See: Object-oriented programming (OOP)

Sources: Java All-In One Desk Reference For Dummies, Second Edition

Classes

Notes

• every class implicitly inherits the Object class
• subclasses don’t inherit constructors

Class structure

• class
  • fields
    • (initializers near the fields they initialize)
  • constructors
  • methods
    • (inner classes after the methods that use them)

Instantiating a class

ClassName variableName = new ClassName();

or:

BaseClass variableName = new ChildClass();

(see below under inheritance for explanation on this one)

To inherit/subclass:

public class MyClass extends YourClass {

(see below under inheritance for more information)

Getting the type of an object

OBJECT.getClass().getName()

Note: this will return the class that the variable has been assigned to. For instance, if you declared your variable like this:

Base b1 = new Child();

Then b1.getClass().getName() will return Child.

Testing the type of an object

instanceof OBJECT

Note: this will return true for all classes that the object may be an instance of.

Ex:

Base b1 = new Child();

if (b1 instanceof Base) {
    // this will evaluate to true
}
if (b1 instanceof Child) {
    // this will also evaluate to true
}

Inner classes

Notes

• Inner classes have access to all variables and methods of the outer class, even private variables and methods

Getting an instance of an outer class from an inner class

MyOuterClass.this

Interfaces

Declaring an interface:

public interface Sortable {
    void sort():
}

Implementing an interface:

• Use the implements keyword in your class declaration to implement the interface
• Your class must implement every method declared by the interface

public class MyClass implements Sortable {
    public void sort() {
        // sorting code
    }
}

Generics

Notes:

• Allows you to create generic classes that can be used with other classes without specifying the class type

  Ex:

  ArrayList is a generic container for arrays of any type.

Using a generic class

ArrayList<Employee> empList;

Creating a generic class

public class GenStack<E> {
    private LinkedList<E> list = new LinkedList<E>();

    public void push(E item) {
        list.addFirst(item);
    }

    public E pop() {
        return list.poll();
    }

    public E peek() {
        return list.peek();
    }

    public int size() {
        return list.size();
    }
}

(source: http://media.wiley.com/product_ancillary/12/04701245/DOWNLOAD/Java_AIO_code_files.zip)

Inheritance

Upcasting and downcasting

• java automatically treats instances/objects of child classes like base/parent classes. this is called upcasting

  Ex:

  class Base extends Child {...
  void baseMethod(Base b1) {...
  
  Child b1 = new Child();
  baseMethod(Child);
  

  • upcasting doesn’t drop the methods and variables of the subclass, it just allows the subclass to be used in methods requiring an instance of a base class

• Downcasting can be done only if the object was assigned an instance of the child class

  Ex:

  void childMethod(Child c1) {...
  
  Base b1 = new Child();
  childMethod((Child) b1);
  

Assigning an object of a base type to an instance of a child type

Ex:

BaseClass variableName = new ChildClass();

Notes:

• This creates a variable of type BaseClass, and then assigns (references) an object of type ChildClass to it
• This is common as it allows you to create code that’s based on the base class and more flexible
• variableName is of type BaseClass, but is assigned an object that is type ChildClass
• You can only see members of the BaseClass, but any methods that have been overridden in the ChildClass will be the ones called
• You can use variableName in any method explicitly requiring ChildClass by casting it to ChildClass:

  childClassMethod((ChildClass) variableName);
  

See here for more information: http://stackoverflow.com/q/4447924/399105