Cheat-Codes

HELLO WORLD :ghost:

//Text file name HelloWorld.java
public class HelloWorld {
  // main() is the method
  public static void main (String[] args)
    //Prints "Hello World" in the terminal window.
    System.out.println("Hello World");
}

COMPILATION & EXECUTING JAVA CODE

javac HelloWorld.java (your program file name)

java HelloWorld (main class name)

DATA TYPES

Type Set of values Values Operators
short integers between -2^15 and + (2^15)-1 + - * / %
int integers between -2^31 and + (2^31)-1 + - * / %
long integers between -2^63 and + (2^63)-1 + - * / %
float integers real numbers 32 bit + - * /
double floating-point numbers real numbers 64 bit + - * /
boolean boolean values true or false && || !
char characters 16 bit  
String sequences of characters it’s not a primitive data type  

DECLARATION AND ASSIGNMENT STATEMENTS

//Declaration statement
int a,b;

//Assignment statement
a = 13212; //a is the variable name; 13212 is the literal which is assign to the variable a

//Initialization statement
int c = a + b;

//Compound assignment expressions
a += b; //a is the variable name; b is the variable name; this expression is an equivalent shorthand notation of a = a + b
a -= b; //a is the variable name; b is the variable name; this expression is an equivalent shorthand notation of a = a - b
a *= b; //a is the variable name; b is the variable name; this expression is an equivalent shorthand notation of a = a * b
a /= b; //a is the variable name; b is the variable name; this expression is an equivalent shorthand notation of a = a / b
a %= b; //a is the variable name; b is the variable name; this expression is an equivalent shorthand notation of a = a % b
a ^= b; //a is the variable name; b is the variable name; this expression is an equivalent shorthand notation of a = a ^ b
a &= b; //a is the variable name; b is the variable name; this expression is an equivalent shorthand notation of a = a & b
a \|= b; //a is the variable name; b is the variable name; this expression is an equivalent shorthand notation of a = a \| b

COMPARISON OPERATORS

Operation Meaning
== equal
!= not equal
< less than
> greater than
<= less than or equal
>= greater than or equal

PRINTING

  String s = "Happy Coding Folks!!"
  System.out.print(String s) //print s
  System.out.println(String s) //print s, followed by a newline
  System.out.println() //print a newline

PARSING COMMAND-LINE ARGUMENTS

  String s = "Java is the best!!"
  int Integer.parseInt(String s) //convert s to an int value
  double Double.parseDouble(String) //convert s to a double value
  long Long.parseLong(String s) // convert s to a long value

MATH LIBRARY

  Public Class Math{
    double abs(double a)    // absolute value of a
    double max(double a, double b)  //maximum of a and b
    double min(double a, dobule a)  //minimum of a and b
    double sin(double theta) //sine of theta
    double cos(double theta) //cosine of theta
    double tan(double theta) //tangent of theta
    double toRadians(double degrees) // convert angle from degrees to radians
    double toDegrees(double radians)  // convert angle from radians to degrees
    double exp(double a)  // exponential (e^a)
    double pow(double a, double p) //raise a to the bth power (a^b)
    double random() //random in [0,1)
    double sqrt(double a)  //square root of a
    }

EXAMPLES OF TYPE CONVERSION

Expression Expression type Expression value
(1 + 2 + 3 + 4) / 4.0 double 2.5
Math.sqrt(4) double 2.0
“123343” + 99 String “12334399”
11 * 0.25 double 2.75
(int) 11 * 0.25 double 2.75
11 * (int) 0.25 int 0
(int) (11 * 0.25) int 2

CONDITIONAL & LOOP STATEMENT

ANATOMY OF CONDITIONAL STATEMENT

IF Statement

  if (x>y) { // x > y is the boolean expression
   //Sequence of statements
   x = y;
  }

IF-ELSE STATEMENT

   if (BOOLEAN EXPRESSION) {
   //Sequence of statements
   } else {
   //Sequence of statements
   }

