Object-Oriented Programming

OOP Class Notes for 9/5/12

Java Introduction

• Java Code Pipeline
  • Java code progression: .java -> javac -> .class -> java -> (program execution)
    • .java is the source files in java
    • .class is the java byte code, not compatible with most processors
    • Does not use registers, stack based per invocation
    • Compiled in a "just in time" manner into standard binary by Java VM

• Java Q&A:
  • Q: how does java VM know the stuff in referenced code in a .class file?(dependencies)
  A: It goes back to the .java files to figure out if it needs stuff
  
  
  • Q: how does it link/load libraries?
  A: Dynamically(only loads class files if they are referenced from other classes)
  
  
  • Q: How does it know where to file those class files?
  A: From the class path(directories containing class files to look for referenced classes in)
  
  
  • Q: What is a jar(Java ARchive) file?
  A: Functionally a zip file
  
  

Code Example

• A four dimensional point class, with constructors, getters, and some other things
• Code from lecture: Point.java
• Code Discussion:
• What data do we need?
  • Four coordinates
    • How should we represent them?
      1. Four doubles
      2. Generic objects(more flexible, but adds complexity and more vague, time consuming, bad idea)
• Good Programming Note(GPN): be "lazy", get things working sooner so you can get feedback, no need to go beyond the requirements
• Need to consider what type to make the coordinates, use your head
• We choose to make them an array of doubles(more ease of reference, but more memory overhead)
• What should they be named?
• Pick "descriptive names" over "convenient names", important for group work, large projects(both common with java)
• Use array initializers, like so:

  coordinates = new double[] { c1, c2, c3, c4 };

• NOT like so:

	    coordinates=new double[];
	    coordinates[0]=c1;
	    coordinates[1]=c2;
	    coordinates[2]=c3;
	    coordinates[3]=c4;
	  

• Q: Do we need to make the coordinates mutable(able to be modified?)
A: No, since that would be a different point. Use final to mark array as immutable(but no way to stop the array's fields from being changed in java however)


• Now we need a "getter" method to retrieve the values(method that returns one of the array fields)
• GPN:
  • choose descriptive method names(do not just call it get())
  • What should the type of the parameter of the getter be?
    • Why not byte?
      • Because people pass it int objects, you have to change them to bytes
    • So we use int
• How do we "get?"

  return coordinates[idx];

  • Throws error if index passed is out of bounds however
    • GPN:
      • java inheritance error hierarchy: ArrayIndexOutOfBoundsException inherits IndexOutOfBoundsException inherits RuntimeException inherits Exception inherits Throwable inherits Object
      • exceptions signal coding errors or things malfunctioning at runtime; need try{}; catch{} cases to resolve them if possibly expected.
      • Also conveniently hides implementation details when you try{}; catch{}
      • Encapsulation and hiding are important
      • Creating an interface for the outside that is simple and clean
      • Error handling is slower than ignoring errors, but here it is more important to be robust(make programming decisions!)
      • Error handling should never be present on the critical path!
      • Also important to get the working version quickly, don't get hung up on issues of performance here(too much)
• Important note: Good testing cases will be important for project.
• Need a method to find the distance from another point
  • How do we compute it?
    • Pythagoras to the rescue! (in 4 dimensions)
  • GPN:
    • balance housekeeping versus efficiency
      • where to we declare variables only used inside a loop?(inside or outside)
        • Trick Question- Correct answer is inside since java handles it such that there is no added performance cost.
    • Don't use hardcoded values(such as defined numbers of iterations for for loops), define a public static final value to store the number(easier to change when used multiple times)
    • Don't add strings in a loop(produces a new string each time, very inefficient)
    • Adding them outside a loop is ok(compiler is smart)