# 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];`