Computer Science- The name used for many computing aspects, generally in developing code.

The Basics

• Commenting, or using comments is your best friend. It’s a very effective tool, and it’s a good idea to get into the habit of commenting.

• It allows programmers to make “notes” in the program so they can reference them later.

• Other people viewing your code won’t have trouble interpreting it.

• Types of commenting:

  • In-line/short comments- They appear after or near a statement and are followed by 2 forward slashes.

    • Ex:

//this is a short comment

• Long comments- They go beyond more than one line of code that is surrounded by special characters; they begin with (/) and end off with (/).

  • Ex:

 /* this is a
long comment*/

Identifiers:

• Identifiers are names that are given to show data that are stored in the memory of a computer when a program is being executed in a computer.

• **Java is special because it lets the program name the identifier based on its role.

• The guidelines below are guidelines to make it easier on us when coding, and its rules that Collegeboard expects us to follow.

  • An identifier needs to have a combination of letters, numbers, and underscores(_). However, they must begin with a letter, and can’t have other characters listed.

    • This includes spaces!!

  • An identifier should be named logically, and it should correspond to the data that it is holding.

  • An identifier should begin with a lowercase letter, and if there are several words used to name the identifier, then the words after the first should be capitalized.

    • Ex: numOfSides, or testScores

• White space- It doesn’t affect the functionality of the program. It just makes the code easily readable, because it lets the programmer space out code to separate statements or tasks.

Compiling and Errors:

• Remember that when a programmer writes in Java, these lines of code are understood by a Java development environment.

• An interpreter is used to change this code into binary(zeros and ones) which is what the computer understands.

• This process is called compiling.

• We use IDEs to write out code, or interactive development environments.

• The code will be checked for programming errors.

• If any sort of errors are found, then the code stops compiling, and an error message appears.

• This would be a compile-time error.  The computer is unable to compile or understand what the code is trying to do, which is why it’s unable to execute properly.

• Logical error- This is based on the logic of the code. The way that the code is structured, is incorrect.

• Run-time error- A compiler doesn’t catch this error. It just causes an error with the execution of the program.

Outputs and Inputs:

• There are 2 ways to produce an output on the screen:

  • Using System.out.print;

  • Using System.out.println;

• What’s the difference?

  • The print() returns the cursor to the same line after it’s done executing. The println() statement moves the cursor to the next line.

• There are different ways you would print out specific types of outputs.

  • Ex:

System.out.println(“Quotes are used because a string is being printed. Otherwise it will simply print whatever is put in the brackets.”);

• If you want to put a line break in a piece of code just use ＼n.

  • Ex:

System.out.print(“The first line＼nThe second line”)

Variables and Assignment:

• To create an identifier we need to assign a value to the identifier.

  • Note: type identifier = data;

  • Ex: int side1 = 2;

  • This specifically tells the compiler that the identifier is side1, and the value that’s being assigned to it is 2. The data has the integer type.

• Note: The equal sign, is an assignment operator, and is very important to have!!

• Note: The semicolon’s that we have at the end of each statement mean that the specific statement is completed.

• Assignment statements DON’T print anything out as output!

• Whenever data is associated with an identifier, it’s referred to as a variable.

  • Ex:

int myFavoriteNumber = 22;
myFavoriteNumber = 78;
System.out.print(“My favorite number is “ + myFavoriteNumber);

Output: My favorite number is 78

• Important things to remember:

  • Once a variable is assigned a given type, it can’t be changed.

  • The quotation is used so that it specifies what sentence to print out, and what variable value to print out.

  • Use concatenation, when you want to combine variables of 2 different types. - The plus sign is used for this, just like in the example above.

The 4 Data Types:

• Primitive Data- The basic type of data.

• Integer(int)- It represents any number that is an integer. So all positive and negative numbers and zero. Integers can’t store decimals!!

• Double(double)- A number that can be positive, negative, or zero, and it can be a fraction or decimal.

• **Boolean(**boolean)- A value that is either true or false. True = 1, False = 0 in machine code.

• **Character(**char)- It represents a single character that can be represented on a computer. It includes any character that you type on the keyboard. All the way from letters, numbers, spaces, and special characters. char values are stored with the use of single quotation marks or an apostrophe.

Arithmetic Operators:

• Note: The different operators that are used in Java are the +. - , ,/, and %.

• The plus sign is used for addition.

  • Ex:

sumOfSides = side2 + side3;

• The subtraction sign is used for subtraction.

  • Ex:

differenceOfSides = side1 - side3;

• The asterisk or the star is used for multiplication.

  • Ex:

side1 * side2 = productSide;

• The slash, or (/) is used for division.

  • Ex:

side3/side2 = quotientSide;

• The percent sign, (%), is called a modulus operator. It does the division of the numbers, and it returns the remainder.

  • Ex:

sumOfSides % differenceOfSides;

• Just like in normal math, Java also follows an order in which it conducts its operations.

• Java performs multiplication and division operators, as well as modulus from left to right, then it does addition and subtraction in the same order.

• If you want to change the order that the operation occurs, just use parentheses to separate the expression out.

• Java likes to do something special with some of its integers. Say that you are dividing 3/2 in reality the answer is actually 1.5, but Java truncates the 0.5 off. It will display the answer as 1, because it was described as in integer type, not a double.

• Negative numbers also work out the same way.

• The only way to get over this portable is with casting.

• Casting- A process in which data is forced to look like another type of data to the compiler.

System.out.print(3 - (double) (4)/5);
System.out.print( 3 - 4/(double)5);

• Both of the examples are the proper way to do casting so that it produces the desired result.

• Note: In the order of operations in Java, casting will be done first because it has higher precedence.

• Other operators:

  • Increment operator(++)- This increases the value of a number by one. It’s just a shorthand of the addition operator that can sometimes be used.

    • Ex: x++; is the same as x = x+1;

  • Decrement operator(- -)- This decreases the value of a number by one.

    • Ex: x- - ; is the same as x = x -1;

• Other Shortcut Commands: