SE_21_Lesson_13: Exceptions, Enums and Memory

-> Exception olanda kod davam elemir

package org.example.part4;

public class Main {
    public static void main(String[] args) {
        System.out.println("Start");
        System.out.println(5 / 0);
        System.out.println("Continue...");
    }

} 


-> try catch bloku

package org.example.part4;

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        System.out.println("Start");
        Scanner in = new Scanner(System.in);
        int a = in.nextInt();
        int b = in.nextInt();
        try {
            System.out.println(a / b);
        } catch (Exception e) {
            // if exception happens trigger this block
            System.out.println("Something went wrong");
        }
        System.out.println("Continue...");
    }
}


-> Oz Exception klasimizi yaratmaq

package org.example.part4;

public class MyException extends Exception {

    public MyException(String message) {
        super(message);
    }
}


-> Multiple catch bloku

package org.example.part4;

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        System.out.println("Start");
        Scanner in = new Scanner(System.in);
        int a = in.nextInt();
        int b = in.nextInt();

        try {
            System.out.println(a / b);
        } catch (ArithmeticException e) {
            e.printStackTrace();
        } catch (NullPointerException e) {
            e.printStackTrace();
        } catch (ArrayIndexOutOfBoundsException e) {
            e.printStackTrace();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}


-> Bir nece exception-u bir blokda tutmaq

package org.example.part4;

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        System.out.println("Start");
        Scanner in = new Scanner(System.in);
        int a = in.nextInt();
        int b = in.nextInt();

        try {
            System.out.println(a / b);
        } catch (ArithmeticException | NullPointerException | ArrayIndexOutOfBoundsException e) {
            e.printStackTrace();
        }
    }
}


-> RuntimeException & Non RuntimeException. Checked & Unchecked exceptions

1. Unchecked Exception. RuntimeException

package org.example.part4;

public class MyException extends RuntimeException {

    public MyException(String message) {
        super(message);
    }
}
package org.example.part4;

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        System.out.println("Start");
        throw new MyException("something went wrong");
        System.out.println("Continue...");
        System.out.println("End");
    }
}


2. Checked exception. Non RuntimeException

package org.example.part4;

public class MyException extends Exception {

    public MyException(String message) {
        super(message);
    }
}
package org.example.part4;

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        System.out.println("Start");
        throw new MyException("something went wrong");
        System.out.println("Continue...");
        System.out.println("End");
    }
}


-> Checked ve Unchecked exception-lar metodda

1. Unchecked

package org.example.part4;

public class MyException extends RuntimeException {

    public MyException(String message) {
        super(message);
    }
}
package org.example.part4;

public class Main {
    public static void main(String[] args) {
        foo();
    }

    public static void foo() {
        throw new MyException("exception");
    }
}


2. Checked

package org.example.part4;

public class MyException extends Exception {

    public MyException(String message) {
        super(message);
    }
}
package org.example.part4;

public class Main {
    public static void main(String[] args) {
        foo();
    }

    public static void foo() {
        throw new MyException("exception");
    }
}


-> Metodun icinde bir exception atmaq, lakin tutanda bashqa exception tutmaq

package org.example.part4;

import java.sql.SQLException;

public class Main {
    public static void main(String[] args) throws SQLException {
        foo();
    }

    public static void foo() throws Exception {
        throw new SQLException("Can not connect to DB");
    }
}

Lakin:

package org.example.part4;

import java.sql.SQLException;

public class Main {
    public static void main(String[] args) throws Exception {
        foo();
    }

    public static void foo() throws Exception {
        throw new SQLException("Can not connect to DB");
    }
}

Niye bele bash verir? Cunki polymorphisme gore SQLException bir Exceptiondur, demeli Exception teleb olunan yere SQLException gondermek olar. Lakin main metoddaki foo() metodunu cagirdiqda o anlamir ki ona gonderilen mehz SQLException -un obyektidir ona gorede problem yaradir.


-> Calishmaq lazimdir ki, Exception atmayaq, atdigimiz Exception daha spesifik olmalidir. (According to Joshua Block - Effecctive Java)

package org.example.part4;

import java.io.IOException;
import java.sql.SQLException;

public class Main {
    public static void main(String[] args) {
        try {
            foo();
        } catch (SQLException | IOException exception) {
            exception.printStackTrace();
        }
    }

    public static void foo() throws SQLException, IOException {
        throw new IOException("Input Output Exception happened");
    }
}

Bu zaman method signature-ye baxan developer aninda anlamalidir ki, metodda ne problem bash vere biler. Eksi oldugu halda Exception-un ne oldugunu anlamir.


-> @SneakyThrows (Lombok) annotation.

