X++ inheritance

Article
08/12/2022

This article describes inheritance in X++, including how to create a subclass and override a method.

Creating a subclass

Subclasses are classes that extend or inherit from other classes. A class can extend only one other class. Multiple inheritance isn't supported. If you extend a class, the subclass inherits all the methods and variables in the parent class (the superclass). Subclasses let you reuse existing code for a more specific purpose. Therefore, they help save you time during design, development, and testing. To customize the behavior of a superclass, override the methods in a subclass. A superclass is often known as a base class, and a subclass is often known as a derived class.

Subclass example

The following example first creates a class that is named Point. It then extends the Point class to create a new class that is named ThreePoint.

class Point
{
    // Instance fields.
    real x; 
    real y; 

    // Constructor to initialize fields x and y.
    void new(real _x, real _y)
    { 
        x = _x;
        y = _y;
    }
}

class ThreePoint extends Point
{
    // Additional instance fields z. Fields x and y are inherited.
    real z; 

    // Constructor is overridden to initialize z.
    void new(real _x, real _y, real _z)
    {
        // Initialize the fields.
        super(_x, _y); 
        z = _z;
    }
}

Preventing class inheritance

You can prevent classes from being inherited by using the final modifier.

public final class Attribute
{
    int objectField;
}

Overriding a method

The methods in a class are inherited by any class that extends the class. To change the functionality of an inherited method, you create a method in the subclass, and then give that method the same name and parameters as the method in the superclass. This process is known as overriding the method.

When you instantiate the subclass, you can assign the reference to either a variable of the superclass type or the subclass type. Regardless of the type of the variable, the overridden method is called.

In the following code example, the subclass overrides the write method. Two variables, both of type Point are created. One is assigned a Point object, the other is assigned a ThreePoint object. When the write method is called on the ThreePoint object, the ThreePoint version of the method is called.

class Point
{
    // Instance fields.
    real x;
    real y;

    // Constructor to initialize fields x and y.
    void new(real _x, real _y)
    {
        x = _x;
        y = _y;
    }

    void write()
    {
        info("(" + any2Str(x) + ", " + any2Str(y) + ")");
    }
}

class ThreePoint extends Point
{
    // Additional instance fields z. Fields x and y are inherited.
    real z;

    // Constructor is overridden to initialize z.
    void new(real _x, real _y, real _z)
    {
        // Initialize the fields.
        super(_x, _y);
        z = _z;
    }

    void write()
    {
        info("(" + any2Str(x) + ", " + any2Str(y) + ", " + any2Str(z) + ")");
    }

}

// Code that creates Point objects and calls the write method.
Point point2 = new Point(1.0, 2.0);
Point point3 = new ThreePoint(3.0, 4.0, 5.0);

point2.write();
// Output is "(1.0, 2.0)".

point3.write();
// Output is "(3.0, 4.0, 5.0)".

Preventing method overrides

Static methods can't be overridden, because they exist per class. To protect other sensitive methods, or core methods, from being overridden, use the final modifier. In the following example, because methodAtt is declared as final, it can't be overridden in any class that extends Attribute. You should not specify new or finalize methods as final.

The following example shows how to use the final keyword.

public class Attribute
{
    int objectVariable;

    final void methodAtt()
    {
        //Some statements
    }
}

Overriding vs. overloading

Overriding occurs when the superclass's implementation of a method is changed by the subclass's implementation of that method, but the signatures of both methods are the same.

By contrast, overloading occurs when more than one method has the same name, but the methods have different signatures (return types, parameter lists, or both). X++ supports overriding, but it doesn't support overloading.

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