Java + OOP concept Cheat Sheet

A Programming › Java cheat sheet in English by son9912

Created by Information Technology, KMITL students #IT14

75,464 views 16,234 downloads 25 Sep 2017 History

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Hello World!

Start your Java day with Hello World program
public class HelloWorld {
public static void main(String[] args) {
// Prints "Hello, World" to the terminal window.
System.out.println("Hello, World");
}
}

When you want to run the program, choose this class as main class.

Run your code

Compile from single class up HelloWorld class
javac HelloWorld.java
java HelloWorld

Compile from multiple classes and choose main class
javac *.java
java HelloWorld // HelloWorld is your preferred main class

Variables

Type	Default Value	Memory Allocation
byte	0	8 bits
short	0	16 bits
int	0	32 bits
long	0L	64 bits
float	0.0F	32 bits (decimal)
double	0.00D	64 bits (decimal)
boolean	False	varies on impliment
String	NULL	depends on character count
char	\u0000	16 bits (unicode)

Operators

Operand	What they do
=	Assign value
==	Check value/address similarity
>	More than
>=	More than or equals
>>>	Move bit to the right by
++	Increment by 1

inverse of these operands still working the same.
For example : != is not equal

Defining variable

Defining new variable attributes
int x = 12;
int x; // will be defined as 0

Define by creating new instances
String x = new String;

Type Casting (decreasing bit use)
Expanding data types will not require type casting. Narrowing does.
double x = 10; // Expanding data types
int y = (int) 10.222222; // Narrowing data types

Conditions

If statement
if (statement) {}
If - else statement
if (statement) {} else{}

Switch

switch (num) {
  case 1: doSomething();
    break;
  default: doThis();
    break;
}

Loop

for (int i: someArray) {}
while (something) {}
do {something} while (true)

Prime number function

if (n < 2) { return false; }
for (int i=2; i <= n/i; i++)
  if (n%i == 0) return false;
return true;

returns a boolean

String Pool - Optimizations

String pool is created to make the same value string use the same address. By doing that, it will save memory and time for compiler to do stuff

Basic testing
String s1 = "Hello World";
String s2 = "Hello World;

Check it using "=="
System.out.println(s1 == s2);
True
"==" will check its address

Allocate a new address using new
String s1 = "Hello World";
String s2 = new String;
s2 = "Hello World";
System.out.println(s1 == s2);
False

Allocate new address by changing its value
String s1 = "Hello World";
String s2 = "Hello World";
s2 = "Hello Thailand";
System.out.println(s1 == s2);
False

Naming Grammars

Naming should be regulated for easier recogition from others

Use Upper Camel Case for classes: VelocityResponseWriter
Use Lower Case for packages: com.company.project.ui
Use Lower Camel Case for variables: studentName
Use Upper Case for constants: MAX_PARAMETER_COUNT = 100

Use Camel Case for enum class names
Use Upper Case for enum values
Don't use '_' anywhere except constants and enum values (which are constants).

Receiving user input

There is normally 2 ways to receive user keyboard input

1. java.util.Scanner
Scanner x = new Scanner(System.in);
String inputString = x.next(); // for String type input
int inputInteger = x.nextInt(); // for Integer type input

2. String[] args from public static void main()
NOTE: args is already in a array. It can receives unlimited amount of arguments.
String inputString = args[0]; // for String type input
Int inputString = (int) args[0]; // for Integer type input

To use Scanner, importing Scanner library is required : import java.Object.Scanner

All types of input can be received. (not just String or int)

Access Modifier

- Java uses <default> modifier when not assigning any.
- public modifier allows same class access
- Works in inherited class means itself and the classes that inherit from it.

Attribute modifier

Attribute Type	Access Grants
Private	Allows only in class where variable belongs
Public	Allows any class to have this attribute
Static	Attribute that dependent on class (not object)
Final	Defined once. Does not allow any change/inheritance

Methods

Methods are fucking easy, dud.
<mod> <return> mthdName (<args>) { }

Example:
public double getAge () {
return someDouble;
}

Constructor

Constructors allow you to create an object template. It consists of complete procedures.

Create a blank constructor to allow its extension classes to inherit this super constructor.
<modifier> Person () {}
But will be created automatically by not writing any constructor

Create an argument-defined constructor
<modifier> Person (String name) {
this.name = name;
}

Abstract Class

Abstract is a type of class but it can consist of incomplete methods.

Create new abstract
<access_modifier> abstract class HelloWorld () {}

Interface

Interface is different from constructor. It consists of incomplete assignments

Interface allows you to make sure that any inherited class can do the following methods. (It's like a contract to agree that this thing must be able to do this shit.) The method is then completed in the class that implements it.

Creating a new interface
interface Bicycle {
  void speedUp (int increment);
}
----
class fuckBike implements Bicycle {
  ...
  void speedUp (int increment) {
    speed += increment;
  }
  ...
}

Encapsulation

Encapsulation allows individual methods to have different access modifier.
Creating setters and getters is one way to use encapsulation

For example
private void setTime(int hour, int minuite, int second){
this.hour = hour;
this.minuite = minuite;
this.second = second;
}

Inheritance

Inheritance helps class to import the superclass' method.

Importing superclass
class HelloWorld extends Object {}

Normally, the class that does not inherit any class will inherit Object class.*

Class can only inherit 1 class/abstract

Importing Interface
class HelloWorld inherits InterfaceThing {}

Class can inherit unlimited amount of interface

Overload

We use overload when you want different input to work differently, but remains the same name.

