day-2

Day 2

Access Modifiers

Used to set the access level for classes, attributes, methods and constructors.

• Default

String name = "Amigoscode"

• Private Only accessible inside same class

private String name = "Amigoscode"

• Public The code is accessible for all classes

public String name = "Amigoscode"

• Protected Accessible in the same package and subclasses.

protected String name = "Amigoscode"

Methods

A method is a block of code which only runs when invoked using (). We can use methods to reuse code that can be shared across the entire application.

Methods are composed of

• Access Modifier

• Return type

• Name

• Optional arguments

• Method body

• Optional return value

For example

/* 
- Access Modifier = Public
- Return type = String
- Name = getName
- Optional arguments = () empty 
- Method body = 
- Optional return value = "Amigoscode"
*/
public String getName() {
    return "Amigoscode";
}

/* 
- Access Modifier = Default
- Return type = String
- Name = getName
- Optional arguments = (int numberOne, int numberTwo) 
- Method body = {}
- Optional return value = "Amigoscode"
*/
String add(int numberOne, int numberTwo) {
    return numberOne + numberTwo;
}

/* 
- Access Modifier = Public
- Return type = void means return nothing
- Name = getName
- Optional arguments = () empty 
- Method body = Prints Amigoscode
- Optional return value = Nonne
*/
public void printBrand() {
    System.out.println("Amigoscode");
}

Enums

An Enum is a type used specifically to represent sets of constants. For example, MALE or FEMALE.

To create an Enum is as following:

public class Main {

    enum Gender {
        MALE,
        FEMALE
    }

    public static void main(String[] args) {
        Gender male = Gender.MALE;
        Gender female = Gender.FEMALE;
    }

}

Working with Dates

Java has a great API for working with dates. You should be aware of these classes:

• LocalDate

• LocalDateTime

• LocalTime

package com.amigoscode;

import java.time.LocalDate;
import java.time.LocalDateTime;
import java.time.LocalTime;

public class Main {
    public static void main(String[] args) {
        LocalDate localDate = LocalDate.now();
        LocalDateTime localDateTime = LocalDateTime.now();
        LocalTime localTime = LocalTime.now();
    }
}

Explore the methods available for each classes.

Error Handling

When building applications things can go wrong and it is best practice to handle scenarios where our code does not behave as expected. Or simply deal with errors that we can anticipate.

I java we errors are called Exceptions and we can deal with them using the following constructs:

• The try statement allows you to define a block of code to be tested for errors while it is being executed.

• The catch statement allows you to define a block of code to be executed, if an error occurs in the try block.

try {
  //  Block of code to try
}
catch(Exception e) {
  //  Block of code to handle errors
}

Checked Exceptions

Checked exceptions represent errors outside the control of the program. For example reading a file. You need to handle this error during compile time otherwise your code will not compile.

File file = new File("file.txt");
try {
    FileInputStream stream = new FileInputStream(file);
} catch (FileNotFoundException e) {
    e.printStackTrace();
}

Unchecked Exceptions

If a program throws an unchecked exception, it reflects some error inside the program logic. For example, if we divide a number by 0, Java will throw ArithmeticException:

System.out.println(10 / 0);

Working with files

package com.amigoscode;

import java.io.*;
import java.util.Scanner;

public class Main {
    public static void main(String[] args) throws IOException {
        // create file
        File file = new File("src/hello.txt");
        if (!file.exists()) {
            file.createNewFile();
        }

        FileWriter fileWriter = new FileWriter(file);
        PrintWriter printWriter = new PrintWriter(fileWriter);
        printWriter.println("hello");

        // once you are done writing to the file
        printWriter.flush();
        printWriter.close();

        // read example 
        Scanner scanner = new Scanner(file);
        while (scanner.hasNext()) {
            System.out.println(scanner.nextLine());
        }
    }
}