Keywords in Java

A keyword is a word that already belongs to Java. Java uses it to understand your program. For example, class tells Java that a class is starting, and if tells Java to make a decision.

Because Java already has a job for these words, you cannot use them as names you create yourself. You can name a variable score, but not class.

See the problem first

Run this example to see its output.

int is a Java keyword that says “this variable stores a whole number.” score is a name chosen by the programmer. It is allowed because Java has no special job for the word score.

The line below is not allowed, so it is written as a comment. If you remove //, Java reports an error:

Java sees class and expects a class declaration, not a variable name. Use a meaningful alternative such as classNumber, courseName, or sectionName.

You do not need to memorise every keyword

There are many Java keywords, but you learn them naturally while learning each topic. When you see a new keyword, ask one question: what job is Java giving this word?

Here are the groups you will meet first:

You will learn the exact behaviour of each keyword in its own lesson. For now, recognise that these words have fixed meanings in Java.

Read keywords inside one program

This program checks whether a student's score reaches the pass mark. Read the explanation below it; each highlighted word has a job.

Run this example to see its output.

Read the important words like this:

1. public class starts a class named ResultChecker.
2. int says that PASS_MARK and score are whole numbers.
3. static lets main use PASS_MARK without first creating a ResultChecker object.
4. final means PASS_MARK should keep the value 40 and not be changed later.
5. if checks a question: is score at least 40?
6. else gives Java the other action when the answer is no.

Do not try to remember the technical details of static and final today. The important point is that Java reads these keywords as instructions, while score and PASS_MARK are names created for this program.

Words that look special

true, false, and null are also words Java reserves for fixed values:

Run this example to see its output.

true and false mean yes/no values. null means that a variable does not currently point to an object. You will learn null in more detail when working with objects and Strings.

Try it yourself

In the ResultChecker program, change score to 32, run it, and see the else part execute. Then try to rename score to a clear name such as studentScore. Do not rename it to if, class, or return; those names are reserved by Java.

Remember this

• Keywords are words Java already uses as instructions.
• You may use your own name such as score, but not a keyword such as class.
• Learn keywords one topic at a time instead of memorising a long list.