Created Cheat sheet for Java language.

Improvement in @raulvillora 's file and also added many new JAVA topics with github md extension suppport.
2024-12-22 01:58:54 +00:00 · 2020-06-05 16:37:59 +05:30 · 2020-06-05 16:37:59 +05:30 · 1bdf7b8a56
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+### HELLO WORLD :ghost:
+
+```java
+//Text file name HelloWorld.java
+public class HelloWorld {
+  // main() is the method
+  public static void main (String[] arfs)
+    //Prints "Hello World" in the terminal window.
+    System.out.pritn("Hello World");
+}
+```
+
+### COMPILATION & EXECUTING JAVA CODE
+
+* Goto your program directory in terminal (Assumed JAVA Path is set)
+* After for compile your code
+
+> **javac HelloWorld.java (your program file name)**
+  
+* For run program 
+
+> **java HelloWorld (main class name)**
+
+
+### DATA TYPES
+
+|   Type  |      Set of values      |            Values            | Operators |
+|:-------:|:-----------------------:|:----------------------------:|:---------:|
+|   int   |         integers        | between -2^31 and + (2^31)-1 | + - * / % |
+|  double |  floating-point numbers |         real numbers         |  + - * /  |
+| boolean |      boolean values     |         true or false        | && \|\| ! |
+|   char  |        characters       |                              |           |
+|  String | sequences of characters |                              |           |
+
+
+### DECLARATION AND ASSIGNMENT STATEMENTS
+
+```java
+//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;
+```
+
+### COMPARISON OPERATORS
+
+| Operation |        Meaning        |
+|:---------:|:---------------------:|
+|     ==    |         equal         |
+|     !=    |       not equal       |
+|     <     |       less than       |
+|     >     |      greater than     |
+|     <=    |   less than or equal  |
+|     >=    | greater than or equal |
+
+
+### PRINTING
+```java
+  String s = "Happy Coding Folks!!"
+  void System.out.print(String s) //print s
+  void System.out.println(String s) //print s, followed by a newline
+  void System.out.println() //print a newline
+```
+
+### PARSING COMMAND-LINE ARGUMENTS
+```java 
+  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
+```java
+  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 toDegreestouble radians)  // convert angle from radians to degrees
+    double exp(doube 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
+```java
+  if (x>y) { // x > y is the boolean expression
+   //Sequence of statements
+   x = y;
+  } 
+```
+
+> IF-ELSE STATEMENT
+```java
+   if (BOOLEAN EXPRESSION) { 
+   //Sequence of statements
+   } else {
+   //Sequence of statements
+   }
+```
+
+> NESTED IF STATEMENT
+```java
+   if (BOOLEAN EXPRESSION) { 
+   //Sequence of statements
+   } else if {
+   //Sequence of statements
+   }
+   .
+   .
+   .
+   else {
+   //Sequence of statements
+   }
+```
+
+>SWITCH STATEMENT
+```java
+  switch (VARIABLE TO EVALUATE ITS VALUE) {
+    case value: Statement; break;
+    ...
+    ...
+    ...
+    default: Statement; break;
+  }
+```
+**Example:**
+```java
+  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
+```java
+  for (declare and initialize a loop control variable; loop-continuation condition/s; increment or decrement of the variable of control)
+  {
+    //Statement
+  }
+``` 
+**Example:**
+```java  
+  for (int i = 0; i <= n; i++) {
+     System.out.println(i);
+  }
+```
+> WHILE LOOP STATEMENT
+```java
+    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:**
+```java
+    int i=1;  
+    do{  
+      System.out.println(i);  
+      i++;  
+    }while(i<=10); 
+```
+
+### ARRAY
+> ARRAY DECLARATION
+
+```java
+    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
+
+```java
+  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
+
+```java
+  //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
+
+```java
+  public class Charge {
+    //Instance variable declarations
+    private final double rx, ry;
+    private final double q;
+  }
+```
+
+### METHODS
+
+```java
+  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
+```java
+class MyClass {
+    // field, constructor, and 
+    // method declarations
+}
+```
+**Example:**
+
+```java
+    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
+```java
+class MyClass extends MySuperClass implements YourInterface {
+    // field, constructor, and
+    // method declarations
+}
+```
+* MyClass is a subclass of MySuperClass and that it implements the YourInterface interface.
+
+> CONSTRUCTORS
+* A class contains constructors that are invoked to create objects from the class blueprint.
+* Constructor declarations look like method declarations—except that they use the name of the class and have no return type
+* Each and every class has defualt No-args constructor.
