Values and expressions

Our job as Java programmers is to write text that can be translated into instructions for a computer. In other words, everything we write needs to have a meaning as defined by Java.

When we learn a human language we first learn a bunch of words and then how to create meaning by combining them in different ways. Similarly, to learn Java we need to learn the basic building blocks and then how to combine them. In this unit we’ll learn about the most basic building blocks of meaning in Java: values and expressions.

In our analogy with human language, values are the words. While it may not be obvious when you use your pocket super computer to play music or watch videos, the fundamental things computers deal with are pretty simple: basically numbers and a special kind of number that can be interpreted as logical values. Everything else is built up from those simple foundations.1

Values in Java, and most other programming languages, are organized into data types. A data type has two aspects: it’s both a set of values and a set of operations that operate on those values. For instance in the next section we’ll learn about two of Java’s data types int and double which both represent numbers. Both of those data types support the usual arithmetic operations of addition, subtraction, multiplication.

Some data types—and all the ones we’ll cover in this unit—have a third aspect: they can be written in our programs as literal values. For instance we can write 1 in a Java program to represent the numeric value one. Most of the time we don’t need to be too particular about distinguishing between literal values and values because the point of a literal value is it stands for the value it represents. But note that a literal value is something that can appear in the text of a program while when we say “a value” we are referring to the actual representation of the value in a computer when the program is running, kind of like the difference between the word on the page of a book (a literal value) and the meaning of the word in your mind when you read it (the actual value).

If values are analogous to words, then expressions are analogous to phrases or sentences: expressions are the parts of our programs that express a computation that produces a value. For instance 1 + 2 is a Java expression that represents the computation of adding together the value one and the value two. When it is run on a computer that computation will produce the value three.

Like literal values which are bits of program text that mean particular values, expressions are bits of program text that mean a particular computation and that computation, when run on a computer, will produce a value in the computer’s memory. Because an expression produces a value, we can also talk about the type of the expression. For instance because the value produced by the expression 1 + 2 is an int so we can say the type of the expression is int.

More complex expressions are made, as in human languages, by combining expressions. In Java we combine values using operators which operate on values to produce new values. You already know about a bunch of Java’s operators because they are the same as the mathematical operators you learned about in elementary school: addition, subtraction, multiplication, and division. In this chapter we will learn how to write those operators in Java, some details about how they work on Java’s numbers, and some other operators that you probably didn’t learn about in 3rd grade.

One final kind of expression that we’ll use a bit in this unit before really diving into them in the next unit, are variables. For now you can just think of a variable as a name for a value, just like in algebra. For instance x could be a stand in for any int in an expression like x * 10.

In the rest of this unit, we’ll learn about four basic data types that we’ll use in almost all our Java programs: int and double which are two different representations of numbers; boolean, the data type used to represent the logical values true and false; and String, a data type used to represent text.

Next: Numbers