NESTED IF STATEMENT

   if (BOOLEAN EXPRESSION) {
   //Sequence of statements
   } else if {
   //Sequence of statements
   }
   .
   .
   .
   else {
   //Sequence of statements
   }

SWITCH STATEMENT

  switch (VARIABLE TO EVALUATE ITS VALUE) {
    case value: Statement; break;
    ...
    ...
    ...
    default: Statement; break;
  }

Example:

  int month = 8;
        String monthString;
        switch (month) {
            case 1:  monthString = "January";
                     break;
            case 2:  monthString = "February";
                     break;
            case 3:  monthString = "March";
                     break;
            case 4:  monthString = "April";
                     break;
            case 5:  monthString = "May";
                     break;
            case 6:  monthString = "June";
                     break;
            case 7:  monthString = "July";
                     break;
            case 8:  monthString = "August";
                     break;
            case 9:  monthString = "September";
                     break;
            case 10: monthString = "October";
                     break;
            case 11: monthString = "November";
                     break;
            case 12: monthString = "December";
                     break;
            default: monthString = "Invalid month";
                     break;
        }

ANATOMY OF A LOOP STATEMENT

FOR LOOP STATEMENT

  for (declare and initialize a loop control variable; loop-continuation condition/s; increment or decrement of the variable of control)
  {
    //Statement
  }

Example:

  for (int i = 0; i <= n; i++) {
     System.out.println(i);
  }

Enhanced for loop/for-each ```java for(dataType item : array) { … }

**Example:**
```java
    // array of numbers
    int[] numbers = {100, 200, 300, 400};

    // for each loop
    for (int number: numbers) {
      System.out.println(number);

WHILE LOOP STATEMENT

    while(condition){  //till condition will be true.
    //code to be executed
    }

Example: ```java //Initialization is a separate statement int power = 1;

while ( power <= 10/2 ) // power <= n/2 is an example of the loop-continuation condition { System.out.println(power); }


> DO-WHILE LOOP STATEMENT

```java
  do{ //always run one time even if condition would be false
    //Statement
  } while(loop-continuation condition);

Example:

    int i=1;  
    do{  
      System.out.println(i);  
      i++;  
    }while(i<=10);

ARRAY

ARRAY DECLARATION

    int[]           ai;        // array of int
    short[][]       as;        // array of array of short
    short           s,         // scalar short
                    aas[][];   // array of array of short
    Object[]        ao;        // array of Object
    Collection<?>[] ca;  // array of Collection of unknown type

DECLARATION OF ARRAY VARIABLE

  Exception ae[]  = new Exception[3];
  Object aao[][]  = new Exception[2][3];
  int[] factorial = { 1, 1, 2, 6, 24, 120, 720, 5040 };
  char ac[]       = { 'n', 'o', 't', ' ', 'a', ' ',
                      'S', 't', 'r', 'i', 'n', 'g' };
  String[] aas    = { "array", "of", "String", };

ACCESS MODIFIERS

  1. defualt(No keyword required)
  2. private
  3. public
  4. protected

NON ACCESS MODIFIERS

  1. static
  2. final
  3. transient
  4. abstract
  5. synchronized
  6. volatile

Object Oriented Programming (OOPs) Concept :clipboard:

OBJECT

  //Declare a variable, object name
  String s;

  //Invoke a contructor to create an object
  s = new String ("Hello World");

  //Invoke an instance method that operates on the object's value
  char c = s.chartAt(4);

INSTANCE VARIABLES

  public class Charge {
    //Instance variable declarations
    private final double rx, ry;
    private final double q;
  }

METHODS

  public static double sum (int a, int b) { //double is the return type, sum is the method's name, a and b are two arguments of type int;
    int result; //local variable
    result = a + b;
    return result;//return statement;
  }

