Recursion

In the Java language the programmer is allowed to call a method within a method, the act of doing so is called recursion. the usefulness of recursion is to repeat methods multiple times without the use of a for loop or in other very specific ways. The benefit is efficiency however it is a more difficult way to program. It is also very useful to use radicals and exponents.
public int pow(int num, int exp){
int answer;
if(exp == 1){
   return num;
}
else{
   answer = num +pow(num, exp - 1);
}
return answer;
}
// this method will return the number to the exp power.
A recursive method can also be used to sort and search like in other tutorials in this blog, it helps it to be more effective but it is considerably more complicated, Trees and Graphs also use these kind of methods.

ArrayLists and Lists

Lists and ArrayLists in Java programming are objects that store other objects inside of them, they are very useful for storing data in multiple levels.

A usual object can store data inside of it as well as objects but ArrayLists keep the data organized in a deeper level. They are somewhat like normal arrays (String[]) however they are elastic instead of solid, meaning that they can increase and decrease in size depending on how much is required, instead of in a set amount.

public static void main(String[]args){

ArrayList<String> names = new ArrayList();// This is an example of instantiating an array list

List<String> moreNames = new ArrayList();//Lists and ArrayLists are similar however ArrayLists //are child objects from the List class. Thus the ArrayList class contains all of the previous methods //and more.

names.add("Joseph");//This is how to add a new object to an array list, it returns a boolean: true;

names.add("Smigla");

names.add(0, "Edward");//This adds a new object in the place of what ever is located in the index 

           //and moves the object in that index to the index higher than itself

           // The array will end up as: Edward, Joseph.

names.remove(1);//This removes an object from the array and and if it is larger than the index it //moves all objects after it downward an index unit, it also returns the object.

for(String n: names){System.out.print(n + " ");}//This will print in the console the objects in the list:

// Edward Smigla

}

Sorting: Quick Sort

Quick sort is a divide and conquer approach to sorting just like Merge sort, however this method is more efficient but it does take up more memory space.
The method divides a list of integers, picks an integer of an array to be a pivot, then moves the integers that are larger in value and moves it to the right of the pivot, and then does that to the smaller ones.

Quick Sort
Algorithm:

public class MyQuickSort {

     

    private int array[];

    private int length;

    public void sort(int[] inputArr) {

         

        if (inputArr == null || inputArr.length == 0) {

            return;

        }

        this.array = inputArr;

        length = inputArr.length;

        quickSort(0, length - 1);

    }

    private void quickSort(int lowerIndex, int higherIndex) {

         

        int i = lowerIndex;

        int j = higherIndex;

        // calculate pivot number, I am taking pivot as middle index number

        int pivot = array[lowerIndex+(higherIndex-lowerIndex)/2];

        // Divide into two arrays

        while (i <= j) {

            /**

             * In each iteration, we will identify a number from left side which

             * is greater then the pivot value, and also we will identify a number

             * from right side which is less then the pivot value. Once the search

             * is done, then we exchange both numbers.

             */

            while (array[i] < pivot) {

                i++;

            }

            while (array[j] > pivot) {

                j--;

            }

            if (i <= j) {

                exchangeNumbers(i, j);

                //move index to next position on both sides

                i++;

                j--;

            }

        }

        // call quickSort() method recursively

        if (lowerIndex < j)

            quickSort(lowerIndex, j);

        if (i < higherIndex)

            quickSort(i, higherIndex);

    }

    private void exchangeNumbers(int i, int j) {

        int temp = array[i];

        array[i] = array[j];

        array[j] = temp;

    }

     

    public static void main(String a[]){

         

        MyQuickSort sorter = new MyQuickSort();

        int[] input = {24,2,45,20,56,75,2,56,99,53,12};

        sorter.sort(input);

        for(int i:input){

            System.out.print(i);

            System.out.print(" ");

        }

    }

}

Sorting: Merge Sort

Sorting is a combination of algorithms or instructions that you give in Java for the program to organize an array of numbers either from smallest to biggest or from biggest to smallest, this helps for your computer to more easily and quickly run a program which has to find a specific number. There are several ways to sort in Java there are Selection, Merge, and Insertion Sorts and each are differently efficient and are used for different reasons.

