Review

Programming

• Machine code is the only language a processor understands
• Assembly language uses words for commands
• High-level languages are compiled or interpreted

Programming methodologies

• Structured programming
• Object oriented programming

Java

• object-oriented
• Stick to "style guide" to enjoy all benefits!
• platform independent
• no recompilation necessary
• even graphics applications platform independent
• robust and secure
• many compiler errors, few(er) wrong programs
• applets run in browser
• simple!!! ;-)

Programming in practice

• save often (and under a different name before major changes!)
• backup sensibly

Errors

• syntax errors (compiler errors)
• runtime errors
• logic errors

Algorithm

• unambiguous
• must execute in finite time

Fundamental Data Types

Number Types

• integer
• floating point (double precision)

Symbolic names (Variables)

• each variable must have a type, e.g., int count = 0;
• assignments, e.g., count = count + 4;
• type conversion
• int_var = "12" // Error
• double_var = int_var // OK
• int_var = (int) double_var //information lost
• constants  final double LB_IN_GRAMS = 453.6;

Arithmetic

• brackets as usual  12.0 / 2.0 * 5.0  is 30.0
• integer division: double total = 3.0 + 5 / 6  is 3.0

Strings

• treated as objects, but you don't need a constructor
• String str = "hello";
• some methods of class String
• int l = str.length();
• String sub = str.substring(start,pastEnd);
• String upper = str.toUpperCase();
• String many = "hello" + l;

Classes

• make Java object-oriented
• purpose: encapsulation (black box)

Objects

• instances of classes (e.g. "bert" is an instance of a button)

Elements of a class

• instance variables: define the state of object
• usually private
• any type (number type or class)
• methods
• usually public
• any return type (or void)
• two common uses of methods:
• either modify instance variables  (return type is void)
• or return information about object
• constructors: create object
• usually public
• no return type
• have to have the same name as the class
• different constructors have to differ in parameter types
• don't use name of an instance variable as parameter
• static variables and methods apply to entire class
• (e.g., Math.round())

Using a class

• methods are commonly called from outside of class
• (method calls are like messages sent from one class to another)
• calling a method with
• return type void:

redNapkin.translate(100,100);
• return type boolean:

boolean in = redNapkin.contains(150,150);
• calling a constructor (creating an object)
• redNapkin = new Rectangle(100,100);

Object References

• redNapkin = blueNapkin;
• copies reference (address) and does not produce a copy of the object!

Applets

• no main method
• you provide a version of methods from class Applet instead

Introduction to graphics

• graphics context
• colors
• text (fonts)

Testing of computations

• visual
• analytical
• numerical

Decisions

General form

if (condition 1)
{ ...
}
else if (condition 2)
{ ...
}
else
{
}

Block statements

• {} not necessary, if only one command is executed
• {} highly recommended for nested if statements

Comparisons

• == for double precision might not be true due to rounding errors
• check instead if |a-b| < e * max(|x|,|y|)
• == for objects, including String checks if objects are identical i.e. same object
• check object1.equals(object2) assuming equals is implemented properly

Boolean expressions

• &&  the result is only true if all conditions are true
• ||  the result is true if at least one of the conditions is true
• ! inverts value of boolean variable / expression
• De Morgan's Law:
• ! (A && B) is the same as !A || !B
• ! (A || B) is the same as !A && !B