CLASS DECLARATION

class MyClass {
    // field, constructor, and
    // method declarations
}

Example:

    public class Bicycle {
        // the Bicycle class has
        // three fields
        public int cadence;
        public int gear;
        public int speed;
        // the Bicycle class has
        // one constructor
        public Bicycle(int startCadence, int startSpeed, int startGear) {
            gear = startGear;
            cadence = startCadence;
            speed = startSpeed;
        }
        // the Bicycle class has
        // four methods
        public void setCadence(int newValue) {
            cadence = newValue;
        }   
        public void setGear(int newValue) {
            gear = newValue;
        }   
        public void applyBrake(int decrement) {
            speed -= decrement;
        }  
        public void speedUp(int increment) {
            speed += increment;
        }
}

DECLARING CLASSESS IMPLEMENTATING AN INTERFACE AND EXTENDING PARENT CLASS

class MyClass extends MySuperClass implements YourInterface {
    // field, constructor, and
    // method declarations
}

CONSTRUCTORS

  public class Bicycle{

      private int gear;
      private int cadence;
      private int speed;

      public Bicycle(int startCadence, int startSpeed, int startGear) { //args-constructor
        gear = startGear;
        cadence = startCadence;
        speed = startSpeed;
      }

      public Bicycle(){//No-args constructor
        super();
      }
  }

POLYMORPHISM

1. Compile Time Polymorphism

INHERITANCE

    public class Superclass{
      // methods and fields
    }
    public interface Superinterface{
      // methods and fields
    }
    public class Subclass extends Superclass implements Superinterface{
      // methods and fields
    }

Abstraction

1. Abstract Class

    abstract class Flower{
        abstract String Smell(); //abstract method.
        String Oil(){  // non-abstract method.
           System.out.println("Flower Oil is good.");
         }
    }

    public class Lily extends Flower{
        private String Smell(){ // implementation of abstarct method.
          System.out.println("Lily smell's lovender.");
        }
    }

2. Interface

    interface print{  
        void printPaper();  
    }  
    public class A4 implements print{  
        public void printPaper(){
          System.out.println("A4 Page Printed. ");
        }  
    }

Encapsulation

    public class User {
        private String username;
        private String password;

        public String getUsername() {
          return username;
        }

        public void setUsername(String username) {
          this.username = username;
        }

        public String getPassword() {
          return password;
        }

        public void setPassword(String password) {
          this.password = password;
        }
    }

ADVANCE DATA TYPE

  public class Stack<Item> implements Iterable <Item>

  Stack()   //create an empty stack
  boolean isEmpty() //return if the stack empty
  void push(Item item) // push an item onto the stack
  Item pop() //return and remove the item that was inserted most recently
  int size() //number of item on stack
  public class Queue<Item> implements Iterable<Item>

  Queue()  //create an empty queue
  boolean isEmpty()  //return if the queue empty
  void enqueue(Item item) // insert an item onto queue
  Item dequeue()  //return and remove the item that was inserted least recently
  int size() //number of item on queue
//import Iterator
import java.util.Iterator;

public class Queue<Item> implements Iterable<Item> {

//FIFO queue
  private Node first;
  private Node last;
  private class Node {
    Item item;
    Node next;
  }

  public void enqueue (Item item)
  ...

  public Item dequeue()
  ...

}
  public class ST<Key extends Comparable<Key>, Value>

  ST()  //create and empty symbol table
  void put(Key key, Value val)  //associate val with key
  Value get(Key key)  //value associated with key
  void remove(Key key) //remove key (and its associated value)
  boolean contains (Key key)  //return if there is a value associated with key
  int size()  //number of key-value pairs
  Iterable<Key> keys()  // all keys in the symbol table
  public class Set<Key extends Comparable<Key>> implements Iterable<Key>
  Set() //create an empty set
  boolean isEmpty()  //return if the set is empty
  void add (Key key)  //add key to the set
  void remove(Key key)  //remove key from set
  boolean contains(Key key) //return if the key is in the set
  int size() //number of elements in set