Merge Sort
Algorithm:

public class MyMergeSort {

     

    private int[] array;

    private int[] tempMergArr;

    private int length;

 

    public static void main(String a[]){

         

        int[] inputArr = {45,23,11,89,77,98,4,28,65,43};

        MyMergeSort mms = new MyMergeSort();

        mms.sort(inputArr);

        for(int i:inputArr){

            System.out.print(i);

            System.out.print(" ");

        }

    }

     

    public void sort(int inputArr[]) {

        this.array = inputArr;

        this.length = inputArr.length;

        this.tempMergArr = new int[length];

        doMergeSort(0, length - 1);

    }

 

    private void doMergeSort(int lowerIndex, int higherIndex) {

         

        if (lowerIndex < higherIndex) {

            int middle = lowerIndex + (higherIndex - lowerIndex) / 2;

            // Below step sorts the left side of the array

            doMergeSort(lowerIndex, middle);

            // Below step sorts the right side of the array

            doMergeSort(middle + 1, higherIndex);

            // Now merge both sides

            mergeParts(lowerIndex, middle, higherIndex);

        }

    }

 

    private void mergeParts(int lowerIndex, int middle, int higherIndex) {

 

        for (int i = lowerIndex; i <= higherIndex; i++) {

            tempMergArr[i] = array[i];

        }

        int i = lowerIndex;

        int j = middle + 1;

        int k = lowerIndex;

        while (i <= middle && j <= higherIndex) {

            if (tempMergArr[i] <= tempMergArr[j]) {

                array[k] = tempMergArr[i];

                i++;

            } else {

                array[k] = tempMergArr[j];

                j++;

            }

            k++;

        }

        while (i <= middle) {

            array[k] = tempMergArr[i];

            k++;

            i++;

        }

 

    }

}




There are different reasons why anyone would use Merge Sort instead of other kinds:

1) it is the most efficient type of sorting for bigger arrays of numbers.

2) it offers performance just bellow Quick Sort but uses less memory space.

Merge Sort Demo.

However it takes more knowledge of coding and a greater understanding of algorithms, and it takes a considerable amount of memory space, if used frequently.

Abstract and Interfaces.

There are ways to assist with inheritance and polymorphism which is done by abstract classes and interfaces these kinds of java files are used for different reasons. They both have abstract methods and those methods cannot be described like normal methods and have to be implemented in any child class of the abstract class and implemented classes.

public interface AnInteface{ // This is how to create an interface.

  public void aMathod(); // This is the way to instaciate an abstract method.

} 

public abstract class AnAbstractClass{ // This is how to create an abstract class.

  public abstract void aMethod(); //This is another way to write an abstract method but abstract 

  // classes can also implement non-abstract methods.

 public String aString(){

  String a = "A string";

  return a;

 }

}

There are two ways to implement these classes to another class:

public class NormalClass extends AnAbstractClass implements AnInterface{

 public void AMethod(){

 }// You have to incorporate the same abstract methods in a class that implements the interface or          // extends the abstract class 

}

Encapsulation and Polymorphism

Encapsulation is a term in Java programming and in general programming that refers to the privatization of files for classes and encapsulation of packages, there are different encapsulation key words: public, private and protected. they all do different things to your variables, classes, or methods. the public word allows to access a class, variable, or methods from one another if they are imported. The private keyword does not allow the interchange of classes, methods or variables even if they are imported. And the protected keyword only allows for methods, classes and variables to be used only within the same package. The way to import packages in any IDE from one class file is the keyword 'import' followed by a space. Ex: import java.util.*; the java.util library imports many very useful classes to the class to which you use this on, such as Random which has methods that create random numbers, or the Scanner class which reads user input from the console and gives it to a variable.
Plymorphism is a way to import classes in Java and assign them to another class as a child of that class, the best way to understand this is - Imagine that you have a dog, well that dog is an animal, but not all animals are dogs. In Java the way to assign an object as a sub-variable of another object is polimorphism.
the way to do that is to use the extends keyword it is used like this:
public class Cats extends Animals()
// all of the public methods from Animals will be usable in the Cats class.

