From 1bdf7b8a563973e3395b4174e6d84d2094d5ce3a Mon Sep 17 00:00:00 2001 From: Dhruv <> Date: Fri, 5 Jun 2020 16:37:59 +0530 Subject: [PATCH 1/2] Created Cheat sheet for Java language. Improvement in @raulvillora 's file and also added many new JAVA topics with github md extension suppport. --- languages/java.md | 619 ++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 619 insertions(+) create mode 100644 languages/java.md diff --git a/languages/java.md b/languages/java.md new file mode 100644 index 0000000..8ee028e --- /dev/null +++ b/languages/java.md @@ -0,0 +1,619 @@ +### 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 implements Iterable + + 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 implements Iterable + + 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 implements Iterable { + +//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, 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 keys() // all keys in the symbol table +``` + +* **SET DATA TYPE** + +```java + public class SET> implements Iterable + 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 +``` From 69a7080ab625c16b475847459c6c9ae01cb17df0 Mon Sep 17 00:00:00 2001 From: iamdhrv <31080769+iamdhrv@users.noreply.github.com> Date: Sat, 6 Jun 2020 14:07:18 +0530 Subject: [PATCH 2/2] Update README.md Added Java extensions in Readme.md file. --- README.md | 1 + 1 file changed, 1 insertion(+) diff --git a/README.md b/README.md index 91c6f7f..9636462 100644 --- a/README.md +++ b/README.md @@ -28,6 +28,7 @@ Feel free to take a look. You might learn new things. They have been designed to - [C](languages/C.txt) - [C#](languages/C%23.txt) - [PHP](languages/php.php) +- [JAVA](languages/java.md) #### Functional