The @SneakyThrows annotation from Lombok allows you to throw checked exceptions without using the throws declaration. This comes in handy when you need to raise an exception from a method within very restrictive interfaces like Runnable.

@SneakyThrows
public static void bar() {
    System.out.println("bla bla bla");
    throw new Exception();
}




*** addition:

package com.appsdeveloperblog.lesson33;

import java.util.Random;

public class Main {
    public static void main(String[] args) {
        try {
            riskyMethod1();
            riskyMethod2();
            riskyMethod3();
        } catch (RuntimeException e) {
            throw new RuntimeException("Something bad happened in main");
        }
    }

    public static void riskyMethod1() {
        Random random = new Random();
        if (random.nextBoolean()) {
            throw new RuntimeException("Something bad happened");
        }
    }

    public static void riskyMethod2() {
        Random random = new Random();
        if (random.nextBoolean()) {
            throw new RuntimeException("Something bad happened");
        }
    }

    public static void riskyMethod3() {
        Random random = new Random();
        if (random.nextBoolean()) {
            throw new RuntimeException("Something bad happened");
        }
    }

}


solution: 

package com.appsdeveloperblog.lesson33;

import java.util.Random;

public class Main {
    public static void main(String[] args) {
        try {
            riskyMethod1();
            riskyMethod2();
            riskyMethod3();
        } catch (RuntimeException e) {
            throw new RuntimeException("Something bad happened in main", e);
        }
    }

    public static void riskyMethod1() {
        Random random = new Random();
        if (random.nextBoolean()) {
            throw new RuntimeException("Something bad happened");
        }
    }

    public static void riskyMethod2() {
        Random random = new Random();
        if (random.nextBoolean()) {
            throw new RuntimeException("Something bad happened");
        }
    }

    public static void riskyMethod3() {
        Random random = new Random();
        if (random.nextBoolean()) {
            throw new RuntimeException("Something bad happened");
        }
    }

}



----------------------------------------------------------------------------------------------------------------------------------------------------------

-> Constructorsuz enumlar

package org.example;

public enum Operation {

    SUM, SUBTRACT, MULTIPLY, DIVIDE, MODULUS
}

package org.example;

public class Main {
    public static void main(String[] args) {
        Operation operation = Operation.SUM;
        System.out.println(operation);
        System.out.println(operation.name());
        System.out.println(operation.ordinal());
        System.out.println(operation.toString());
    }
}


-> Constructorlu enumlar

package org.example;

public enum Operation {
    SUM('+'),
    SUBTRACT('-'),
    MULTIPLY('*'),
    DIVIDE('/'),
    MODULUS('%'),
    DEFAULT();

    private char c;

    Operation() {
    }

    Operation(char c) {
        this.c = c;
    }

    public char getC() {
        return c;
    }
}

package org.example;

public class Main {
    public static void main(String[] args) {
        Operation operation = Operation.SUM;
        System.out.println(operation);
        System.out.println(operation.name());
        System.out.println(operation.ordinal());
        System.out.println(operation.toString());
        System.out.println(operation.getC());
    }
}


-> Enumlarin ustunluyu

package org.example;

public enum Operation {
    SUM('+'),
    SUBTRACT('-'),
    MULTIPLY('*'),
    DIVIDE('/'),
    MODULUS('%'),
    DEFAULT();

    private char c;

    Operation() {
    }

    Operation(char c) {
        this.c = c;
    }

    public char getC() {
        return c;
    }

    public static Operation getOperation(char c) {
        if (c == '+') return Operation.SUM;
        else if (c == '-') return Operation.SUBTRACT;
        else if (c == '*') return Operation.MULTIPLY;
        else if (c == '/') return Operation.DIVIDE;
        else if (c == '%') return Operation.MODULUS;
        else return Operation.DEFAULT;
    }
}
package org.example;

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        char c = in.nextLine().charAt(0);
        Operation operation = Operation.getOperation(c);
        
    }

    public static Double calculate(Double a, Double b, Operation operation) {
        if (operation == Operation.SUM) {
            return a + b;
        } else if (operation == Operation.SUBTRACT) {
            return a - b;
        } else if (operation == Operation.MULTIPLY) {
            return a * b;
        } else if (operation == Operation.DIVIDE) {
            return a / b;
        } else if (operation == Operation.MODULUS) {
            return a % b;
        } else return null;
    }
}