Example of Overload
public printer(String x){}
public printer(String x, String y){}

If the input is 2 string, it will go to the second method instead of first one.

But you cannot overload by using the same input type sequence. For example
public printer(String x){}
public printer(String x, String y){} // conflict
public printer(String y, String x){} // conflict

Java will not allow this to be run, because it cannot determine the value.

Override

When you have inherit some of the class from parents, but you want to do something different. In override feature, all the subclass/class object will use the newer method.

To make sure JDK knows what you are doing, type @Override in front of the public name. If the override is unsuccessful, JDK will returns error.

Example of overriden helloWorld() method :
Class Student
public void helloWorld(){
System.out.println("Hello");
}

Class GradStudent extends Student
@Override
public void helloWorld(){
System.out.println("Hello World");
}

Rules of Overridden methods
1. Access modifier priority can only be narrower or same as superclass
2. There is the same name method in superclass / libraries

java.io.PrintStream

Print with new line
System.out.println("Hello World");
Print
System.out.print("Hello World");

java.util.Scanner

Create a Scanner object
Scanner sc = new Scanner(System.in);
Accept input
double d = sc.nextDouble()

java.lang.Math

Methods	Usage
`Math.max(<value1>, <value2>)`	Return maximum value
`Math.min(<value1>, <value2>)`	Return minimum value
`Math.abs(<value>)`	Return unsigned value
`Math.pow(<number>, <exponent>`	Return value of a number^exponent
`Math.sqrt(<value>)`	Return square root of a value

java.lang.String

Find the length -> int
msg.length()
To lower/uppercase -> String
msg.toLowerCase()
msg.toUpperCase()
Replace a string -> String
msg.replaceAll(String a, String b)
Split string between delimeter -> array
msg.split(String delimeter)
Start/end with -> boolean
msg.startsWith(String pre)
msg.endsWith(String post)
String format -> String
String.format(String format, Object... args)

java.lang.String

Methods	Description
charAt(int index)	Returns the char value at the specified index
compareTo(String otherString)	Compare 2 strings lexicographically
concat(String str)	Concatenate specified string
endsWith(String suffix)	Test if the string ends with specified suffix
equals(String andObject)	Test if strings values are the same
toCharArray()	Convert string to character array
toLowerCase()	Convert string to lowercase
toUpperCase()	Convert string to uppercase
toString()	Convert things to string
valueOf(<value>)	Return the representation of argument
length()	Return length of the string
replaceAll(String a, String b)	Replace string a to string b
split(String delimeter)	Split string between delimeter
startsWith(String prefix)	Test if string starts with specified prefix
format(String format, Object arg)	Format strings to the format given

There is many more in Java documents : https://docs.oracle.com/javase/9/docs/api/java/lang/String.html

java.util.Collection (CollectionAPI)

Provides ways to keep variables and access it faster

Ways to keep data
1. Set - Care about duplicity, not queue (eg. HashSet)
2. List - Care about queue, not duplicity (eg. LinkedList)
3. Map - Care about both queue and key duplicity (eg.HashMap)

Methods that will be included
boolean add(Object element);
boolean remove(Object element);
int size();
boolean isEmpty();
boolean contains(Object element);
Iterator Iterator();

HashList - CollectionAPI

Method	Usability
void add (int index, Object element)	Add value to list
Object remove(int index)	Remove item #index from list
Object get(int index)	Retrieve item #index from list
void set(int index, Object element)	Set data to correspond #index
int indexOf(Object element)	Find the #index from element
ListIterator listIterator()

It also includes all CollectionAPI methods

Create new HashList by using
List x = new HashList();

HashMap - CollectionAPI

Method

Usability

Collections

Create List of 1, 2, 3 on-the-fly
Arrays.asList(1, 2, 3)
Convert primitive array to Stream
Arrays.stream(primitiveArray)
Convert ArrayList to Stream
arrayList.stream()

LinkedList - CollectionAPI

Create empty LinkedList of Integer
LinkedList myList = new LinkedList<Integer>t()
Create LinkedList with values in it
new LinkedList<>(Arrays.asList(1, 2, 3)))
Add an object to LinkedList
myList.add(50)

Badges

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Comments (2)

hirousto 09:03 22 Jan 2019

thanks a lot

Jaydizm 19:25 2 Aug 2018

Thanks

Java + OOP concept Cheat Sheet

Hello World!

Run your code

Variables

Operators

Defining variable

Conditions

Switch

Loop

Prime number function

String Pool - Optimi­zations

Naming Grammars

Receiving user input

Access Modifier

Attribute modifier

Methods

Constr­uctor

Abstract Class

Interface

Encaps­ulation

Inheri­tance

Overload

Override

java.i­o.P­rin­tStream

java.u­til.Sc­anner

java.l­ang.Math

java.l­ang.String

java.l­ang.String

java.u­til.Co­lle­ction (Colle­cti­onAPI)

HashList - Collec­tionAPI

HashMap - Collec­tionAPI

Collec­tions

LinkedList - Collec­tionAPI

Badges

Comments (2)

String Pool - Optimizations

Constructor

Encapsulation

Inheritance

java.io.PrintStream

java.util.Scanner

java.lang.Math

java.lang.String

java.lang.String

java.util.Collection (CollectionAPI)

HashList - CollectionAPI

HashMap - CollectionAPI

Collections

LinkedList - CollectionAPI