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Java All About

List of the Java class

01. Hello World Program Hello

  • Code example

    public class Hello {
public static void main(String[] arg) {
    System.out.println("Hello World");
    }
}

02. Comments in Java Comments

  • Code example

    public class Comments {
// To returning my name
public static String name() {
        return "Sabbir Hossain"; // it's return name as string
    }

/*
* For showing my name
*/
public static void main(String[] arg) {
        System.out.println(name());
    }
}

03. Variable in Java Variable

The variable syntax looks like this.

type variableName = value;

Types of varibale

  • The first is a type: type is means which kind of data you want to store, on this varibale. based on your went to stored value, you have to call a type like

    1. String - stores text, such as "Hello". String values are surrounded by double quotes or single quote.
    2. int - stores integers (whole numbers), 111, -111
    3. float - stores number with decimals like: 11.34, -55.10
    4. char - Single characters like: a or "A"
    5. boolean - Store true or false value only

    Code example

    public class Variable {
  public static void main(String[] arg) {
      /*
       * Variable in java
       */
      String friendName = "Rafiul"; // this is varibale to store friend name
      System.out.println(friendName);

      final int myNumber = 20;
      System.out.println(myNumber);

      // perform mathmetical task
      int fNumber = 20;
      int lNumber = 10;
      // make a varibale for storing result inside the result variable
      int result = fNumber + lNumber; // answer should be 30
      System.out.println(result);
  }
}

04 Data type data-type

Java data type

04.1 Number Data Type Number

05 Defarent between primitive & Non-Primitive

Primitive Non-Primitive
Primitive types are predefined (already defined) in Java Non-primitive types are created by the programmer and is not defined by Java (except for String)
Non-primitive types can be used to call methods to perform certain operations while primitive types cannot
A primitive type has always a value while non-primitive types can be null
A primitive type starts with a lowercase letter while non-primitive types starts with an uppercase letter

05. Java String

Strings are used for storing text.

  • 05.1 String Length

    A String in Java is actually an object. which contain methods that can perform certain operations on strings. For example, the length of a string can be found with the length() method:

    String txt = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
System.out.println("The length of the txt string is: " + txt.length());

  • 05.2 More String Methods There are many string methods available, for example toUpperCase() and toLowerCase():

    String txt = "Hello World";
System.out.println(txt.toUpperCase());   // Outputs "HELLO WORLD"
System.out.println(txt.toLowerCase());   // Outputs "hello world"

  • 05.3 Finding a Character in a String The indexOf() method returns the index (the position) of the first occurrence of a specified text in a string (including whitespace):

    String txt = "Please locate where 'locate' occurs!";
System.out.println(txt.indexOf("locate")); // Outputs 7

36. Java Method

Java method work as like same as normal method that can help us to perform any kind of task

Deffrent between public & Static Keyword

  • Public
    • Public methods must be called by creating objects
  • Static
    • Static methods can be called without creating objects

Java Polymorphism

The word polymorphism means having many forms. In simple words, we can define polymorphism as the ability of a message to be displayed in more than one form.

Real-life Illustration Polymorphism: A person at the same time can have different characteristics. Like a man at the same time is a father, a husband, and an employee. So the same person possesses different behavior in different situations. This is called polymorphism.

What is Polymorphism in Java?

Polymorphism is considered one of the important features of Object-Oriented Programming. Polymorphism allows us to perform a single action in different ways. In other words, polymorphism allows you to define one interface and have multiple implementations. The word “poly” means many and “morphs” means forms, So it means many forms.

Types of Java polymorphism

In Java polymorphism is mainly divided into two types:

  • Compile-time Polymorphism

    Compile-Time Polymorphism

    It is also known as static polymorphism. This type of polymorphism is achieved by function overloading or operator overloading.

    Note: But Java doesn’t support the Operator Overloading. Function Overloading

    class JavaPolymorphism {
  public static void main(String[] args) {
      System.out.println(Helper.multiply(2, 3));
      System.out.println(Helper.multiply(2.3, 3.8));
  }
}

class Helper {
    static int multiply(int a, int b) { // we reveive 2 number a and b which is integer types
        return a * b;
    }

    static double multiply(double a, double b) {
        return a * b;
    }
}

    Output

    6
1.1399999999999992

  • Runtime Polymorphism

java basic