How to best explain and use empty Constructors in Java? [duplicate]

Answer 1

If you write a constructor with parameters, then you need to declare the default one (if you want to use it)

class Person {
    String name;
    int age; 

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

You can't do this now:

Person p = new Person();

In order to use the "default" constructor (with no parameters) you will need to declare it:

class Person {
    String name;
    int age; 

    public Person(){
        name = "Man With No Name";  //sometimes you will want to set the variables 
        age = 21;     //to some default values
    }

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