-> Enumlar arxa planda nece ishleyir

package enums;

public final class Operation2 {

    public static final Operation2 SUM = new Operation2('+');
    public static final Operation2 SUBTRACT = new Operation2('-');
    public static final Operation2 MULTIPLY = new Operation2('*');
    public static final Operation2 DIVIDE = new Operation2('/');
    public static final Operation2 DEFAULT = new Operation2();

    private char c;

    private Operation2() {
    }

    private Operation2(char c) {
        this.c = c;
    }

    public char getC() {
        return c;
    }

}
package org.example;

public class Main2 {
    public static void main(String[] args) {
        Operation operation = Operation.SUM;
        Operation2 operation2 = Operation2.SUM;
    }
}



-> Some examples:

package org.example.part5;

public enum Numbers {
    ZERO(0, "Sifir", "Zero"),
    ONE(1, "Bir", "One"),
    TWO(2, "Iki", "Two"),
    THREE(3, "Uc", "Three"),
    FOUR(4, "Dord", "Four"),
    FIVE(5, "Besh", "Five"),
    SIX(6, "Alti", "Six"),
    SEVEN(7, "Yeddi", "Seven"),
    EIGHT(8, "Sekkiz", "Eight"),
    NINE(9, "Doqquz", "Nine"),
    TEN(10, "On", "Ten"),
    UNDEFINED;

    private Integer value;
    private String nameAze;
    private String nameEng;

    Numbers() {
    }

    Numbers(Integer value, String nameAze, String nameEng) {
        this.value = value;
        this.nameAze = nameAze;
        this.nameEng = nameEng;
    }

    public Integer getValue() {
        return value;
    }

    public String getNameAze() {
        return nameAze;
    }

    public String getNameEng() {
        return nameEng;
    }

//    public static Numbers getNumber(Integer number) {
//        return Arrays.stream(Numbers.values())
//                .filter(n -> n.getValue().equals(number))
//                .findFirst()
//                .orElse(Numbers.UNDEFINED);
//    }

    public static Numbers getNumber(Integer number) {
        return number >= 0 && number < 11 ? Numbers.values()[number] : Numbers.UNDEFINED;
    }
}


-----------------------------------------------------------------------------------------------------------------------------

 Heap - JVM terefinden obyekt yaratmaq ucun iwlenilen yaddaw yeridir. Buna dynamic memory area da deyilir. Normalda stack - dan daha boyukdur. 

Stack - LIFO(last in first out) prinsipi ile iwleyir. Stack proqramda o anda caliwan, aktiv olan metodlar ucun ayrilan yaddawdir. main-den davam ederek, cagrilan her yeni metod ucun stackda uste yeni bir pencere acilir. Metod iwini bitirende, onu cagiran metoda qayidarken stack-daki pencereleride silir. Stack-daki yaddaw pencereleri onun ucun acilan metodun parametrlerini local deyiwenlerini saxlayir. One gore de local variable -larina stack variable deyilir. 


*** Bir variable eger obyektin bir hissesidirse yeni object variable - dirsa(field) bu zaman heap yaddawdadir. Yox eger local variable-dirsa stack yaddawdadir. 


Mes: 

Car car = new Car();

car.make = "Mercedes";

car.model = "C200";

int speed = car.speed;

Burada: speed, car - stackda yerlewir, Car obyekti ise heap yaddawdadir, hemcinin ona mensub olan make, model de heap da yerlewir. 

Obyektin hissesi olan instance varialbe-lar tipden asili deyil daima heapda olur. Local variable-lar tipden asili olmadan daima stack yaddawda olur. 


-----------------------------------------------------------------------------------------------------------------------------Application start olduqda klasslar haqqinda butun informasiya class loader terefinden metaspace yazilir. Metaspace Java memory-nin bir hissesidir. Java memory ozu 3 hisseye bolunur: stack, heap ve metaspace.  Class variable(static) saxlanilacaq metaspace-de. Eger primitiv variable - dirsa o zaman hem ozu hem deyeri metaspacede saxlanilacaq. Eger reference variable-dirsa o zaman variable ozu metaspace-de lakin deyeri heap yaddawda saxlanilacaq. 

-----------------------------------------------------------------------------------------------------------------------------






Metaspace.

Metodlarin kodlari, JVM-in klaslari yuklediyi metaspace deyilen yaddawda tutulur. 



-> How Garbage Collector works

package org.example;

public class Main {
    public static void main(String[] args) {
        Person one = new Person();
        Person two = one;
        one = null;
    }
}

Is object one erased from heap? No. Because object two still refers to the same object.

To prove it we write:

package org.example;

public class Person {
    @Override
    protected void finalize() throws Throwable {
        System.out.println("Person is erased");
    }
}
package org.example;

public class Main {
    public static void main(String[] args) {
        Person one = new Person();
        Person two = one;
        one = null;
        System.gc();
    }
}


package org.example;

public class Main {
    public static void main(String[] args) {
        Person one = new Person();
//        Person two = one;
        one = null;
        System.gc();
    }
}


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