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Java Classes and

Objects

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Object Oriented Terms

Abstraction

Before abstraction: Porche, Pinto, Semi,Motorcycle

After abstraction: Vehicle

Encapsulation

All vehicles have: number of wheels, color,

start(), accelerate(), decelerate(), turnOff()

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Object Oriented Terms

Inheritance

Polymorphism Method Overloading

start(Key key)

start (Screwdriver s, Hammer, h)

The list of input variables must be unique for eachoverloaded method

Method Overriding For vehicle: start() { engineStatus = ON }

For bicycle: start() { throw new DoesNotStartEx()}

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Access Specifiers/Modifiers

Private, Protected, Public, (none)

Accessed By Inherited?

Private

this class only NoProtected

this class only Yes

Public All Yes

Package Yes

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Returning values from Methods

If your method returns something, use return ;

If your method returns nothing (void),

you can use return alone. return;

Note: only use this when you want to exit themethod early. No need to put it at the end!

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Constructors

No return type

If a class does not have a constructordefined, the compiler automatically adds a

default constructor with no input values.

Constructors are NOT inherited!

If you wish to inherit a constructor, you mustwrite out the signature and call the parentsconstructor using the super keyword

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Variable Scope

{} define the scope of a variable When using catch, for, and while,

remember that items declared within the

catch, for, or while are only in scopewithin the following {}{

Try { //ex out of scope}

Catch (Exception ex) {// ex in scope}Finally { // ex out of scope }

//ex out of scope

}

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The Stack and the Heap

The stack is an orderly storage of datathat is currently in scope

The heap is an disorderly storage of datathat may or may not be in scope

Primitive data types exit on the stack

Objects exist in the heap, but if they arein scope, pointer(s) to them exist in thestack

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The stack

Once popped from the stack, the

variables are out of scope

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Garbage Collection

Cleans the heap

Done sporadically by JRE after an object is outof the scope of the stack

finalize() method of an object is called beforethe memory occupied by that object is released

System.gc() makes a request that garbage

collection take place, but there is no guaranteethat it will happen

Setting reference variables to null marks theassociated object for garbage collection

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Passing Data To Methods

If you call a method with a primitivevariable, a copy of it is made on the stack,so changing it in the method has no effect

on your copy.

If you call a method with an object, youpass a reference, so any modifications to

the object are done to the same data usedby your pointer.

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The return ofreturn

In addition to modifying classes that arepassed into a method, you can return areference to an item created in the

method with the return statement When calling the method you can:

Use the return value: Dog dog =Kennel.getDog()

Use it directly: Kennel.getDog().speak();

Disregard it: Kennel.getDog()

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Building your own packages

Normally, your projects will include morethan a single class. Therefore, it is best toorganize them in packages

Create a folder hierarchy that makessense with classes in each folder

The package keyword is used on the first

line (above import statements!) of eachclass to define its package structure to beused in import statements

package com.mypackage;

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-classpath parameter

When compiling and running classes that useother classes, the compiler and JRE need toknow the location of the binaries of the classes

in your import clauses. javac -classpath . MyClass.java

java -classpath . MyClass

When using multiple packages, you must

do it from the root root by package

Javac classpath mypackage/* *.java

Java classpath mypackage/* ClsName

http://java.sun.com/j2se/1.4.2/docs/tooldocs/windows/classpath.html