Showing posts with label Inheritance. Show all posts
Showing posts with label Inheritance. Show all posts

Monday, May 9, 2011

Published 5/09/2011 by with 0 comment

interface expected here

The code below leads to an error "interface expected here"

public interface MyInterface extends Thread
{
}

Here an interface is created (MyInterface) and it inherits(extends) the Thread class which is not possible at all. Why? Because an interface can inherit only another interface, not any class. As Thread is a class, it cannot be inherited by any interface.

The code below is correct as the interface inherits another interface Runnable



public interface MyInterface extends Runnable
{
}


A similar type of error is posted at  no interface expected here

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Published 5/09/2011 by with 0 comment

no interface expected here

Any Java code as below will lead an error "no interface expected here"

import java.io.Serializable;
public class Person extends Serializable
{
private String firstName;
private String lastName;
//...
}

See that, here Person is a class (public class Person) and it inherits an interface (extends Serializable). Note that a class can inherit another class only, not any interface. A class can implement one or more interface only. Serializable is an interface, not a class.

Actually the correct code of the above will be:


import java.io.Serializable;
public class Person implements Serializable
{
private String firstName;
private String lastName;
//...
}

Here it is notable that, only an interface can inherit another interface.

A similar type of error is posted at interface expected here
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Sunday, April 10, 2011

Published 4/10/2011 by with 1 comment

call to super must be first statement in constructor

The error message is very clear actually:

i) We can call super in the constructor of a subclass
ii) If we call super in the constructor of a subclass, it must be the first statement in that constructor; i.e, before writing any other statement we call to super must be made.

It is mentionable that, super(parameters) calls the constructor of the super class.

See the code below:

The super class...


public class Employee
{
private int id;
private String name;
public Employee()
{
}
public Employee(int id,String name)
{
this.id = id;
this.name = name;
}
}

The subclass...


public class SalariedEmployee extends Employee
{
private double salary;
public SalariedEmployee()
{
}
public SalariedEmployee(int id,String name,double salary)
{
this.salary = salary; //this is the first statement now
super(id,name); //error : call to super must be first statement in constructor
}
}

The correct subclass will be...


public class SalariedEmployee extends Employee
{
private double salary;
public SalariedEmployee()
{
}
public SalariedEmployee(int id,String name,double salary)
{
super(id,name); // now call to super is the first statement in the constructor
this.salary = salary; // it is now the second statement
}
}



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Friday, August 28, 2009

Published 8/28/2009 by with 0 comment

cyclic inheritance involving ...

public class Person extends Employee
{
private String firstName;
private String lastName;

public Person(String firstName,String lastName)
{
this.firstName=firstName;
this.lastName=lastName;
}

}

Person is a class which inherits Employee and see the code below where Employee also inherit the Class Person.


public class Employee extends Person
{
private double salary;

public Employee(String firstName,String lastName,double salary)
{
super(firstName,lastName);
this.salary=salary;
}
}

In the above code we will see the error message "cyclic inheritance involving Person

public class Person extends Employee"

Actually Person should be the super class here and Employee is the subclass. But here the relationsship is such that Employee is the subclass of Person, again Person is the subclass of Employee. It's a cyclic inheritance which is not possible.

The solution of this problem is identifying the proper relationship i.e, finding out the super class and the subclass. In our case Person is the super class, so it should not try to inherit Employee and Employee is the subclass which will inherit Person. Correction is ommiting "extends Employee" from the Person class signature as below:

public class Person
{
private String firstName;
private String lastName;

public Person(String firstName,String lastName)
{
this.firstName=firstName;
this.lastName=lastName;
}

}

public class Employee extends Person
{
private double salary;

public Employee(String firstName,String lastName,double salary)
{
super(firstName,lastName);
this.salary=salary;
}

}
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