How to call a method that belongs to another class? JAVA POO

A regular method is not called in the class Person, you call it on an object of the person class, technically speaking, on an instance of the class 1 .

This, if you think about it, makes a lot of sense. The class is like the mold with which a person is represented and constructed . But there is no weight of all people, but each person has its own weight.

Then, if you need the weight of a particular person, you must first have built an object-an instance-of that class, and then you can establish and obtain its weight.

Let's make a small example:

{
  //declaramos una _variable_ para una persona
  Persona juan;
  //ahora, creamos el objeto y lo inicializamos...
  juan = new Persona();
  juan.setNombre("Juan Antonio");
  juan.setEdad(43);
  juan.setPeso(125.12);
  //.... hacemos otras cosas
  //y ahora, recuperamos el peso
  double peso;
  peso = juan.getPeso();
}

Now, assuming you have this class EstadisticasDePersonas , I am not very clear about its use or its purpose, but it could be that this class has a list of people who are adding and then we can get the weight of any of these people. Here I will assume, that internally the class has a list or an arrangement of people and that it has a agregarPersona(Persona persona) method that is adding people to this arrangement or list, for example, I can do something like this:

{
  Persona juan;
  Persona mario;
  EstadisticaPersonas estadistica;
  estadistica = new EstadisticaPersonas();

  juan = new Persona();
  juan.setNombre("Juan Antonio");
  juan.setEdad(43);
  juan.setPeso(125.12);
  estadistica.agregarPersona(juan);

  mario = new Persona();
  mario.setNombre("Mario Alberto");
  mario.setEdad(44);
  mario.setPeso(132.15);
  estadistica.agregarPersona(mario);
  }

Then, within this class, you could implement the getPesoPedia method, using a foreach doing something like:

public double getPesoPromedio() {

    int cantidadPersonas = 0;
    double sumatoriaPeso = 0;

    for (Persona persona : listaPersonas) {
      cantidadPersonas++;
      sumatoriaPeso += persona.getPeso();
    }

    return sumatoriaPeso / cantidadPersonas;
}

Of course, here we are in the field of assumptions and I will stop ... I hope that the information provided helps clarify your doubts.

1 : I have said that it is usually called on an object, but java (and many other object-oriented languages) if they allow to call a method on a class, but this is a special type , or less common, method, which is called static method or class method . This method is called about the class, but as such, it is in a different context and does not have access to the members or regular methods of the class, which always require an instance to be accessed / invoked.

So you can access the methods and properties that exist in the .java file but from the .java file; you should consider the following

Persona.java

package stackover;

public class Persona {

private String nombre ;
private int edad;
private double peso;
private String genero ;
private double altura;

public Persona(String nombre , int edad , double peso , String genero , double altura) {

    this.nombre = nombre ;
    this.edad = edad ;
    this.peso = peso;
    this.genero = genero;
    this.altura = altura;

}

public String toString() {
    return "EstadisticasDePersonas [nombre=" + nombre + ", edad=" + edad + ", peso=" + peso + ", genero=" + genero
            + ", altura=" + altura + "]";
}



public String getNombre () {

    return this.nombre;
}

public void setNombre (String nombre) {

    this.nombre = nombre;
}

public int getEdad () {

    return this.edad;
}

public void setEdad(int edad) {

    this.edad = edad;
}

public double getPeso() {

    return this.peso ;
}

public void setPeso() {

    this.peso = peso;
}

public String getGenero() {

    return this.genero;
}

public void setGenero (String genero) {

    this.genero = genero;
}

public double getAltura() {

    return this.altura;
}

public void setAltura() {

    this.altura = altura;
}
}

Person Statistics.java

package stackover;

public class EstadisticasDePersonas
{
    public static void main(String args[])
    {
        Persona personita = new Persona("Alfredo", 23, 23.2, "masculino", 12.3);
        personita.setGenero("Femenino");
        personita.getGenero();

    }
}