Skip to content

Latest commit

 

History

History
129 lines (89 loc) · 2.87 KB

lecture02-introduction-to-java.md

File metadata and controls

129 lines (89 loc) · 2.87 KB

Lecture 02 Introduction to Java

"Hello World" explained

  • must use a class even if not doing OO programming
  • main must be public
  • main must be static
  • main must return void
  • main must declare command line arguments even if unused
  • psvm = public static void main

Execution

  • Compile
    • javac HelloWorld.java
  • Launch
    • java HelloWorld

Type System

  • Primitives

    • int 32-bit signed integer
    • long 64-bits signed integer
    • byte 8-bits signed integer
    • short 16-bit signed integer
    • char 16-bit signed integer
    • float 32-bit IEEE 754 floating point number
    • double 64-bit IEEE 754 floating point number
    • Boolean Boolean value: true or false

  • Object Reference Types

    • Have identity distinct from value
    • Some mutable, some immutable
    • On heap, garbase collected
    • Unity of expression with generics
    • More costly

Tips

  • Minimize scope of local variable

  • Initialize variables in declaration

  • Prefer for-each loops to regular for-loops

  • Use common idoms

  • Watch out bas smell of code

Objects

  • a bundle of state and behavior
  • State - fields of the object
  • Behavior - methods

Class

  • Every object has a class
  • Class defines both type and implementation
  • Methods of a classs are its Application Programming Interface (API)

The class hierarchy

  • The root is Object
  • All classes except Object have one parent class
  • A class is an instance of all its superclasses

Implementation inheritance

  • A class
    • Inherits visible fields and methods from its superclasses
    • Can override methods to change their behavior
  • Overriding method implementation must obey contracts of its superclasses
    • Liskov Substitution Principle (LSP)

Interface types

  • Defines a type without an implementation
  • Much more flexible than class types

Enum types

  • Java has object-oriented enums

Boxed primitives

  • Immutable containers for primitive types
  • Boolean, Integer, Short, Long, Character, Float, Double
  • Canonical use case is collections
  • Do not use boxed primitives unless you have to

Comparing values

  • x == y compares the contents of x and y

    • primitive values: returns true if x and y have the same value
    • objects refs: returns true if x and y refer to same object

  • x.equals(y) compares the values of the objects referred to by x and y

    • Always use .equals to compare object refs
    • Except for enums, which are special

Quick 'n' dirty I/O

  • Output
    • Unformatted
    • Formatted

A brief introduction to collections

Primary collection interfaces

  • Collection<-(Set, List, Queue<-(Deque))

  • Map

Primary collection implementations

  • Set - HashSet
  • List - ArrayList
  • Queue - ArrayDeque
  • Deque - ArrayDeque
  • Stack - ArrayDeque
  • Map - HashMap

Other noteworthy collection implementations

  • Set - LinkedHashSet/TreeSet/EnumSet
  • Queue - PriorityQueue
  • Map - LinkedHashMap/TreeMap/EnumMap