class-03.md

Maps, primitives, File I/O

Primitives vs. Objects

• in java I can create object corresponding to each primitve datatype.

• for example int, char double are primitives but i can declare them as objects using Integer, Double Character

• The wrapper class allow me to create these corresponding objects.

• I need them because sometimes.

• To consider working with objects I have 2 things to put in mind:

  • The amount of memory size i have to work with.
  • The application performance I am trying to achieve.

• Primitives impact on memory.

  Type	Primitive
  int	16 bit
  char	8 bit
  short	16 bit
  long	32 bit
  double	32 bit
  float	16 bit
  boolean	16 bit
  No reference i built it myself

memory and Performance

• Variables of these types live in the stack therefore they are accessed fast in memory.
• The reference types are objects, they live on something called heap in memory and it's reference variable in stack and are relatively slow to access.
• this will consume huge memory thus will make proccess slower.

Default Values primitive vs Wrapper Class

• all wrapper objects default is null value.
• as for primitives, each type will contain default value when it's declared. e.g. int = 0,char \u0000 etc..
• as for wrappers variables they are initialized to null.

Usage

• because primitives are much faster we should use them always, but there are cases i will need to use wrappers.
• Java langauge does not allow using primitves on collections.

Exceptions

• Java use expections to handle errors.
• Exceptions in java are events occur during the execution of a program and interrupt the execution and crash the application.
• throwing exception is when a method run and error happen, it will create an object (called exception object) contain information about the event and then send it to the runtime system.
• Example from java documentaion: .
• The runtime search for method handler to handle error, when it find it, it invoke a method which is the exception handler, then give it the information of error.
• exception handler method handler a block of code handle the exception ( catch the exception).
• Java documentation on handling error .

try and catch

• a code that throw exception should be in try and catch statement.
• the code should be in the try block and it catch the error and deliver it to exception.
• the catch block receive an object from Exception class will tell the runtime system what to do with the error.
• if catch fail to deal with the error it will throw error thus app crash.
• catch(Class object){} in params section i either specifiy object from specific class like NullPointerException or i use (Execption excep) which is the general class
• the Final block contain a statement should be executed wherether the error occurs or not.

Kinds of Exceptions

• checked exception: when user try to open file that does not exist it will throw me error:
  • java.io.FileNotFoundException.
• second kind of exception is the error.
  • The unsuccessful read will throw java.io.IOError.
• runtime exception: application usually cannot catch them, indicate programming bugs, such as logic errors or improper use of an API.
  • If a logic error causes a null to be passed to the constructor, the constructor will throw NullPointerException.
  • try{}catch(){}final{} Check Java Documentation to see how they work as explain above

Advantages of Exceptions

1. Separating Error-Handling Code from "Regular" Code.
2. Propagating Errors Up the Call Stack.
3. Grouping and Differentiating Error Types. Java Documentation.

Scanner

• format inputs and translate them to tokens, then translate token according to it's type.
• I use object scanner to take input from user.
• to use it first i have to include it.
  • import java.utl.Scanner;
  • Scanner scan = new Scanner();
• Scanner support all datatypes expect for char, bigIntger, BigDecimal.