System.out.print("Strings");

Strings are identifiers that are composed of one or many char identifiers, they can hold a big amount of characters and can be modified within a program in an amount of ways. a usual string is created in this way: String str1 = "Hello World";   It is very important that String is capitalized, that it's variable is stated with a quotation mark for both starting and beginning the string and that it ends with a semicolon.
Print method.
The print method is a method from the java.util.Object class in java that is the main class from which you import statements and methods from. It allows you to use String, String manipulation methods, the Print methods, and other equally important methods.
The print method is stated like this:
System.out.print("This is a print");
This will print the words "This is a print" to console. This is often used outside of the Graphic interfaces of programs for debugging purposes.
There are different methods that are used to manipulate String objects: charAt(), substring(), contains(), indexOf(), concat(), equals(), length(), trim(), etc.
These are used for different purposes, charAt(int) will find a char type data at a certain index from the string on which is used, it has parameters of int which means that you need to introduce an int and it will find the character from that position.
The method substring(int, int) requires two integers one for the beginning of the sub string and one for the end, it will take a chunk from between the two given indexes.
The method contains(String) has a String parameter and will find if the string it's used on has a sub string with the exact composition of the parameter string and will give it back as a boolean.
To find an index of a sub-string or a char then you use the indexOf(String) method which will return an int.
The method concat(String) allows you to concatenate two strings together, it will add the parameter string into the string to which the method is used.
equals(String) is the most secure way to see if two strings are equal to each other instead of using the == identifier.
length() will return an int with the number of characters that are in a single string variable.
trim() will erase from a string any excess white space from before or after a string.
REMEMBER: every time you modify a String object the String class does not dump the previous object instead it creates a new one and saves the other one.

String a = "\'a small fox sleeps\'"; if(a.contains("fox")){System.out.print(a);}
This will print: 'a small fox sleeps' to the console.

Apart from manipulating Strings with those three methods it is also possible to turn other primitive data to Strings. The method toString() from the class of any primitive data will turn it to Strings:
Integer.toString(), Double.toString(), Boolean.toString(), etc.

Loops.

The while and for loops are the most important types of loops, and the most commonly used, there is also an advanced for loop called the for each loop but arrays are needed for it.
The while loop is a loop that keeps doing the code inside it until it's conditional is false.

boolean end = false;
int point = 0;
while(!end){
point ++;
if(point > 10){
end;
}
}
//this will run the loop until points is 10

As well as there being a for loop which runs the loop until the conditional say x < 10 reaches false, but is written differently than the while loop and is more controllable.

int point = 0;
for(int i = 0; i < 10; i++){
point++;
System.out.println(point);
}
//this program will run the loop ten times and will print on the console numbers from 1 to 11
// this is because the value of point is increased before the print statement

Loops are allowed to be inside of each other and you can use multiple combinations of the code to achieve different outcomes.

while(!end){
for(int i = 0; i < 10; i++){
point++;
System.out.print(point);
}
System.out.println();
if(point > 30){
end = true;
}
}
//this will run the loops until point is 30, the for loop will run 30 times and the while loop 3 times

For the next loop you will need to know about arrays, which is another post in this blog. After knowing about arrays you can use the for each loop, which loops for every array object or value.

int[] array = new int[4];
for(int arr : array){
arr = 3;
}
//this sets every int of the array to the number 3
//for-each loops are useful for scrolling around arrays quickly

