Lecture for January 18
Variables and Assignment
Think about variables as buckets that hold information. Once the bucket is created, only one type of item can go in the bucket.
We can say that bucket1 is of type
sand, that means the only thing that can go in the bucket is sand.
int bucket1; double bucket2;
From the two lines above, we have declared the variable.
Variables store state, they are a name for a location in memory.
Always remember to initialize your variables. Otherwise there’s nothing in the bucket!
bucket1 = 5;
You can combine both the declaration and initialization
int count = 15;
Remember when dealing with variables to stay true with the type, don’t mix a bucket of water with a bucket of sand.
We can update
count to contain a true value
count = 55;
count no longer has the value of
15 in it. There’s no record of it! It has been overwritten with the value
There are 8 primitive types in Java
byte through double are all numeric types
boolean can only be equal to
boolean student = true;
Stores a single character from the Unicode set
There are 16 bits per character which adds up to 65,536 characters
ASCII is the US subset of the characters. You can look this up online when needing to deal with ASCII values
char firstLetter = 'A';
The different numeric types determine the precision of your number. Since numbers are not represented the same in the computer as they are in real life, there are some approximations.
The default type you can use your code is
int for integers and
double for numbers with a decimal point
There are certain types of operations you can perform on numeric type
|+||addition||43 + 8||51|
|*||multiplication||43 * 8||344|
|/||division||43.0 / 8.0||5.375|
|%||remainder / mod||43 % 8||3|
There are two types of in/decrementers postfix and prefix
int x = 0; int y = 7; x++; // Shortcut for x = x + 1 y--; // Shortcut for y = y - 1
int x = 0, y = 7, z; z = y * x++; // Equivalent to (y * x) + 1 = 7 * 0 z = y * ++x; // Equivalent to y * (x + 1) = 7 * 1
There are two types of data conversion, implicit and explicit
The compiler can perform implicit data conversion automatically.
Performing an explicit data conversion requires additional work on the programmer’s part
A conversion is implicit if you do not lose any information in it
double price = 6.99; int sale = 3; double total = price - sale;
A cast is an explicit data conversion. This is requested by a programmer, this can lead to loss of information
int nextChar = 'b'; Character.isAlphabetic( (char) nextChar); // Let's you print the actual letter 'b' instead of the number corresponding to it float price = 6.99; int cost = (int) price; // cost is now 6
You can print the values of variables using
The difference is that
System.out.println adds a new line at the end. Meaning the next print out will be on the next line.
int cost = 5; double sale = .30; System.out.print(cost); System.out.print(sale); // Prints out '5.30` System.out.println(cost); System.out.println(sale); // Prints out '5' // Prints out '0.30'
To add a space between two variables in a print, add
" " to the expression in between the two variables
System.out.println("The total cost is " + 5 " dollars and" + " " + 93 + " cents"); // The total cost is 5 dollars and 94 cents
Input from User
You can get import from the user, we can do this using the
First import it at the top of your file
All you can do with
Scanner is outlined in the Java API at this link https://docs.oracle.com/javase/8/docs/api/index.html?java/util/Scanner.html
Create a Scanner object
Scanner input = new Scanner(System.in); System.out.print("Please enter an integer: "); price = input.nextInt(); // The integer that the user inputs is now stored in price System.out.println("Your input: " + price);