public Car(int theCapacity, String theMake, String theType, String theModel) {
      super(theCapacity, theMake);
      type = theType;
      model = theModel;
   }

   @Override
   public void print() {
      super.print();
      System.out.println("  type = " + type);
      System.out.println("  model = " + model);

   }

     @Override
     public void setCapacity() {
       super.print();
       System.out.println("Cannot change capacity of a car");
     }       
 }

class Task3 {

   public static void main (String[]args){
      Car car1 = new Car (1200,"Holden","sedan","Barina" );
      Vehicle v1 = new Vehicle (1500,"Mazda");
      v1.setCapacity(1600);
      v1.print();
      car1.setCapacity(1600);
      car1.print();
   }
}

Answer 1

There is a mismatch in the child and parents method signature of setCapacity() . If you want to override a method from the parent class in the child class then it must have the same signature.

Change

public void setCapacity() { //... }

to

public void setCapacity(int setCapacity) { // ... }

in the Car class.

In your code you have missed the parameter setCapacity and thus the compiler complains.

Answer 2

The void setCapacity(int setCapacity) is not overriden. void setCapacity() and void setCapacity(int setCapacity) are two different methods. Therefore generates the @Override annotation the compile error.

Regarding the terminology, the setCapacity is said to be overloaded in this scenario.

Answer 3

The signature of createCapacity is different in Vehicle and Car classes. So, there is a compilation error. In Vehicle class you have an argument setCapacity but in the Car class the argument list to the method is empty. So, overriding is not possible.

@Override
public void setCapacity(   int capacity   ) { --> **adding this argument here will fix the issue.**
    super.print();
    System.out.println("Cannot change capacity of a car");
}

public void setCapacity(int setCapacity) {
    this.capacity = setCapacity;
    System.out.println("New Capacity = " + setCapacity);
}