1. Class Inheritance (Cont.)

2. Keyword: this Use this to refer to the constructor
Use this to refer to the current object

3. Keyword: this Example 1: Use this to refer to the constructor
public class MyInteger {
private int value;
public MyInteger(int init)
{ value = init;}
public MyInteger()
{ this(0);}
... }

4. Keyword: this Example 2: Use this to refer to the current object
public class MyInteger {
private int value;
public MyInteger(int value)
{ this.value = value;}
public MyInteger()
{ this(0);}
... }

5. Keyword: super
The constructor of the parent class is usually implicitly called
However, what if the constructor of the parent class has formal parameters?

6. Keyword: super MyChildInteger integer = new integer (2);
public class MyParentInteger {
private int value;
public MyParentInteger(int value)
{this.value = value;}
public MyParentInteger()
{this(0);}
...//some other method definitions }
public class MyChildInteger
extends MyParentInteger {
private int value2;
public MyChildInteger(int value2)
{this.value2 = value2;}
public MyChildInteger()
{this(0);}
...//some other method definitions }
MyChildInteger integer = new integer (2);
what is the value of integer.value? How to give it a different value??

7. Keyword: super public class MyParentInteger { private int value;
public MyParentInteger(int value)
{this.value = value;}
public MyParentInteger()
{this(0);}
...//some other method definitions }
public class MyChildInteger
extends MyParentInteger {
private int value2;
public MyChildInteger(int value, int value2)
super(value);
this.value2 = value2; }
public MyChildInteger()
{this(0);}
...//some other method definitions

8. Keyword: super public class MyParentInteger { private int value;
public MyParentInteger(int value)
{this.value = value;}
public MyParentInteger()
{this(0);}
...//some other method definitions }
public class MyChildInteger
extends MyParentInteger {
private int value2;
public MyChildInteger(int value, int value2)
super(value);
this.value2 = value2; }
public MyChildInteger()
{this(0);}
...//some other method definitions

9. Abstract Classes

10. Abstract Classes Java allows abstract classes
use the modifier abstract on a class header to declare an abstract class abstract class Vehicle { … }
An abstract class is a placeholder in a class hierarchy that represents a generic concept
Vehicle
Car
Boat
Plane

11. Abstract Class: Example
An abstract class often contains abstract methods, though it doesn’t have to
Abstract methods consist of only methods declarations, without any method body
public abstract class Vehicle
{ String name;
public String getName()
{ return name; } \\ method body
abstract public void move();
\\ no body! }

12. Abstract Classes
An abstract class often contains abstract methods, though it doesn’t have to
Abstract methods consist of only methods declarations, without any method body
The non-abstract child of an abstract class must override the abstract methods of the parent
An abstract class cannot be instantiated
(why?)
The use of abstract classes is a design decision; it helps us establish common elements in a class that is too general to instantiate