Java Programming Keywords Summary

Here are the summary of the available keywords in the Java programming language. Keywords are reserved words that already taken and internally used by Java, so we cannot create variables and name it using this keyword.

Keyword Meaning
abstract an abstract class or method
assert used to locate internal program errors
boolean the Boolean type
break breaks out of a switch or loop
byte the 8-bit integer type
case a case of a switch
catch the clause of a try block catching an exception
char the Unicode character type
class defines a class type
const not used
continue continues at the end of a loop
default the default clause of a switch
do the top of a do/while loop
double the double-precision floating-number type
else the else clause of an if statement
extends defines the parent class of a class
final a constant, or a class or method that cannot be overridden
finally the part of a try block that is always executed
float the single-precision floating-point type
for a loop type
goto not used
if a conditional statement
implements defines the interface(s) that a class implements
import imports a package
instanceof tests if an object is an instance of a class
int the 32-bit integer type
interface an abstract type with methods that a class can implement
long the 64-bit long integer type
native a method implemented by the host system
new allocates a new object or array
null a null reference
package a package of classes
private a feature that is accessible only by methods of this class
protected a feature that is accessible only by methods of this class, its children, and other classes in the same package
public a feature that is accessible by methods of all classes
return returns from a method
short the 16-bit integer type
static a feature that is unique to its class, not to objects of its class
strictfp Use strict rules for floating-point computations
super the superclass object or constructor
switch a selection statement
synchronized a method or code block that is atomic to a thread
this the implicit argument of a method, or a constructor of this class
throw throws an exception
throws the exceptions that a method can throw
transient marks data that should not be persistent
try a block of code that traps exceptions
void denotes a method that returns no value
volatile ensures that a field is coherently accessed by multiple threads
while a loop

How do I write Hello World program in Java?

Hello World is a classic program example to start with when we begin to learn a new programming language. The main purpose is to know the basic syntax of the language. Below is the Java version of a Hello World program, it simple enough to start.

package org.kodejava.example.basic;

public class HelloWorld {
    public static void main(String[] args) {
        System.out.println("Hello World");
    }
}

The code contains one class called HelloWorld, we declare a class using the class keyword. A main(String[] args) method with a public static void signature and an argument of string array is a special method in Java class. This method is the execution entry point of every Java application. When a class have this method it will be executable. And finally in the body of the method we have a single line of code that write a Hello World string to the console.

The HelloWorld class must be saved in a file named HelloWorld.java, the class file name is case sensitive. In the example we also define a package name for the class. In this case the HelloWorld.java must be placed in a org.kodejava.example.basic directory.

To run the application we need to compile it first. I assume that you have your Java in your path. To compile it type:

javac -cp . org.kodejava.example.basic.HelloWorld.java

The compilation process will result a file called HelloWorld.class, this is the binary version of our program. As you can see that the file ends with .class extension because Java is everything about class. To run it type the command bellow, class name is written without it extension.

java -cp . org.kodejava.example.basic.HelloWorld

And you will see a string Hello World! is printed out in the console as the output of your first Java program, the HelloWorld program.

To install Java Development Kit (JDK) and configure Java environment you can see the following tutorial: