function - Purpose of a constructor in Java?

Question

What is the purpose of a constructor? I've been learning Java in school and it seems to me like a constructor is largely redundant in things we've done thus far. It remains to be seen if a purpose comes about, but so far it seems meaningless to me. For example, what is the difference between the following two snippets of code?

public class Program {    
    public constructor () {
        function();
    }        
    private void function () {
        //do stuff
    }    
    public static void main(String[] args) { 
        constructor a = new constructor(); 
    }
}

This is how we were taught do to things for assignments, but wouldn't the below do the same deal?

public class Program {    
    public static void main(String[] args) {
        function();
    }        
    private void function() {
        //do stuff
    }
}

The purpose of a constructor escapes me, but then again everything we've done thus far has been extremely rudimentary.

Answer 1

Constructors are used to initialize the instances of your classes. You use a constructor to create new objects often with parameters specifying the initial state or other important information about the object

From the official Java tutorial:

A class contains constructors that are invoked to create objects from the class blueprint. Constructor declarations look like method declarations—except that they use the name of the class and have no return type. For example, Bicycle has one constructor:

public Bicycle(int startCadence, int startSpeed, int startGear) {
    gear = startGear;
    cadence = startCadence;
    speed = startSpeed;
}

To create a new Bicycle object called myBike, a constructor is called by the new operator:

Bicycle myBike = new Bicycle(30, 0, 8);

new Bicycle(30, 0, 8) creates space in memory for the object and initializes its fields.

Although Bicycle only has one constructor, it could have others, including a no-argument constructor:

public Bicycle() { gear = 1; cadence = 10; speed = 0; }

Bicycle yourBike = new Bicycle(); invokes the no-argument constructor to create a new Bicycle object called yourBike.

Answer 2

In Java, a constructor is a special type of method used to initialize the states of objects. It allows you to provide initial values for the object’s fields (attributes) when the object is created. I have studied constructor chapter from here without any confusion. This tutorial has explained very well with through various example programs.