+
+
+```java
+  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
+* Polymorphism is the concept where an object behaves differently in different situations.
+*  There are two types of polymorphism 
+    1. compile time polymorphism 
+    2. runtime polymorphism.
+
+#### 1. Compile Time Polymorphism
+* Compile-time polymorphism is achieved by method overloading.
+* method overloading is creating multiple method with methods name is same and arguments are different. 
+```java
+  public class Circle {
+
+    public void draw(){
+      System.out.println("Drwaing circle with default color Black and diameter 1 cm.");
+    }
+
+    public void draw(int diameter){ //method draw() overloaded.
+      System.out.println("Drwaing circle with default color Black and diameter"+diameter+" cm.");
+    }
+
+    public void draw(int diameter, String color){ //method draw() overloaded.
+      System.out.println("Drwaing circle with color"+color+" and diameter"+diameter+" cm.");
+    }
+  }
+```
+#### 2. Run Time Polymorphism
+* Run-time polymorphism is achieved by method overriding.
+* Runtime polymorphism is implemented when we have an **“IS-A”** relationship between objects.
+* method overriding is the subclass has to override the superclass method.
+```java
+    public interface Shape {
+
+	    public void draw();
+    }
+```
+```java
+    public class Circle implements Shape{
+
+      @Override
+      public void draw(){
+        System.out.println("Drwaing circle");
+      }
+
+    }
+```
+```java
+    public class Square implements Shape {
+
+      @Override
+      public void draw() {
+        System.out.println("Drawing Square");
+      }
+
+    }
+```
+* `Shape` is the superclass and there are two subclasses `Circle` and `Square`
+* Below is an example of runtime polymorphism.
+```java
+    Shape sh = new Circle();
+    sh.draw();
+
+    Shape sh1 = getShape(); //some third party logic to determine shape
+    sh1.draw();
+```
+
+### INHERITANCE
+
+* Inheritance is the mechanism of code reuse. 
+* The object that is getting inherited is called the superclass and the object that inherits the superclass is called a subclass.
+* We use `extends` keyword in java to implement inheritance from class.
+* We use `implements` keyword in java to implement inheritance from interface.
+
+```java
+    public class Superclass{
+      // methods and fields
+    }
+```
+```java
+    public interface Superinterface{
+      // methods and fields
+    }
+```
+```java
+    public class Subclass extends Superclass implements Superinterface{
+      // methods and fields
+    }
+```
+
+### Abstraction
+
+* Abstraction is the concept of hiding the internal details and describing things in simple terms.
+* Abstraction can be achieved by two ways.
+  1. Abstract Class
+  2. Interface
+  
+#### 1. Abstract Class
+* An abstract class must be declared with an `abstract` keyword.
+* It can have abstract and non-abstract methods.
+* It cannot be instantiated.
+* It can have constructors and static methods also.
+* It can have final methods which will force the subclass not to change the body of the method.
+
+```java
+    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 is a blueprint of a **class**.
+* It can have only abstract methods. [Except Java 8 and next versions.]
+* Since Java 8, we can have **default and static** methods in an interface.
+
+
+```java
+    interface print{  
+        void printPaper();  
+    }  
+    public class A4 implements print{  
+        public void printPaper(){
+          System.out.println("A4 Page Printed. ");
+        }  
+    }
+```
+
+### Encapsulation
+
+* Encapsulation is used for access restriction to class members and methods.
+* Encapsulation is the technique used to implement abstraction in OOP.
+* As in encapsulation, the data in a class is hidden from other classes, so it is also known as **data-hiding**.
+* Encapsulation can be achieved by Declaring all the variables in the class as private and writing public methods in the class to set and get the values of variables.
+* Best example of Encapsulation is POJO (Plain-Java-Object-Class).
+  
+ ```java
+    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
+* **STACK DATA TYPE**
+
+```java
+  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
+```
+
+* **QUEUE DATA TYPE**
+
+```java
+  public class Queue<Item> implements Iterable<Item>
+
+  Queue()  //create an emptyh queue
+  boolean isEmpthy()  //return if the queue empthy
+  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
+```
+
+* **ITERABLE**
+
+```java
+//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()
+  ...
+
+}
+```
+
+* **SYMBOL TABLE DATA TYPE**
+
+```java
+  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
+```
+
+* **SET DATA TYPE**
+
+```java
+  public class SET<Key extends Comparable<Key>> implements Iterable<Key>
+  SET() //create an empthy set
+  boolean isEmpthy()  //return if the set is empthy
+  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
+```