Online Reading: 03 - Maps, primitives, File I/O

Primitives vs. Objects

Java Type System

• Java has a two-fold type system consisting of primitives and reference type. Primitives correspond to a reference type.Every object contains a single value of the corresponding primitive type.
  • Examples of primative: int, boolean
  • Examples of reference: Integer, Boolean
• Autoboxing - process of converting a primitive type to a reference type
• Unboxing - opposite process of autoboxing
• Primitive type variables live in the stack and can be accessed fast. They have the following impact on the memory:
  • boolean – 1 bit
  • byte – 8 bits
  • short, char – 16 bits
  • int, float – 32 bits
  • long, double – 64 bits
• Reference types are objects that live in the heap and slow to access.
• Default values of the primitive types are 0 for numeric types, false for the boolean type, \u0000 for the char type, and null for the wrapper classes.
  • Primitive types may acquire values only from their domains.
  • Reference types might acquire a value that doesn’t belong to their domains.

Exceptions in Java (read the first three sections on the left: What is an Exception, The Catch or Specify Requirement, Catching and Handling Exceptions)

What is an Exception?

• Exception - is an event that disrupts the normal flow of the program’s instructions during the execution of a program. Exception short hand for exceptional event.
• Exception object - object contains information about the error
• Throwing an exception - creating an exception object and handing it to the runtime system
• Call stack - list of methods
• Exception handler - block of code that can handle and catch the exception The Catch or Specify Requirement
• Catch or Specify Requirement - code that might throw certain exceptions must be enclosed by either a try statement that catches the exception or a method that specifies that it can throw the exception
• 3 kinds of execeptions
• Checked exception - are conditions that a well-written application should anticipate and recover from, and are subject to the Catch or Specify Requirement.
• Error - are conditions that are external to the application, and that the application usually cannot anticipate or recover from, and are not subject to the Catch or Specify Requirement.
• Runtime exception - are exceptional conditions that are internal to the application, and that the application usually cannot anticipate or recover from, and are not subject to the Catch or Specify Requirement.
• Unchecked exceptions - errors and runtime exceptions

How to Throw Exceptions

• Any code can throw an exception.
• Throwable class - All the classes are descendants of it, and all allow programs to differentiate among the various types of exceptions that can occur during the execution of a program.
• throw statement - Requires a single argument called a throwable object, which is a instance of any subclass of the Throwable class.
• Subclasses - error class and execption class

Using Scanner to read in a file in Java

Scanning

• Scanner - an objects of type useful for breaking down formatted input into tokens and translating individual tokens according to their data type.
• Breaking Input into Tokens
  • a scanner uses white space to separate tokens
  • white space characters - blanks, tabs, and line terminators
• Translating Individual Tokens
  • Scanner supports tokens for all of the Java language’s primitive types with the exception of char.
  • Numeric values can use thousands separators.