Constructor

 Constructor - A constructor in Java is similar to a method that is invoked when an object of the class is created. 

Not clean version:

package main;

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

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

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

    public Tree(double newHeitht) {
        type = "Pine";
        height = newHeitht;
    }

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

Clean version:

package main;

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

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

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

    public Tree(double newHeitht) {
//        type = "Pine";
//        height = newHeitht;
        this("Pine", newHeitht);
    }

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

*this keyword-u birinci gelmelidir, eks halda compile time xeta alacayiq. 
---------------------------------------------------------------------------------------------
menasiz kod parcasi:
    public Tree(double newHeitht) {
        System.out.println("height " + newHeitht);
        type = "Pine";
        height = newHeitht;
//        this("Pine", newHeitht);
    }
---------------------------------------------------------------------------------------------
*Tricky
package main;

public class CreditCard {
    String no;
    double balance;

    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 main;

public class Customer {
    int id;
    String name;

    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 main;

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

        // Alternatively
        Customer c2 = new Customer(2, "Kemal");
        CreditCard cc2 = new CreditCard("2", 2500, c2);
        System.out.println(c2);
    }
}