Study Guide

Multi Dimensional Array

• Two dimensional array:
    int[][] twoD_arr = new int[10][20];
  

• Three dimensional array:
    int[][][] threeD_arr = new int[10][20][30];
  

• The total number of elements that can be stored in a multidimensional array can be calculated by multiplying the size of all the dimensions.

  • ```

    public static int[] rowSum(int[][] arr2D){ int[] arr = new int[arr2D.length]; for(int i = 0; i < arr2D.length; i++){ arr[i] = arraySum(arr2D[i]); } for(int i = 0; i < arr.length; i++){ } return arr; }

Substring

• This method has two variants and returns a new string that is a substring of this string. The substring begins with the character at the specified index and extends to the end of this string. And index of substring starts from 1 and not from 0.

•   String str = new String("navodit");
    System.out.println(str.substring(3));
    System.out.println(str.substring(3, 5));
  

  Output:
  odit
  od
  

Interface

• In Java, an interface is an abstract type that contains a collection of methods and constant variables. It is one of the core concepts in Java and is used to achieve abstraction, polymorphism and multiple inheritances.

  • ```java public interface Electronic {

    // Constant variable String LED = “LED”;

    // Abstract method int getElectricityUse();

    // Static method static boolean isEnergyEfficient(String electtronicType) { if (electtronicType.equals(LED)) { return true; } return false; }

    //Default method default void printDescription() { System.out.println(“Electronic Description”); } }

Implements

• The implements’ keyword is useful to implement the interfaces in classes.
• We know that an interface in Java is defined as a special type of class that only contains abstract methods inside it.
• Methods just provides a plan, contract, or a prototype that the classes must follow.

• //Defining the Shape interface
  interface Shape {
  //declaring an abstract method of the interface
  public double getArea();
  }
  //class implements the Shape interface using the implements keyword
  class Circle implements Shape {
  public double radius = 5;
  public double getArea() {
  return 3.142 * radius * radius;
  }
  }
  class Rectangle implements Shape {
      public double length = 5;
      public double breadth = 10;
      //providing implementation to the interface methods
  
  public double getArea() {
  return length * breadth;
  }
  }
  
  public class Demo {
  public static void main(String args[]) {
  Circle c = new Circle();
  double circleArea = c.getArea();
  System.out.println("Area of Circle is: " + circleArea);
  Rectangle r = new Rectangle();
  double rectangleArea = r.getArea();
  System.out.println("Area of Rectangle is: " + rectangleArea);
       }
    }
  

  Output: Area of Circle is: 78.55 Area of Rectangle is: 50.0 ```