CS1021 - More Inheritance Review

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class: taylor msoe
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.title[
  # More Inheritance Review
  ## Abstract Classes/Methods, Casting References, Visibility Modifiers, Interfaces
]

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# Abstract
* If a class is declared abstract, cannot create objects from it

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* E.g., `public abstract Shape {`

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* A class **must** be declared abstract if it has one or more abstract methods

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* An abstract method is declared but not implemented

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* E.g., `public abstract void draw();`

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* A subclass of an abstract class must either:
    + be declared abstract, or
    + implement all inherited abstract methods

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# Motivation for Abstract Classes
* Provide common functionality for subclasses (code reuse)

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* Use general reference to point to specific object type
    + Reveal common behavior even if the behavior varies across subclasses
    + E.g., all shapes can be drawn, but implementation depends on specific type

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# Casting References
* Java will do an implicit type cast from subclass reference to superclass reference, e.g.,
```
Shape shape = ___;
Circle circle = ___;
shape = circle;
```
--
    + Implicit cast from `Circle` to `Shape` 
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* Must explicitly cast from superclass to subclass
```
circle = (Circle)shape;
```
--
* If the `shape` reference does not refer to a `Circle` object, a `ClassCastException` will be thrown at runtime

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# Visibility Modifiers
* Every attribute and method has a visibility level associated with it
    + `public` - All classes have access [UML: **`+`**]
    + `private` - Only the class has access [UML: **`-`**]
--
    + package - Only classes in the same package have access [UML: **`~`**]
--
    + `protected` - Only subclasses and classes in the same package have access [UML: **`#`**]

--
```
public int publicVisibility;       // Most visible
protected int protectedVisibility;
int packageVisibility;
private int privateVisibility;     // Least visible
```

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# Interfaces
* An interface advertises functionality but doesn't provide an implementation

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* Cannot create objects from an interface

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* An interface reference can point to any object from a class that implements the interface

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* Here too, the reference determines what methods can be called

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* The implementing class **can-do** what the interface claims for functionality

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# Inteface Example
* [`CharSequence`](https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/CharSequence.html) - a readable sequence of `char` values

```
public interface CharSequence {
    char charAt(int index);
    int length();
    CharSequence subSequence(int start, int end);
    String toString();
}
```
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* Methods are `public` by default

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* The `String` class implements the `CharSequence` interface
```
public class String implements CharSequence {
```
--
    + This is a simplified declaration of the `String` class

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# Intefaces Continued...
* Other classes also implement the `CharSequence` interface: `StringBuffer`, `StringBuilder`, `CharBuffer`, `Segment`

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* Whenever a `CharSequence` is needed, an object from any of the implementing classes will work

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* Summary:
    + Inheritance describes an **is-a** relationship
    + A class that implements an interface has a **can-do** relationship