SE_21_Lesson_7: Constructor

Constuctor - is a special method that is used to initialize objects. The constructor is called when an object of a class is created. 


-> Menasiz constructor (dummy contructor):

package org.example;

public class Tree {
    public String type;
    public float height;

    public Tree() {
        type = "Oak";
        height = 1.0F;
    }
}

Bu zaman butun obyektler standart wekil alacaqdir.


-> Smart constructor:

package org.example;

public class Tree {
    public String type;
    public float height;

    public Tree(String newType, float newHeight) {
        type = newType;
        height = newHeight;
    }
}


-> Constructor overloading:

package org.example;

public class Tree {
    public String type;
    public float height;

    public Tree(String newType, float newHeight) {
        type = newType;
        height = newHeight;
    }

    public Tree(String newType) {
        type = newType;
        height = 1.0F;
    }

    public Tree(float newHeight) {
        type = "Pine";
        height = newHeight;
    }

    public Tree() {
        type = "Pine";
        height = 1.0F;
    }
}


-> Constructorlarin bir birini cagirmasi:

package org.example;

public class Tree2 {
    public String type;
    public float height;

    public Tree2(String newType, float newHeight) {
        type = newType;
        height = newHeight;
    }

    public Tree2(String newType) {
        this(newType, 1.0F);
    }

    public Tree2(float newHeight) {
        this("Pine", newHeight);
    }

    public Tree2() {
        this("Pine", 1.0F);
    }
}


-> this keyword constructor-da ilk setirde olmalidir, eks halda compile time error alariq.package org.example;


public class Tree2 {
    public String type;
    public float height;

    public Tree2(String newType, float newHeight) {
        type = newType;
        height = newHeight;
    }

    public Tree2(String newType) {
        this(newType, 1.0F);
    }

    public Tree2(float newHeight) {
        System.out.println(height);
        this("Pine", newHeight);
    }

    public Tree2() {
        this("Pine", 1.0F);
    }
}

Burada obyekti yaradib bitirmeden filed-ini print edirik. 


-> Qarshiliqli recursive this constructorlarda ishletmek olmaz


package org.example;

public class Tree2 {
    public String type;
    public float height;

    public Tree2(String newType, float newHeight) {
        this();
        type = newType;
        height = newHeight;
    }

    public Tree2(String newType) {
        this(newType, 1.0F);
    }

    public Tree2(float newHeight) {
        this("Pine", newHeight);
    }

    public Tree2() {
        this("Pine", 1.0F);
    }
}


->Class-in icinde static int deyisheni yaratmaq ve constructor ile onu 1 vahid artirmaq. Yeni her defe obyekt yaradilanda hemin deyishen artacaq.

package org.example;

public class Test {
    public static int count;

    public Test() {
        count++;
    }
}
package org.example;

public class Main {
    public static void main(String[] args) {
        new Test();
        new Test();
        new Test();
        new Test();
        new Test();

        System.out.println(Test.count);
    }
}


-> this keyword. illegal forward reference problemi ve this keyword-u ile bu problemi aradan qaldirmaq, ve method ile aradan qaldirmaq:

package org.example;

public class ThisDemo {
    int i = k;
    int k = 5;
    int j = 2;
}
package org.example;

public class ThisDemo {
    int i = this.k;
    int k = 5;
    int j = 2;
}
package org.example;

public class Main2 {
    public static void main(String[] args) {
        ThisDemo one = new ThisDemo();
        System.out.println(one.i);
        System.out.println(one.k);
        System.out.println(one.j);
    }
}


package org.example.test;

public class Main {
    int i = getJ();
    int j = 10;

    int getJ() {
        return j;
    }

    public static void main(String[] args) {
        Main main = new Main();
        System.out.println(main.i);
    }
}


Lakin:

package org.example;

public class ThisDemo {
    int k = 5;
    int i = this.k;
    int j = 2;
}

Bu wekilde olduqda cavabda deyiwir.


-> this keywordu metodlarda:

package org.example;

public class ThisDemo {
    int k = 5;
    int i = this.k;
    int j = 2;

    public void foo1() {
        this.foo2();
        System.out.println("foo1");
    }
    
    public void foo2() {
        System.out.println("foo2");
    }
}


-> this keywordunu menali iwledilmesi:

package org.example;

public class ThisDemo {
    int k = 5;
    int i = this.k;
    int j = 2;

    public void foo() {
        int i = this.i;
        System.out.println(i);
    }
}


-> this keywordunun obyektin ozunu qaytarmasi:

package org.example;

public class Person {
    public int height;

    public Person grow() {
        height++;
        return this;
    }
}
package org.example;

public class Main2 {
    public static void main(String[] args) {
        Person person = new Person();
        System.out.println(person.height);
        person = person.grow();
        System.out.println(person.height);
    }
}


*** Bunu yoxlamaq ucun cari adresleini cap etmek kifayet edecek.


-> ***

package org.lesson4;

public class Person {
    public int height;

    public Person grow() {
        height++;
        return new Person();
    }
}
package org.lesson4;

public class Main {
    public static void main(String[] args) {
        Person one = new Person();
        System.out.println(one.height);
        Person two = one.grow();
        System.out.println(one.height);
        System.out.println(two.height);
    }
}


->***

package org.lesson4;

public class Person {
    public int height;

    public Person(int height) {
        this.height = height;
    }

    public Person grow() {
        height++;
        return new Person(height);
    }
}
package org.lesson4;

public class Main {
    public static void main(String[] args) {
        Person one = new Person(5);
        System.out.println(one.height);
        Person two = one.grow();
        System.out.println(one.height);
        System.out.println(two.height);
    }
}




-> this keywordune en gozel numune:

package org.example;

public class Customer {
    public int id;
    public String name;

    public CreditCard cc;

    public Customer(int id, String name) {
        this.id = id;
        this.name = name;
    }

    public void setCc(CreditCard cc) {
        this.cc = cc;
    }

    @Override
    public String toString() {
        return "Customer{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", cc=" + cc +
                '}';
    }
}
package org.example;

public class CreditCard {
    public String no;
    public double balance;

    public Customer owner;

    public CreditCard(String no, double balance) {
        this.no = no;
        this.balance = balance;
    }

    public CreditCard(String no, double balance, Customer owner) {
        this.no = no;
        this.balance = balance;
        this.owner = owner;
        owner.setCc(this);
    }

    public void setOwner(Customer owner) {
        this.owner = owner;
    }

    @Override
    public String toString() {
        return "CreditCard{" +
                "no='" + no + '\'' +
                ", balance=" + balance +
                '}';
    }
}


package org.example;

public class Main2 {
    public static void main(String[] args) {
        Customer c1 = new Customer(1, "Parvin");
        CreditCard cc1 = new CreditCard("1", 2000);
        cc1.setOwner(c1);
        c1.setCc(cc1);
        System.out.println(c1);

        // Alternative
        Customer c2 = new Customer(2, "Narmin");
        CreditCard cc2 = new CreditCard("2", 3000, c2);
        System.out.println(c2);
    }
}

Yuxaridaki numunede 2 setr koddan azad olduq. 


-> == obyektlerin beraberliyinin yoxlanilmasi

package org.example;

public class Main2 {
    public static void main(String[] args) {
        Person person = new Person();
        System.out.println(person.height);
        Person grow = person.grow();
        System.out.println(grow.height);

        System.out.println(person == grow);
    }
}









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