Inheritance

Subclasses and superclasses

Inheritance is a technique that allows one class to be derived from another.
A derived class inherits all of the data and methods from the original class

Example: Suppose that class Y is inherited from class X.

class X is the superclass. Also known as base class or parent class
class Y is the subclass. Also known as the derived class, or child class, or extended class
class Y consists of anything created in class Y, as well as everything from class X, which it inherits

Declaring a subclass

Use the keyword extends to declare the derived class

  // Example 1
  public class AAA		// AAA is the base class
  { ... }

  public class BBB extends AAA  // BBB is the derived class
  { ... }


  // Example 2
  public class Employee {...}				// base class
  public class HourlyEmployee extends Employee { ... }  // derived

The keyword `super`

When you create a derived object, the derived class constructor needs to invoke the base class constructor

Do this with the keyword super -- in this context, it acts as the call to the base class constructor

  super();		// invokes base class default constructor
  super(parameters);    // invokes base class constructor with parameters

  // Example, for a class called HourlyEmployee, derived from Employee
  public class HourlyEmployee extends Employee
  {
    public HourlyEmployee()	// default constructor
    {
	super();		// invokes Employee() constructor
    }

    public HourlyEmployee(double h, double r)
    {
	super(h,r);		// invokes Employee constructor w/ 2 parameters
    }
    
    // ... more methods and data

  } // end class HourlyEmployee

The call to super() must be the first line of the derived class constructor
If explicit call to parent constructor not made, the subclass' constructor will automatically invoke super(). (the default constructor of the base class, if there is one)
Can also use super to invoke a method from the parent class (from inside the derived class). Format:
```
  super.method(parameters)
```

The `protected` modifier

Recall that public data and methods can be accessed by anyone, and private data and methods can be accessed only by the class they are in.
protected data and methods of a public class can be accessed by any classes derived from the given class (this is also true in C++)
In Java, a protected member can also be accessed by any class in the same package (to be discussed later)

The `final` modifier

In addition to creating constant variable identifiers, the keyword final can be used for a couple of special purposes involving inheritance:

When used on a class declaration, it means that the class cannot be extended. (i.e. it cannot become a parent class to a new subclass)
When used on a method declaration, it means that the method cannot be overridden in a subclass. (i.e. this is the final version of the method)

Method Overriding

Although the derived class inherits all the methods from the base class, it is still possible to create a method in the derived class with the same signature as one in the base. Example:

Suppose a class Rectangle is derived from class Shape.
Shape has a method:
```
  void Draw() { ... }  
```

We can define a method in class Rectangle with the same signature. The derived class version will override the base class version, when called through an object of type Rectangle.

  Rectangle r = new Rectangle();  // create a Rectangle object
				  //  which has all the Shape methods available

  r.Draw();			  // invokes the Draw method from the
				  //  Rectangle class

Note that the Rectangle class' Draw() method can still invoke the superclass' method, with the keyword super

  public void Draw()
  {
    super.Draw();		// invoke parent's Draw()

    // continue with any processing specific to Rectangle
  }

In-class Example

Here's a link to the code example we wrote up in class. Base class is Shape, and Circle is derived from Shape. The Tester.java file contains some sample calls

Shapes

Abstract Classes

Superclasses are more general and subclasses are more specific.
Sometimes a base class is so general that it doesn't make sense to actually instantiate it (i.e. create an object from it).
- Such a class is primarily a grouping place for common data and behaviors of subclasses -- an abstract class.
To make a class abstract, use the keyword abstract (which is a modifier)
```
  public abstract class Shape
```

Now that Shape is abstract, this would be illegal:

  Shape s = new Shape();        // specifically, it's  new Shape();
                                //   that is illegal

Methods can be abstract as well:

An abstract method is a method signature without a definition
Abstract methods can only be created inside abstract classes
The main purpose of an abstract method is to be overridden in derived classes (with the same signature)

Example:

  public abstract class Shape		// Shape is an abstract class
  {
    public abstract double findArea();	// findArea is an abstract method

    // other methods and data
  }

The `Object` class

In Java, every class is derived automatiacally from a class called Object. If no specific inheritance is declared for a class, it automatically has Object as a superclass.

While there are several methods in class Object, here are three important such methods, inherited by every Java class

public boolean equals(Object object)
public String toString()
public Object clone()

Let's look at each.

`public boolean equals(Object object)`

tests whether two objects are equal

  object1.equals(object2)	// returns true if equal, false if not
				// (object1 and object2 same class type)

Default implementation is:

  public boolean equals(Object obj)
  {
	return (this == obj);
  }

Note that this default implementation is equivalent to the == operator, since it only tests the reference variables for equality. The intent is that subclasses of Object should override the equals method whenever they want a test of equality of two objects' contents.

`public String toString()`

returns a string that represents the object. Call format:

  objectName.toString();

The default version of the string might not always be useful, but this can be overridden in any derived class. Example for a class called Fraction:

  public String toString()
  {
    return numerator + "/" + denominator;
  }

Assuming the above function for a Fraction class, the following illustrates its usage:

  Fraction f1 = new Fraction(4,5);	// create the fraction 4/5
  System.out.print(f1.toString());	// will print "4/5"

  System.out.print(f1);		// also prints "4/5" as this
				//  always invokes a class' toString method

Examples:

Example of default operation of equals and toString -- This uses the Fraction class from a previous notes set
Example of overriding equals and toString -- in the Fraction class

`public Object clone()`

Remember, direct assignment between object names will only copy one reference variable to another. Use the clone() method to make copies of objects.

  newObject = someObject.clone();

Not all objects can be cloned. Only objects imeplementing the java.lang.Cloneable interface (which will be discussed later) can use the clone method.

The clone() method from the object class does a "shallow copy" (i.e. copies reference variables verbatim). If a "deep copy" is needed (a la copy constructors in C++), you should override clone() for a class.

Other methods from class `Object`

finalize -- called by garbage collector to perform to perform cleanup on an object. Can be overridden, but rarely done
getClass -- returns an object of type Class, with information about the calling object's type
hashCode -- returns hash value that can be used as a key for the object (for use in a hash table, for example)
notify, notifyAll, wait -- related to multithreading

Inheritance

Subclasses and superclasses

Declaring a subclass

The keyword super

The protected modifier

The final modifier

Method Overriding

In-class Example

Abstract Classes

The Object class

public boolean equals(Object object)

public String toString()

public Object clone()

Other methods from class Object