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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
