Cheat Sheet

Java Coding Standards

• Always include class documentation
• Class names start with upper-case letters
• Variable names start with lower-case letters
• Align { and matching } in same column
• Indent methods and statements
• Use meaningful variable names
• Use camelNotation

Program Structure

Every Java program is a class

Program execution begins with the main method (if it exists)

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

• class tells Java that this is a class definition
• public who can access the class
• HelloWorld the name of the class and the file
• { } defines the start and end of the class code

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

• main name of the method
• public who can access the method
• static type of method
• void the method doesn't return a result
• { } defines the start and end of the method code
• (String[] args) parameter (data passed to the method)

System.out.println("Hello World!");

• System.out.println built-in method to display data
• "Hello World!" argument passed to println method
• ; tells Java where the statement ends

public class Foobar {
  public static void main(String[] args) {
    int totalScore = 0;
    System.out.println("Score" + totalScore);
  }
}

• You must declare all variables
• This means telling Java the:
  • type: int
  • identifier name: totalScore
  • initial value (optional): 0
• By convention variables start with lowercase letters in Java
• This is called camelCase

Objects

• An object is something about which you have data
• Could be a Person:
  • Student
  • Employee
  • a Place:
    • City
    • State
  • or a Thing:
    • Screen Display
    • Game Character
    • Bank Account

Classes

public class Hello {
  String myString = "";
  public Hello () {
    this.myString = "Default string";
  }
  public Hello (String myString) {
    this.myString = myString;
  }
  public String getMyString () {
    return myString;
  }
  public void printMyString () {
    System.out.println(myString);
  }
}

• A class defines the behaviors associated with an object by means of methods
• A class stores the state of an object by means of member variables
• A class consist of code…
  • statements
    • System.out.println();
  • member variables
    • int theAnswer;
  • zero, one, or more constructors…
    • constructors create objects
  • and zero, one, or more methods
    • methods modify or return object data

Member Variables

public class River {
  private String name;
  private int length;
  private int locks;
  ...
}

• Member variables contain data associated with an object
• Specify private access to member variables
  • prevents class users from randomly modifying member variable values

Constructor

public class River {
  ...
  public River(String inName, int inLen) {
    name = inName;
    length = inLen;
  }
  ...
}

• Special method that creates an instance of a class, this runs when the class is instantiated
  • Instantiated - represent as or by an instance.
• Must have same name as the class
• May include parameters

Instantiating an Object

public class RiverTest {
  public static void main(String[] args) {
    River riverA;
    riverA = new River("Ohio",981);
    River riverB = new River("Kanawha",3000);
  }
}

riverA contains hash-code identifier of a particular River object, located somewhere in memory

Accessor Methods

public class River {
  private String name;
  private int length;
  …
  public String getName() {
    return name;
  }
}

• retrieves value of object member variable
• usually doesn't accept parameters
• naming convention:
  • getFooBar - return a value
  • findFooBar - return a calculated result

Mutator Methods

public class River {
  private String name;
  private int length;
  …
  public void setName(String name) {
    this.name = name;
  }
}

• changes value of object member variable
• usually doesn't return a value
• naming convention:
  • setFooBar

Arrays

• Arrays are a sequence of values of the same datatype or class
• Use standard variable declaration except include [ ]
• int[] grades;
• Instantiate the array with the desired size using the new operator:

grades = new int[25];
String[] myNames = new String[42];

• Array index values start at 0
• length contains the size of an array:
• System.out.println("Size: " + myNames.length);
• Multi-dimensional arrays specified by repeated [ ]
  • int[][] matrix = new int[25][42];