Error in conditional

The condition does apply but you think it does one thing, when it does another:

public void insertarAlFinal(int elemento) {

        System.out.println("" + this.elementosEncolados);
        System.out.println("" + this.cola.size());

        if (this.elementosEncolados <= this.cola.size()) {

        this.cola.add(elemento);
        this.elementosEncolados++;

        }

The above would have an exit more or less like this:

0
0
1
1
2
2
3
3

You can see that both elements and size grow at the same time, now without making many changes and seeing how the vector initializes you could use the following:

public void insertarAlFinal(int elemento) {     

        if (this.elementosEncolados + 1 < this.cola.capacity()) {

        this.cola.add(elemento);
        this.elementosEncolados++;

        } else {
            System.out.println("No se puede insertar. Cola llena");
    }
    }

We change this.cola.size() by this.cola.capacity() and <= by this.elementosEncolados + 1 < although the latter is only for your code can do what you are looking for you can change it and adjust it better. I think his main mistake was that maybe he was confused with size() and capacity()

The previous thing would have an exit thus:

0
3
1
3
2
3
No se puede insertar. Cola llena

By the way, you need a } in the method insertarAlFinal

• size() returns the number of components on the vector in this case.
• capacity() returns the current capacity of the vector.

Vector class

First, you must use capacity () to get your initial capacity in your vector, then make some adjustments so that if you correctly validate the 3 elements that should enter the queue:

Note: The Vector class indicates an initial capacity and as a second argument the size that will increase:

When we create a vector or object of the Vector class, we can specify its initial dimension, and how much it will grow if we exceed that dimension.

Vector vector=new Vector(20, 5);

package com.stack.over.flow.ejemplos;

import java.util.NoSuchElementException;
import java.util.Vector;

public class ColaVector {

    private Vector<Integer> cola = new Vector<Integer>(3);
    public int elementosEncolados = 0;
    static int capacidadCola = 0;

    public void insertarAlFinal(int elemento) {
        try {

            if (this.elementosEncolados < capacidadCola) {

                this.cola.add(elemento);
                this.elementosEncolados++;

            } else {
                System.out.println("No se puede insertar. Cola llena");
            }
        } catch (NoSuchElementException e) {
            e.printStackTrace();

        } catch (Exception e2) {
            e2.printStackTrace();

        }

    }

    public int primerValor() {
        return cola.firstElement();
    }

    public static void getCapacidadInit() {
        ColaVector obj = new ColaVector();
        capacidadCola = obj.cola.capacity();
        System.out.println("capacidad es : " + capacidadCola);
    }

    public static void main(String[] args) {
        getCapacidadInit();
        ColaVector c1 = new ColaVector();

        c1.insertarAlFinal(1);
        c1.insertarAlFinal(2);
        c1.insertarAlFinal(3);
        c1.insertarAlFinal(4);
        c1.insertarAlFinal(5);
        c1.insertarAlFinal(6);
        c1.insertarAlFinal(7);

        System.out.println(c1.primerValor());

    }

}