String[] array = {"This", "is", "an", "array"}

Arrays and matrices are essential in Java programming, and programming in general. they are groups of variables or objects that hold different data in them.
a single array holds as many variables of the same data requirements that you need.

int[] array = new int[7];
int x = 1;
for(int i : array){
 i = x;
x++;
}
//this program will create an array of integers

There are different ways to instantiating an array
String[] names = {"Edward", "Joseph", "Smigla"};
for(String name : names){
System.out.print(name + " ");
}
//this code will print: Edward Joseph Smigla on the console
//you can also pull any variable from an array using the index
names[1] = "E.";
names[1] = "J.";
names[1] = "Shmigs";
for(String name : names){
System.out.print(name + " ");
}//this code will print: E .J. Shmigs on the console
//the index of the arrays is in programmer numbers 0 - 2 for arrays with 3 indexes
//always decreasing the index number by one from the amount of indexes

you can also create arrays of objects such as this:
public Integer[] array = new Integer[32];
for(int i = 0; i < array.length(); i++){
array[i] = (int)(Math.random()*32);
}//Remember to cast because Math.random() give a double //variable
// you do not need to use the for-each loop to cycle through arrays, you can also use the length()
//method, which finds the length of the array given  and returns it

You can also create arrays of arrays which are called matrices or 2D arrays (you can create as many arrays of arrays as needed)

int[][] numbers = new int[5][5];//this will create a 2D array containing 25 int variables
for(int i = 0; i < array.length(); i++){
for(int j = 0; j < array[i].length(); j++){
array[i][j] = (int)(Math.random() * 25);
System.out.print(array[i][j] + " ");
}
System.out.println();
}//to give values to multiple index arrays you need multiple loops
//this will print random values from 0 - 24, 25 times (with a space in between and a new line every //five variables)

Matrices can be instantiated in different ways like the simple array.

int[][] numbers = {{1,2,3},{4,5,6},{7,8,9}};
//this instantiates a 2D array with the variables 1,2,3,4,5,6,7,8, and 9. in different levels of the array

If, Else Statements, and Switch.

If are ways to chose conditionals this can relate directly into real life, for example the problem about milk and eggs, when you go to the store and want to buy milk and eggs you first have to know whether there are milk and eggs. This problem in code would be solved with conditional statements:
int milk = 0;
boolean eggs = true;
if (eggs){
milk = 6;
}
//That indicates what the programmer in the picture thought of //when going to the store
else{
milk = 1;
}
After this you have to make up an alternative to your condition just to make sure that if the condition is false then you are able to get a controlled result this is achieved with an else statement, for the else statement you do not need a condition because one was already stated, however you can use another if statement to add a condition.
int milk = 0;
int eggs = 20;
if (eggs > 0){
milk = 6;
}else if (eggs < 0){
milk = 1;
}
//This will identify that if there is more than 0 eggs there will be 6 bottles of milk, however if there is //no eggs the you'll only buy 1 bottle of milk.
Switch statements help identify different conditionals.
int milk = (int)(Math.random() * 6);//Will randomize the amount of milk from 0-6, remember casting
int eggs;
switch(milk){
case 1: eggs = 1;
          break;
case 2: eggs = 2;
          break;
case 3: eggs = 3;

default : eggs = 4;
          break;
}
This conditional will check the value of milk and will compare it to the different cases that it's given, which will help indicate what block of code to activate, the "break" statement helps finishing the statement, without it the program would do the cases under the one without break until finding a break or a default statement, for example in the case 3 it would assign eggs to the value of 3, but then it would do the default case which assign eggs to 4.

public class JavaClass { public static void main(String[]args){ } }

Classes and Methods:
A class is a set of rules that will define an object in a later program, they are made to hold methods which are consequently the rules that formulate every object and these methods are written with different parameters and manipulated to benefit the code.
Ex. of a class:
public class Blog{

}
this will be what contains the rules and settings to create an object called Blog. They are usually started with a capital letter and are written with brackets which are what holds the code inside of the class.
Ex. of methods:
public class Blog{
int amountOfPosts
  public static void main(String[]args){

