The Java programming language

(in slow motion)

In the last class...

These situations are unpredictable

Thanks for helping out!

Variables in Java have types

Declaring variables

The int type

// if we know the initial number of apples beforehand int number_of_apples = 3; // if we don't know it int number_of_apples;

Operations with numerical variables

Binary: variable_1 OPERATOR variable_2

• =: Assignment
• + , -: Addition , Subtraction
• * , /: Multiplication , Division
• %: Remainder of division

Unary:OPERATORvariable_1, variable_1OPERATOR

• ++ , --: Increase variable value by 1, decrease variable value by 1

Operations with variables

Assigning values to variables

// declare a variable int number_of_apples_in_box; // assign a value to the variable number_of_apples_in_box = 5;

Operations with variables

// we initially have 5 apples int number_of_apples_in_box = 5; // we get 2 extra apples from somewhere int additional_apples = 2; // we put the apples in the box number_of_apples_in_box = number_of_apples_in_box + additional_apples; // I eat one apple number_of_apples_in_box -= 1;

Operations with variables

We can also do some operations with Strings

// we declare variable for the labels of our boxes String label_of_box_1 = "McIntosh"; String label_of_box_2 = "Cortland"; // we "concatenate" the two labels for the new box String label_of_new_box = label_of_box_1 + " and " + label_of_box_2;

Important thing to remember

• Declarations:
  
  int number_of_apples_in_box = 5; double weight_of_single_apple = 0.3; double weight_of_apples_in_box; String label_of_box_1 = "McIntosh";
• Anything that changes the value of a variable:
  
  number_of_apples_in_box -= 1; weight_of_apples_in_box = number_of_apples_in_box*weight_of_single_apple; label_of_box_1 = label_of_box_1+" apples";

These statements must end with a ; in Java

Logical Expressions

Statements for which we can answer yes or no

Determine the truth value of a statement: true or false

A variable that can hold truth values is called a boolean variable

Logical Expressions

The simplest logical expressions

• The true value
• The false value
• A boolean variable boolean is_left_apple_fresh = true; boolean is_right_apple_fresh = false;

Logical Expressions

The next simplest logical expressions, Comparisons

• a > b , a < b: Greater than, Less than
• a >= b , a <= b: Greater or equal, Less than or equal
• a == b , a!= b: Equal, Different
  [for Primitive Types]
• a.equals(b) , !(a.equals(b)): Equal, Different
  [for Object Types]

The value of a comparison is either true or false

Logical Expressions

Comparisons

// we declare variables for the weight of our apples double weight_of_big_apple = 0.45; double weight_of_small_apple = 0.2; /* We declare a boolean variable that will hold the value of our test */ boolean test1 = weight_of_big_apple > weight_of_small_apple;

// we declare variable for the weight of our apples double weight_of_big_apple = 0.45; double weight_of_small_apple = 0.2; /* We declare boolean variables that will hold the value of our tests */ boolean test1 = weight_of_big_apple > weight_of_small_apple; boolean test2 = weight_of_big_apple < weight_of_small_apple;

Logical Expressions

We can build complex logical expressions using boolean operators

• &&: AND
• ||: OR
• !: NOT

A tasty apple : is NOT rotten AND it is a McIntosh

A tasty apple : it is a McIntosh OR weights more than 0.3 lb

Conditional statements

We can use the truth values of logical expressions (or operations on them) to conditionally execute code

Example: A computer program for putting apples in the appropriate shelf space

Conditional Execution

if ( the apple is rotten )
then it goes in the garbage bin;

Alternative Execution

if ( the apple is not rotten )
then it goes in one of the shelves;

else
then it goes in the garbage bin;

Chained conditional statements

if ( the apple is not rotten AND it is big )
then it goes in one of the top shelves;

else if ( the apple is not rotten AND it is small )
then it goes in one of the bottom shelves;

else
then it goes in the garbage bin;

Nested conditional statements

Visualizing conditional statements

Visualizing conditional statements

How do we repeat executions?

The while loop

What is the difference?

The while loop

• The program will do something until the logical_expression is false
• If the logical_expression is never false, the program never stops

  An infinite loop

The while loop

while ( there are apples left in the box )
do something;

The while loop

while ( there are apples left in the box )
if ( the apple is not rotten )
then it goes in one of the shelves;
else
then it goes in the garbage bin;

Visualizing the while loop

There are other kinds of loops!

• The while loop
• The for each loop
• The indexed for loop

Before going into for loops, let's move on from our apples example

Let's leave that for next class

More on Variables

Imagine we want to write a program that uses a calendar...

A friend wants to come visit Montreal

Your friend wants to come when the weather is nice

We will use variables to manipulate information about the weather

Declaring variables for September

Declaring variables January

Declaring variables for the rest of the months

Resources

• The Java data types:
  http://docs.oracle.com/javase/tutorial/java/nutsandbolts/datatypes.html
• The Java operators:
  http://docs.oracle.com/javase/tutorial/java/nutsandbolts/operators.html
• Suggested reading:
  How to think like a Computer Scientist, Chapter 4.1 and 7.2