By Barry A. Burd

Part of Java For Dummies Cheat Sheet

When you write a Java program, you can divide the program’s words into several categories. This cheat sheet tells you all about those categories.

Keywords

The Java programming language has 50 keywords. Each keyword has a specific meaning in the language. You can’t use a keyword for anything other than its pre-assigned meaning.

The following table lists Java’s keywords.

Keyword What It Does
abstract Indicates that the details of a class, a method, or an interface are given elsewhere in the code.
assert Tests the truth of a condition that the programmer believes is true.
boolean Indicates that a value is either true or false.
break Jumps out of a loop or switch.
byte Indicates that a value is an 8-bit whole number.
case Introduces one of several possible paths of execution in a switch statement.
catch Introduces statements that are executed when something interrupts the flow of execution in a try clause.
char Indicates that a value is a character (a single letter, digit, punctuation symbol, and so on) stored in 16 bits of memory.
class Introduces a class — a blueprint for an object.
const You can’t use this word in a Java program. The word has no meaning but, because it’s a keyword, you can’t create a variable named const.
continue Forces the abrupt end of the current loop iteration and begins another iteration.
default Introduces a path of execution to take when no case is a match in a switch statement.
do Causes the computer to repeat some statements over and over again (for instance, as long as the computer keeps getting unacceptable results).
double Indicates that a value is a 64-bit number with one or more digits after the decimal point.
else Introduces statements that are executed when the condition in an if statement isn’t true.
enum Creates a newly defined type — a group of values that a variable can have.
extends Creates a subclass @@md a class that reuses functionality from a previously defined class.
final Indicates that a variable’s value cannot be changed, that a class’s functionality cannot be extended, or that a method cannot be overridden.
finally Introduces the last will and testament of the statements in a try clause.
float Indicates that a value is a 32-bit number with one or more digits after the decimal point.
for Gets the computer to repeat some statements over and over again (for instance, a certain number of times).
goto You can’t use this word in a Java program. The word has no meaning. Because it’s a keyword, you can’t create a variable named goto.
if Tests to see whether a condition is true. If it’s true, the computer executes certain statements; otherwise, the computer executes other statements.
implements Indicates that a class provides bodies for methods whose headers are declared in an interface.
import Enables the programmer to abbreviate the names of classes defined in a package.
instanceof Tests to see whether a certain object comes from a certain class.
int Indicates that a value is a 32-bit whole number.
interface Introduces an interface. An interface is like a class but, for the most part, an interface’s methods have no bodies.
long Indicates that a value is a 64-bit whole number.
native Enables the programmer to use code that was written in a language other than Java.
new Creates an object from an existing class.
package Puts the code into a package — a collection of logically related definitions.
private Indicates that a variable or method can be used only within a certain class.
protected Indicates that a variable or method can be used in subclasses from another package.
public Indicates that a variable, class, or method can be used by any other Java code.
return Ends execution of a method and possibly returns a value to the calling code.
short Indicates that a value is a 16-bit whole number.
static Indicates that a variable or method belongs to a class, rather than to any object created from the class.
strictfp Limits the computer’s ability to represent extra large or extra small numbers when the computer does intermediate calculations on float and double values.
super Refers to the superclass of the code in which the word super appears.
switch Tells the computer to follow one of many possible paths of execution (one of many possible cases), depending on the value of an expression.
synchronized Keeps two threads from interfering with one another.
this A self-reference — refers to the object in which the word this appears.
throw Creates a new exception object and indicates that an exceptional situation (usually something unwanted) has occurred.
throws Indicates that a method or constructor may pass the buck when an exception is thrown.
transient Indicates that, if and when an object is serialized, a variable’s value doesn’t need to be stored.
try Introduces statements that are watched (during runtime) for things that can go wrong.
void Indicates that a method doesn’t return a value.
volatile Imposes strict rules on the use of a variable by more than one thread at a time.
while Repeats some statements over and over again (as long as a condition is still true).

Literals

In addition to its keywords, three of the words you use in a Java program are called literals. Each literal has a specific meaning in the language. You can’t use a literal for anything other than its pre-assigned meaning.

The following table lists Java’s literal words.

Literal What It Does
false One of the two values that a boolean expression can possibly have.
null The “nothing” value. If you intend to have an expression refer to an object of some kind, but the expression doesn’t refer to any object, the expression’s value is null.
true One of the two values that a boolean expression can possibly have.

The keywords and literal words are all called reserved words because each of these words is reserved for special use in the Java programming language.

Restricted keywords

With the release of Java 9, the language has ten new words called restricted keywords. A restricted keyword has a specific meaning in the language, but only if you use that word in a specific way. For example, if you write

requires other.stuff;

you tell Java that your program won’t run unless it has access to some other code (the code contained in other.stuff). But if you write

int requires = 10;

then requires is an ordinary int variable.

The following table lists Java’s restricted keywords.

Restricted Keyword What It Does
exports Indicates that the code in a particular package is available for use by code in other modules.
module A bunch of packages.
open Indicates that all the packages in a module are, in a certain way, available for use by code in other modules.
opens Gets access to all the code in another module. This access uses Java reflection (which tends to be messy).
provides Indicates that a module makes a service available.
requires Indicates that the program won’t run unless it has access to the some other code.
to Names the code that has permission to use a particular piece of code.
transitive When my code requires use of the A code, and the Z code requires use of my code, the word transitive means that Z code automatically requires A code.
uses Indicates that a module uses a service.
with Specifies a particular way of using a service.

Identifiers in the Java API

The Java API (Application Programming Interface) has thousands of identifiers. Each identifier is the name of something (a class, an object, a method, or something like that). These identifiers include System, out, println, String, toString, JFrame, File, Scanner, next, nextInt, Exception, close, ArrayList, stream, JTextField, Math, Random, MenuItem, Month, parseInt, Query, Rectangle, Color, Oval, paint, Robot, SQLData, Stack, Queue, TimeZone, URL, and so many others.

You can reuse any of these names for any purpose in your code. But if you do, you might have trouble using a name with its normal meaning from the Java API. For example, you can write

int System = 7;

java.lang.System.out.println(System);

But you can’t write

int System = 7;

System.out.println(System);

Identifiers that you (the programmer) declare

In your own Java program, you can make up names to your heart’s delight. For example, in the code

double multiplyByTwo(double myValue) {

return myValue * 2;

}

the names multiplyByTwo and myValue are your very own identifiers.

When you create a new name, you can use letters, digits, underscores (_), and dollar signs ($). But don’t start the name with a digit. If you try to start a name with a digit, Java replies with a “Please don’t do that” message.