Arrays in Java

Arrays in Java work similar as we have discussed in pseudocode. They are linear, they have indexes, they cannot change in size.

int[] numbers = {3, 45, 522, 1, 1, 9};

The length or size of the array is the number of elements, in the case of numbers is 6.

The index of 3 is 0.
The value of numbers[2] is 522.

The maximum index is always equal to [nameOfThArray].length -1. In this case is 5, whose value is 9.

We can also not now the values of the numbers:

int[] numbers = int[399];

This is an arry of 399 elements.

Loop through an array

We usually use for loop this way:

int[] numbers = {3, 45, 522, 1, 1, 9};
for (int i = 0; i < numbers.length; i++){
    //do something
    //we use numbers[i] to access the possible values
    //we use i to access the possible indexes
}

There is the possibility of using for each.

int[] numbers = {3, 45, 522, 1, 1, 9};
for (int number:numbers){
    //to access the possible values we use "number"
}

I suggest don't use it unless you're too deep into python

More reference on this:
https://www.w3schools.com/java/java_foreach_loop.asp

Array of objects

It's very common in classes that they have an array of Objects instead of primitive.

For example, makes sense that a IBClass should have an array of IBStudent

Student[] students = new Student[100];

Counter Variables in IB

In IB you will find scenarios where they use arrays. In this scenarios they need to handle sometimes that the number of objects is variable, for example the number of students in a subject may vary from subject to subject. If we have an AirCompany, it may have a variable number of Planes in their fleet. So to deal with that you will find counter variables.

In these cases you will have an array of objects that it's over the limit the maximum size of the actual model (so the program doesn't get out of elements). For example in the class "Subject" we might find an array of 50 students (Because 50 is already too much in IB), if we talk about a parking lot we might have an array of Cars that are entering that is just the size of the parking or in the case of a fleet of an air company we can go with 200 planes (that's already a big number).

Counter variables are int that they are related to an array and count how many elements we have in that oversize array.

For example:

  private Plane[] fleet = new Plane[200];
  private int numberOfPlanes; //counter Variable

This counter variable sometimes has an accessor but it shouldn't have a mutator.

Arrays in the constructor

We don't usually have the array as a variable in the constructor, but you might find it.

Encapsulation of the array

We don't have direct accessors and mutators of the array. You won't see, in the case of AirCompany methods like "getFleet" or "setFleet". This is because we want to make sure that AirCompany has full control of fleet and also the counter variable.

"Accessor" of one element of the array

This accessor is different from regular accessors because they require a parameter. The index of the element that we're looking for. They have to return the object.

  public Plane getPlane(int index) {
    if (index >= 0 && index < numberOfPlanes) {
      return fleet[index];
    }
  }

"Mutator" of one element of the array: AddElement()

  public void addPlane(Plane plane){
    fleet[numberOfPlanes] = plane;
    numberOfPlanes++;
  }

"Remover" of one element of the array.

This is different depending of the context. I've seen this as part of the difficult question to implement (7 marks). The idea can be "find one element, then take it out, reorder the rest of the elements so there is no extra space, update the counter".

If they ask this in an exam the will give you the specific instructions to construct the method.

The easier way is the input is the index.

Construct the method removePlane(index) in AirCompany that given a specific index, if it's valid it's going to update the number of planes and create an array that keeps the same order and returns the deleted Plane.

    public Plane removePlane(int index){
        //checking if valid
        if(index < 0 || index >= this.numberOfPlanes) {
            return null; //if the index is out of bound we return null
        }
        //we take the first plane before deleting it
        Plane deletedPlane = fleet[index];
        //we loop through the only part of the array that is going to shift (move one less)
        for(int i = index; i < numberOfPlanes -1) {
            fleet[i] = fleet[i++];
        }
        //we put the last one to null
        fleet[numberOfPlanes-1] = null;
        numberOfPlanes --;
        //we return the Plane that we reserved before
        return deletedPlane;
    }

Quite a mess, huh?

Let's do the next twist

Remove a plane given a specific attribute (the first one that we find)

Construct the method removePlane(index) in AirCompany that given a specific modelName, if it's valid it's going to update the number of planes and create an array that keeps the same order and returns the deleted Plane.

    public Plane removePlane(String modelName){
        //let's supose that we don't have the plane in the array, that would be that the index is -1
        int index = -1; 
        //find the plane
        for (int i = 0; i<numberOfPlanes; i++)
        //loop through the array 
        //notice that we use the counter variable 
        {
        if (fleet[i].getModelName.equals(modelName)) {
            index = i;
            break; //if we find the value we don't need to keep looking for it
          }
        }
        //check if we have found something
        if (index == -1) {
            return null;
        }
        //we loop through the only part of the array that is going to shift (move one less)
        for(int i = index; i < numberOfPlanes -1) {
            fleet[i] = fleet[i++];
        }
        //we put the last one to null
        fleet[numberOfPlanes-1] = null;
        numberOfPlanes --;
        //we return the Plane that we reserved before
        return deletedPlane;
    }

Another way of implementing the same method. Both are valid but the second one is more complicated to understand and it's a bit more oneliner

    public Plane removePlane(String modelName){
        //find the plane
        for (int x = 0; x<numberOfPlanes; x++)
        {
            if (fleet[i].getModelName.equals(modelName)) {
                //if found do stuff
                deletedPlane = fleet[i];
                //rearrange
                for(int i = index; i < numberOfPlanes -1) {
                    fleet[i] = fleet[i++];
                }
                //we put the last one to null
                fleet[numberOfPlanes-1] = null;
                numberOfPlanes --;
                //we return the Plane that we reserved before
                return deletedPlane; 
            }
        }
        //if not found we arrive here
        return null;
        }
     }

"Finder" in the array

Sometimes one of the methods that you can find in an class that has an array (or any given collection) is a "finder". A method that tries to finde in that array a specific element that complies with a specific characteristic. This types of methods usually will have the parameter (or parameters) the finding criteria and they usually return the object itself. This is specified in the statement.

