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Java Programming Cheat Sheet

Larson Reever
I am the Vice President @ *** Solutions. | Technology Expert | Author | Speaker | Small & Medium Business IT Support - We founded ***IT Solutions in 2007 to provide IT services
・3 min read

Java runs in a Java Virtual Machine (JVM), a layer that translates Java code into bytecode compatible with your operating system. Get an open source JVM from openjdk.java.net or developers.redhat.com/products/openjdk/download

Java Packages

Related classes are grouped into a package. Declare a

package name at the top of your code.

package com.opensource.hello;
/* @author your-name */

When creating libraries, you can create packages within a package to provide access to each unique class.

package com.opensourc.greeting;
/* @author your-name */

Java Imports

When importing libraries needed for your Java code, use
the import key word. Imports work based on your
environment's Java path, and open source libraries can be
bundled with your application (license permitting.)

package com.opensource.hello;
/* @author your-name */

import java.lang.System;
import javax.swing.*;

//code...

Java Variables

Java is strongly typed, meaning all variable types must be
declared when created. Local variables may be created
inside a method, instance variables may be created inside
a class, and static variables may be shared across
instances.

//code...
public class Greeting {
static int num = 42;
public static String namer() {
String name = "Java";
int number =1;

double bignum = 3.1415926535897932384;
float smallnum = 3.141592;

char character = 'a';
boolean toggle = true;
return name; }
}

Java Classes

A class file must contain one public class. Other classes
may exist elsewhere and may not be public. By
convention, class names start with a capital letter.

package com.opensource.hello;
/* @author your-name */

import java.lang.System;

public class Greeting {
public static String namer() {
//code...
}
}

Java Methods

Java methods may be public (accessed by any other
class), private (known only within a class), protected
(unavailable to an unrelated class), or default. Provide the
type of returned data, such as void, int, float, etc.

//code...
public static String namer() {
String name = "Java";
return name; }
}
public static void main(String[] args) {

String player = greeting.namer();
System.out.printf("%s,%s", named, num);
}
}

Try and Catch

To catch errors in Java, start with try, fall back on catch, and end with finally. Should the try clause fail, then catch is invoked, and in the end, there's finally to perform some action regardless of the results.

try {

cmd = parser.parse(opt, args);

if(cmd.hasOption("help")) {

HelpFormatter helper = new HelpFormatter();

helper.printHelp("Hello ", opt);

System.exit(0);

}

else {

if(cmd.hasOption("shell") || cmd.hasOption("s")) { String target = cmd.getOptionValue("tgt"); } // else

} // if

} catch (ParseException err) { System.out.println(err); System.exit(1);

} //catch

finally {

new Hello().helloWorld(opt);

} //finally

} //try

Arguments

Passing arguments into a Java application is done with
the args keyword, preceded by the type of acceptable
arguments (such as String, int, and so on).

//code...
public static void main (String[] args) {
System.out.printf("You rolled a ");
DiceRoller App =

new DiceRoller(

Integer.parseInt(args[0]) );

App.Roller();

}

}

Run a .java file

Java files, usually ending in .java, can be run from your
IDE or in a terminal using the java command. If an
application is complex, however, running a single file may
not be useful. Add arguments as appropriate.

$ java ./Example.java
$ java ./diceroller.java 20
You rolled a 17

Run a JAR file

Usually, Java applications are distributed as Java
Archives (JAR) files, ending in .jar. To run a JAR file,
double­click its icon or launch it from a terminal.

$ java -jar /path/to/Example.jar

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