The Java programming language

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In the last class...

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Variables

Imagine we want to write a program about apples...

We will use variables to manipulate information about them

Variables will store information (about apples in our example)

Variables in Java have types

Is the apple on the left fresh? Is the apple on the right fresh?

Declaring variables

To use variables in Java we need to declare them

TYPE variable_name;

TYPE variable_name = INITIAL_VALUE;

Sidenote: /* * This is * a multiple line * comment */ // this is a single line comment

Comments are ignored by the computer

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;

Declaring variables

The float type

// if we know the weight of the apple beforehand float weight_of_apple_lb = 0.45; // if we don't know it float weight_of_apple_lb;

Declaring variables

The double type

// if we know the weight of the apple beforehand double weight_of_apple_lb = 0.45; // if we don't know it double weight_of_apple_lb;

Declaring variables

The char type

char first_apple_code = 'M'; char second_apple_code = 'C'; char third_apple_code = 'S';

char values go in single quotes

Declaring variables

The String type

String first_apple_kind = "McIntosh"; String second_apple_kind = "Cortland"; String third_apple_kind = "Spartan";

String values go in double quotes

Declaring variables

The boolean type

boolean is_apple_fresh = false; boolean is_apple_rotten = true;

Declaring variables

The boolean type

boolean is_apple_fresh = true; boolean is_apple_rotten = false;

Operations with variables

We can assign 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 can accumulate values in variables

// declare a variable int number_of_apples_in_box; // assign a value to the variable number_of_apples_in_box = 5; // increase the value of a variable by 2 number_of_apples_in_box += 2;

Operations with variables

We can do mathematical operations with number types

Binary: variable_1 OPERATOR variable_2

• + , -: Addition , Subtractions
• * , /: Multiplication , Division
• %: Remainder of division

Unary:OPERATORvariable_1, variable_1OPERATOR

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

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;

Operations with variables

// we initially have 5 apples int number_of_apples_in_box = 5; // we measure the weight of an apple double weight_of_apple = 0.45; // we compute the weight of the box double weight_of_box = number_of_apples_in_box * weight_of_apple;

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";

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;

Logical Expressions

Statements for which we can answer yes or no

Determine the truth value of a statement: true or false

Logical Expressions

Comparisons are a kind of logical expressions

• > , <: Greater than, Less than
• >= , <=: Greater or equal, Less than or equal
• == , !=: Equal, Different

Logical Expressions

Comparisons are a kind of logical expressions

// we declare variable for the labels of our boxes int apples_big_box = 100; int apples_small_box = 10;

// we declare variable for the labels of our boxes int apples_big_box = 100; int apples_small_box = 10; boolean test1 = apples_big_box > apples_small_box;

// we declare variable for the labels of our boxes int apples_big_box = 100; int apples_small_box = 10; boolean test1 = apples_big_box > apples_small_box; boolean test2 = apples_big_box < apples_small_box;

// we declare variables for the weight of our apples double weight_of_big_apple = 0.45; double weight_of_small_apple = 0.2; boolean test1 = weight_of_big_apple > weight_of_small_apple;

// we declare variable for the labels of our boxes double weight_of_big_apple = 0.45; double weight_of_small_apple = 0.2; boolean test1 = weight_of_big_apple > weight_of_small_apple; boolean test2 = weight_of_big_apple < weight_of_small_apple;

Logical Expressions

We can combine multiple logical expressions

• &&: AND
• ||: OR

R: The box contains red apples
G: The box contains green apples

What is the truth value of R && G for each box?

What is the truth value of R || G for each box?

Conditional statements

We can use the truth values of logical expressions to conditionally execute code

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

Conditional statements

if the apple is rotten
then it goes in the garbage bin

else it goes in one of the shelves

Conditional statements

if the apple is rotten
then it goes in the garbage bin

else if it is not rotten
then it goes in one of the top shelves

Let's write that Java program!

The example code is available here!

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 1