Computers operate on data which is stored as bits, usually described as “zeros and ones”, in the computer’s memory. Though even that’s a bit of a fairy tale: in the computer each bit is actually represented by the state of a tiny electrical circuit.
However sequences of zeros and ones—let alone microscopic circuitry—aren’t convenient for humans to deal with, so programming languages like Java give us higher-level abstractions for dealing with data that let us treat those sequences of bits as values that have some meaning to us humans. The kinds of values, called data types, supported by a programming language and what we can do with them are the most basic building blocks of our programs
In a program the bit of code that produce values are called expressions. Every expression evaluates to a value. In this section we’ll deal with two of the most basic kinds of expressions. First, literal values are the way we write specific values in our program, that will be turned into the appropriate bits in memory when the program runs. These are things like 123 and 3.14. Second, we can create names in our program, called variables that represent a place in the computer’s memory where a value can be stored which we can then refer to by the variable’s name. In a later unit we’ll talk about expressions that use operators that cause the computer to perform computations using existing values to produce new values.
In Java, every value has a data type that determines how it is represented in the computer’s memory (which mostly we don’t have to worry about) and what we can do with it. There are two categories of data types in Java. One, primitive types, hold the kinds of values that the computer can directly operate on such as numbers which we can do math with and logical values that we can do, well, logic with. The other, reference types hold a reference to a more complex value called an object. For now we’ll focus on three of the main primitive data types in Java. We’ll come back to reference types in Chapter 8 Classes.
double which can represent non-integer numbers like 6.3 -0.9, and 60293.93032. Computer people call these “floating point” numbers because the decimal point “floats” relative to the magnitude of the number, similar to the way it does in scientific notation like \(6.5 ✕ 10^8\text{.}\) The name double comes from the fact that double values are represented using sixty-four bits, double the thirty-two bits used for the type float which used to be the normal size floating point number when most computers did math in units of 32-bits. (float is rarely used these days and is not part of the AP curriculum.)
Another way to think of a data type is as a set of values and a set of operations on those values. For example, the int datatype represents the set of integers that can be represented in thirty-two bits (-2,147,483,648 to 2,147,483,647) and they support the normal arithmetic operations like addition, subtraction, multiplication, and division as well as a few that you probably didn’t learn about in elementary school.
While you could use an int, this would throw away any digits after the decimal point, so it isn’t the best choice. You might want to round up a grade based on the average (89.5 or above is an A).
An average is calculated by summing all the values and dividing by the number of values. To keep the most amount of information this should be done with decimal numbers so use a double.
While you could use an int and use 0 for false and 1 for true this would waste 31 of the 32 bits an int uses. Java has a special type for things that are either true or false.
The integer type (int) can’t be used to represent numbers with fractional parts and the difference between gold and silver in the Olympics is often measured in just thousandths of a second.
There are two aspects to a variable. To the computer a variable is a location in its memory that can store a value. The value stored at that location can change, or vary, while the program is running which is why we call them variables. To us human programmers, on the other hand, the important aspect of a variable as its name that we can use to refer to the data without having to know about the variable’s exact location in memory.
For example, in a computer game we might need to keep track of the player’s score, which will start at 0 and increase as the player earns points. The score can be stored somewhere in memory but in our program we’ll refer to it with the name score since that’s much more convenient than something like “address 66a3ffec”.
To create a variable, you must tell Java its data type and its name. Creating a variable is also called declaring a variable. The type is a keyword like int, double, or boolean, but you get to make up the name for the variable. When you create a variable that will hold a primitive type, Java will set aside enough memory to hold a value of that type and will associate that memory location with the name you used.
Computers store all values using bits (binary digits). A bit can represent two values and we usually say that the value of a bit is either 0 or 1. When you declare a variable, you have to tell Java the type of the variable because Java needs to know how many bits to use and how to represent the value. The 3 different primitive types all require different number of bits. An integer gets 32 bits of memory, a double gets 64 bits of memory and a boolean could be represented by just one bit.
To declare (create) a variable, you specify the type, leave at least one space, then the name for the variable and end the line with a semicolon (;). Java uses the keyword int for integer, double for a floating point number (a double precision number), and boolean for a Boolean value (true or false).
Or you can set an initial value for the variable in the variable declaration. Here is an example that shows declaring a variable and initializing it all in a single statement.
The equal sign here = doesn’t mean the same thing as it does in a mathematical equation where it is an assertion that the two sides are equal. In programming it means make them equal by setting the variable named on the left to the value of the expression on the right. The line above sets the value of the variable score to the literal value 4. The variable always has to be on the left side of the = and the value expression on the right.
While we can declare a variable without giving it an initial value, a variable must be initialized before it can be used in an expression. A variable is initialized the first time it is assigned a value.
The following code declares and initializes variables for storing a number of visits, a person’s temperature, and if the person has insurance or not. It also includes extra blocks that are not needed in a correct solution. Drag the needed blocks from the left area into the correct order (declaring numVisits, temp, and hasInsurance in that order) in the right area. Click on the “Check Me” button to check your solution.
While you can name your variable almost anything, there are some rules. A variable name should start with an alphabetic character (like a, b, c, etc.) and can include letters, numbers, and underscores _. It must be all one word with no spaces.
The name of the variable should describe the data it holds. A name like score helps make your code easier to read. A name like x is not a good name for a variable holding a score, because it gives no clues what the variable is used for. On the other hand, don’t use names that are too long like theNumberThatHoldsTheScore as those are also hard to read. Choose names that make your code easier to understand, not harder.
Variable names are case-sensitive and spelling sensitive! Each use of the variable in the code must match the variable name in the declaration exactly.
The convention in Java and many programming languages is to always start a variable name with a lower case letter and then uppercase the first letter of each additional word, for example gameScore. Variable names can not include spaces so uppercasing the first letter of each additional word makes it easier to read the name. Uppercasing the first letter of each additional word is called camel case because it looks like the humps of a camel. Another option is to use underscore _ to separate words, but you cannot have spaces in a variable name.
Debug the following code that reads out a weather report. Make sure the data types match the values put into the variables. Can you find all the bugs and get the code to run? Work with a programming buddy if you get stuck.
(AP 1.2.B.2) Every variable has a name and an associated data type that determines the kind of data it can hold. A variable of a primitive type holds a primitive value from that type.
(AP 1.2.A.1) A data type is a set of values and a corresponding set of operations on those values. Data types can be primitive types (like int) or reference types (like String).
(AP 1.2.A.2) The primitive data types used in this course define the set of values and corresponding operations on those values for numbers and Boolean values.
Which of the following pairs of declarations are the most appropriate to store a student’s average course grade in the variable GPA and the number of students in the variable numStudents?
