Java method call in method

The joy with Java is it's Object Oriented nature. You can achieve what you're Ruby does, but let a List do all the heavy lifting for you.

If you create and maintain a list of visitors, then you can simply create your Visitor object and add it to the list. The list then has a size() method that will allow you get the number of visitors whenever you need it.

In my example, I have used ArrayList (the most common type) but there are a number of lists (and indeed other collections) that could be used. This page could be useful.

import java.util.ArrayList;
import java.util.List;

public class Visitors {

    private final List<Visitor> visitors;

    public Visitors() {
        this.visitors = new ArrayList<Visitor>(); //Initialise the list within the constructor here.
    }

    public void AdmitNewVisitor(final String theName, final String theGender, final int theAge, final String theCitizenship ) {
        //create your new visitor with the new keyword. Then add it to the list of visitors.
        final Visitor visitor = new Visitor(theName, theGender, theAge, theCitizenship);
        this.visitors.add( visitor );
    }

    public Integer getNumberOfVisitors() {
        //There's no need to maintain the number of visitors yourself, you can let the List do it.
        return this.visitors.size();
    }
}

Edit:

Of course, this relies on having a Visitor object with the correct constructor:

public class Visitor {

    private final String theName;
    private final String theGender;
    private final int theAge;
    private final String theCitizenship;

    public Visitor(final String theName, final String theGender, final int theAge, final String theCitizenship) {
        this.theName = theName;
        this.theGender = theGender;
        this.theAge = theAge;
        this.theCitizenship = theCitizenship;
    }

    public String getTheCitizenship() {
        return theCitizenship;
    }

    public int getTheAge() {
        return theAge;
    }

    public String getTheGender() {
        return theGender;
    }

    public String getTheName() {
        return theName;
    }
}

Edit:

You could do it maintaining your own array and number of visitors, but you'd need to think about the following:

• The number of visitors would need to be defined as an instance/static field. (Personally, I'd go with instance and have the Visitors class instantiated when using it.)
• You're only adding one visitor at a time, so there's no need to have a loop in your addNewVisitor method.
• Arrays are indexed from 0, so the first visitor is at index 0 and the last visitor is at index ( this.visitors.length - 1 ). Incrementing the number of visitors after adding the visitor to the array will avoid any index arithmetic.
• When maintaining the array, keep in mind that it won't expand by itself. If you have an array of size 10, but then you try to add a visitor to index 11 (you get a surprise 11th visitor!) then you will need to copy the array contents into a bigger array. This is why we use Lists in Java. They handle this dynamic size issue for us.

As requested in the comments, no code solution, just the above tips.

Edit:

Adding a solution using arrays and maintaining the number of visitors by hand. I will ignore the dynamic size of arrays problem for now. (The array will just be initialised to be large)

From what I can gather, you need the visitor class that I have given previously. This is a separate class to you overall class that monitors the array (or list) of visitors. This class represents your visitor object. In your Ruby, you have Visitor.new(the_name, the_gender, the_age, the_citizenship). This becomes new Visitor(theName, theGender, theAge, theCitizenship) in Java.

The @num_visitors needs to be defined outside of the method that adds a visitor because it is a globally maintained value, it does not just exist within the admitNewVisitor method.

The only other thing of note is that I increment the numberOfVisitors after adding the visitor to avoid doing index arithmetic.

public class Visitors {

    private int numberOfVisitors = 0;
    private final Visitor[] visitors;

    public Visitors() {
        this.visitors = new Visitor[250];
    }

    public void AdmitNewVisitor(final String theName, final String theGender, final int theAge, final String theCitizenship ) {
        final Visitor visitor = new Visitor(theName, theGender, theAge, theCitizenship);
        this.visitors[this.numberOfVisitors] = visitor;
        this.numberOfVisitors++;
    }

    public Integer getNumberOfVisitors() {
        return this.numberOfVisitors;
    }
}

And a test for this would look maybe like this:

@Test
public void thatnumberOfVisitorsIncreasesWhenAVisitorIsAdmitted() {
    final Visitors visitors = new Visitors();

    Assert.assertEquals(visitors.getNumberOfVisitors(), (Integer) 0);

    visitors.AdmitNewVisitor("name", "male", 50, "British");

    Assert.assertEquals(visitors.getNumberOfVisitors(), (Integer) 1);

    visitors.AdmitNewVisitor("name2", "female", 48, "British");

    Assert.assertEquals(visitors.getNumberOfVisitors(), (Integer) 2);
}

Hope that helps. Testing is always important, but don't get too bogged down with format/syntax of the test method while you are learning.