Lesson: Constructor

 A constructor is a special method that is used to initialize objects.

*** Obyekt yaradilan zaman ilk ishe dushen hisse constructor sayilir.

Ashagidaki misal clean code deyil.

package org.example;

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

    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;
    }
}

DRY:

package org.example;

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

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

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

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

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

Tree tree1 = new Tree("Pine", 2.0F);
tree1.printInfo();

Tree tree2 = new Tree("Oak");
tree2.printInfo();

Tree tree3 = new Tree(8.0F);
tree3.printInfo();

Tree tree4 = new Tree();
tree4.printInfo();

-- this() keyword constructor-daki ilk cagrilan hisse olmalidir, eks halda compile time error verecek:

package org.example;

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

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

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

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

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

    public void printInfo() {
        System.out.println("Type is : " + type + ", and height is : " + height);
    }
}

Niye bu problem yaradir?

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

Ferz edinki obyekti yaradib bitirmeden biz onun fieldini cap edirik.

-- Lakin bele kod(menasiz) yaza bilerik:

public Tree(float newHeight) {
    System.out.println("height : " + height);
    type = "Pine";
    height = newHeight;
}

-- Recursive constructor call:

package org.example;

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

    public Tree(String name) {
        this(name, 5.5F);
    }

    public Tree(String name, float newHeight) {
        this(name);   
    }
}

-- this()

package org.example;

public class Tree {
    public float height;

    public Tree grow() {
        height++;
        return this;
    }
}

Tree tree = new Tree();
tree.grow();
System.out.println(tree.height);

-- Excellent example

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;
    }
}

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;
    }
}

package org.example;

public class Main {
    public static void main(String[] args) {
        Customer c1 = new Customer(1, "Parvin");
        CreditCard cc1 = new CreditCard("1", 1000);
        cc1.setOwner(c1);
        c1.setCc(cc1);

        // Alternatively
        Customer c2 = new Customer(2, "Ali");
        CreditCard cc2 = new CreditCard("2", 2000, c2);
    }
}