  Blog myBlog = new Blog(2);
  Blog myBlog2 = new Blog();

  //Will do the same as myBlog
  myBlog.setAmountOfPosts(3);

  }
  public Blog(){
  }
  public Blog(int a){
  amountOfPosts = a;
  }
}
the main method is the most important method in Java programming, it helps define where a program starts and inside of it's brackets is where the program should be defined for the JRE(Java Run-time Environment) to read and run the program. The parameter of String[]args inside the parenthesis next to main is in itself quite complicated, it is an array of Strings that are assigned to variable args, it helps the program know that it is running.
public Blog() is a method that is defined as a Constructor, it defines what an object of class Blog must have when it is defined. In this case myBlog is an object of class Blog and has an amountOfPosts of two, this is assigned in the parenthesis next to Blog when creating a new object
and is ultimately assigned on the constructor, if the parameter of an object is empty such as myBlog2 it will use the constructor that has the same parameters.
There are different ways of writing a method that have to do with encapsulation (private, public, protected) but there is different types of methods as well depending on whether you want a return or not.
public void setAmountOfPosts(int a){
  amountOfPosts = a;
}
this method will not give you a return because is a void method, this means that it does not by itself hold to any data but it will modify the program in any way you want
public static String getATitle(String t){
int title = t;
return title;
}
this is a method that requires a return because it holds on to data, in this case is a String data. It is not required to use the static modifier but for this type of method it is recommended because of recursion (Will probably write another post about this theme.)

int primitiveData = 0;

In Java programming, Primitive Data is the most basic storage of data. Java is an Object oriented programming language, which means that most of the programming will be aimed towards and shaped around creating and modifying objects. The first step for that is Primitive Data.
There are several different ways to identify primitive data with variables. Java like every other programming language requires an IDE(Integrated Development Environment) which is any text editor and a Compiler such as Eclipse (http://www.eclipse.org/downloads/) to build and test programs, every IDE will need a copy of the Java Development Kit (JDK) (http://www.oracle.com/technetwork/java/javase/downloads) to access it download it and the IDE should automatically find it. I personally enjoy using Eclipse because as well as being a compiler it is also an IDE as well. Every line of code should be finished with a semicolon and any comment that you want to make shall be written with two forward slashes Ex: //This is a comment.
Different ways to identify variables in Java:
int (This identifies a variable to an integer or whole number. Ex: int x = 10; this will set variable x as an integer to the number 10, creating primitive data.)
boolean (This identifier will set a variable to either a true or false, the use of this is varied but it is mostly used for when a loop or a conditional requires a boolean, but is also not necessary at that point either. Ex: boolean end = false; <= Will set variable end to false)

For the next Identifiers you'll need to understand a basic concept of bit data, which means that a bit is a number in binary system (a 0 or 1), this makes up everything in programming, but it is unnoticeable. a byte is a gathering of 8 bits and everything is saved on bytes.
double (This identifier will set a variable to any rational number and can reach up to 64 bits or 8 bytes. Ex: double x1 = 10.32; It is also not necessary to always set an identifier with a number value as a number but you are also allowed to utilize operations such as x1 = 2.3 * 14; which will output 32.2, if an int is used for this same operation it will round to the nearest whole number it will set x1 to 32.)
float (This identifier will set a variable just like a double but is then used for different purposes, and is saved as 32 bits or 4 bytes.)

The next identifier deals with characters and it consequently called a char, a char identifier is used for single characters that can be typed on your keyboard. (Ex. char a = 'A'; to identify a char variable you have to type whichever character you want to assign the variable to inside single quotes.)
Strings: a string identifier is a special one because it is a combination of a seemly unlimited amount of char identifiers. It allows for an extensive use of characters. (Ex. String b = "This is String b :)";)
there are multiple special characters that you cannot use within strings but that will be talked about at a later time.