Java Basics Cheat Sheet

Class Declaration and Object Creation

import javax.swing.*;

class Ch2Sample1 {
    public static void main(String[] args) {
        JFrame myWindow;
        myWindow = new JFrame();
        myWindow.setSize(300, 200);
        myWindow.setTitle("My First Java Program");
        myWindow.setVisible(true);
    }
}

• Declare a name.
• Create an object.
• Use an object.

Object Declaration

JFrame myWindow;
Account customer;
Student jan, jim, jon;
Vehicle car1, car2;

• One object is declared here.
• Class name must be defined before this declaration.

Object Creation

myWindow = new JFrame();
customer = new Customer();
jon = new Student("John Java");
car1 = new Vehicle();

• Name of the object we are creating here.
• An instance of the class is created.
• No arguments are used here.

Identifiers

• A Java identifier is a sequence of letters, digits, underscores (_), and dollar signs ($) with the first one being a non-digit.
• Valid identifiers: MyFirstApplication, FunTime, DEFAULT_VALUE.
• Invalid identifiers: 45678, Sample Program, hello,there.

Java Standard Naming Convention

Robot robot;

• Class name and object name follow the Java naming convention.

Declaration vs. Creation

Customer customer;
customer = new Customer();

1. The identifier customer is declared, and space is allocated in memory.
2. A Customer object is created, and the identifier customer is set to refer to it.

Sending a Message

myWindow.setVisible(true);
account.deposit(200.0);
student.setName("john");
car1.startEngine();

• Name of the object to which we are sending a message.
• Method name, the name of the message we are sending.
• Argument, the argument we are passing with the message.

Standard Input

import java.util.Scanner;

Scanner scanner;
scanner = new Scanner(System.in);
String input;
System.out.print("Enter your input(s): ");
input = scanner.nextLine();
System.out.println("I entered: " + input);

• Using a Scanner object to input data from the standard input (System.in).
• Reading input using nextLine().

Standard Output

System.out.print("I Love Java");
System.out.println("How do you do?");
System.out.println("My name is Jon Java.");

• Using print and println methods of System.out to display output.

String

String text = "Espresso";
text.substring(6, 8);
text.length();
text.indexOf("gram");

• Using substring, length, and indexOf methods with String objects.
• Concatenating strings with + operator.

Date

import java.util.Date;
Date today;
today = new Date();

• Using the Date class to represent a time instance.

SimpleDateFormat

import java.text.SimpleDateFormat;

SimpleDateFormat sdf1, sdf2;
sdf1 = new SimpleDateFormat("MM/dd/yy");
sdf2 = new SimpleDateFormat("MMMM dd, yyyy");
sdf1.format(today);
sdf2.format(today);

• Using SimpleDateFormat to format Date objects.