Why would you need to use more than one constructor?

Question

I am learning Java as of right now and have just been learned what constructors are. I do not understand why you would need more than one constructor if you need to initialize all variables.

Answer 1

To put it simply, you use multiple constructors for convenience (1st example) or to allow completely different initialization methods or different source types (2nd example.

You might need multiple constructors to implement your class to simply allow omitting some of the parameters that are already setup:

//The functionality of the class is not important, just keep in mind parameters influence it.
class AirConditioner{
   enum ConditionerMode{
      Automatic, //Default
      On,
      Off
   }
   public ConditionerMode Mode; //will be on automatic by default.
   public int MinTemperature = 18;
   public int MaxTemperature = 20;

   public AirConditioner(){ //Default constructor to use default settings or initialize manually.
      //Nothing here or set Mode to Automatic. 
   }

   //Mode
   public AirConditioner(ConditionerMode mode){ //Setup mode, but leave the rest at default
      Mode = mode;
   }
   //setup everything.
   public AirConditioner(ConditionerMode mode, int MinTemp, int MaxTemp){
      Mode = mode;
      MinTemperature = MinTemp;
      MaxTemperature = MaxTemp;
   }
}

---

Another example is when different constructors follow different procedures to initialize the variables. For instance you could have a data table that simply displays a table of text. The constructor could get the data from either database OR a file:

class DataTable{
   public DataTable(){} //Again default one, in case you want to initialize manually

   public DataTable(SQLConnection con, SQLCommand command){
      //Code to connect to database get the data and fill the table
   }

   public DataTable(File file){
      //Code to read data from a file and fill the table
   }
}

Answer 2

A class can have multiple constructors, as long as their signature (the parameters they take) are not the same. You can define as many constructors as you need. When a Java class contains multiple constructors, we say that the constructor is overloaded (comes in multiple versions). This is what constructor overloading means, that a Java class contains multiple constructors.

Having said that, it is completely dependent upon your implementation whether or not you want to create more than one constructor in your class but having more than one constructor can ease your life in many instances. Suppose below class doesn't have a default constructor:

public class Employee {

    private int age;
    private String name;

    Employee(int age, String name){
        this.age=age;
        this.name=name;     
    }
}

So, while creating object of this class user would not be able to do so until he has age and name parameters handy which restricts the true functionality of Java objects as Objects' state should be able to be modified and populated at any time once initialized.