mirror of
https://github.com/namibia/awesome-cheatsheets.git
synced 2024-11-25 06:07:34 +00:00
620 lines
16 KiB
Markdown
620 lines
16 KiB
Markdown
### HELLO WORLD :ghost:
|
|
|
|
```java
|
|
//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
|
|
|
|
* 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 toDegrees(double 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
|
|
```
|