They usually loop through the array and inside has an if clause with the specific condition that needs to meet.

Construct the method findPlaneByPassengerCapacity() that given a given passengerCapacity returns the first plane in the fleet that complies with that value.

  public Plane findPlaneByModelName(String modelName) {
    for (int i = 0; i<numberOfPlanes; i++)
    //loop through the array 
    //notice that we use the counter variable 
    {
      if (fleet[i].getModelName.equals(modelName)) {
        return fleet[i];
      }
    }
    return null;
    
  }

Construct the method findPlaneByPassengerCapacity() that given a given passengerCapacity returns the first plane in the fleet that complies with that value.

public Plane findPlaneByPassengerCapacity(int passengerCapacity){
    for (int i = 0; i<numberOfPlanes; i++)
    {
      if (fleet[i].getPassengerCapacity() == passengerCapacity) {
        return fleet[i];
      }
    }
    return null;
  }

Counting exercise

A specific version of the "finder" methods where you count elements that has a specific propierty. The idea is that you initialize the counter, you loop through the array, you add 1 to the counter then you return the counter.

Construct a method planesAvailable in the AirCompany that counts how many planes in their fleet are active at any given time and returns that value.

 public int planesAvailable() {
    int count = 0;
    for (int i = 0; i < numberOfPlanes; i++){
     if (fleet[i].isAvailable()) {
       count++;
     } //end if
    } //end loop 
    return count;
  }

Driver class exercises

This is very common in exercises in OOP exams. They give you the whole code and they say you that in the driver or main class you will have some code. Then your task is to be able to know what would be the output.

public class Main {
    public static void main(String[] args) {
      AirCompany ryanair = new AirCompany("Ryanair");
      System.out.println(ryanair.getName());
      System.out.println(ryanair.getNumberOfPlanes());
      //one way of adding a new plane
      Plane plane = new Plane("A380");
      ryanair.addPlane(plane);
      //another way of adding a plane
      ryanair.addPlane(new Plane("Concorde"));
      System.out.println(ryanair.getNumberOfPlanes());
      System.out.println(ryanair.getPlane(1).getModelName());
      Plane plane2 = new Plane("A380");
      System.out.println(ryanair.findPlaneByModelName("Concorde").isActive());
  }
}

Output
0
2
Concorde
true

Second exercise:

import java.util.*;

public class Main {
    public static void main(String[] args) {
      AirCompany ryanair = new AirCompany("Ryanair");
      AirCompany vueling = new AirCompany("Vueling");
      Plane plane1 = new Plane ( "Boeing", "747", 
true ,  12,230 );
      Plane plane2 = new Plane ( "Airbus", "380", 
true ,  7,  123 );
      ryanair.addPlane(plane1);
      ryanair.addPlane(plane2);
      vueling.addPlane(new Plane("Concorde"));
      ryanair.addPlane(new Plane("4433"));
      System.out.println(ryanair.getNumberOfPlanes());
      System.out.println(vueling.getPlane(0).getCrewNeeded());
      System.out.println(ryanair.getPlane(1).getManufacturer());
      
       }
}

Output:

3
5
Airbus

I suggest to use one of the scenario and tweak it and try to see what happens using the compiler.

Scenario of reference. AirCompany and plane

These are the classes that I'm using as context here during May2025

Main.Java:

import java.util.*;

public class Main {
    public static void main(String[] args) {
      AirCompany ryanair = new AirCompany("Ryanair");
      AirCompany vueling = new AirCompany("Vueling");
      Plane plane1 = new Plane ( "Boeing", "747", 
true ,  12,230 );
      Plane plane2 = new Plane ( "Airbus", "380", 
true ,  7,  123 );
      ryanair.addPlane(plane1);
      ryanair.addPlane(plane2);
      vueling.addPlane(new Plane("Concorde"));
      ryanair.addPlane(new Plane("4433"));
      System.out.println(ryanair.getNumberOfPlanes());
      System.out.println(vueling.getPlane(0).getCrewNeeded());
      System.out.println(ryanair.getPlane(1).getManufacturer());
      
       }
}

AirCompany.java:

public class AirCompany {
  private String name;
  private Plane[] fleet = new Plane[200];
  private int numberOfPlanes; //counter Variable
  
  public AirCompany() {
    this.name = "Default Name";
  }

  public AirCompany(String name) {
    this.name = name;
    this.numberOfPlanes = 0;
  }
  
  public void addPlane(Plane plane){
    fleet[numberOfPlanes] = plane;
    numberOfPlanes++;
  }
  
  public Plane getPlane(int index) {
    if (index >= 0 && index < numberOfPlanes) {
      return fleet[index];
    }
    return null;
  }
  
  public Plane findPlaneByModelName(String modelName) {
    for (int i = 0; i<numberOfPlanes; i++)
    //loop through the array 
    //notice that we use the counter variable 
    {
      if (fleet[i].getModelName().equals(modelName)) {
        return fleet[i];
      }
    }
    return null;
  }
  
  public Plane findPlaneByPassengerCapacity(int passengerCapacity){
    for (int i = 0; i<numberOfPlanes; i++)
    {
      if (fleet[i].getPassengerCapacity() == passengerCapacity) {
        return fleet[i];
      }
    }
    return null;
  }
  
  public int planesActive() {
    int count = 0;
    for (int i = 0; i < numberOfPlanes; i++){
     if (fleet[i].isActive()) {
       count++;
     } //end if
    } //end loop 
    return count;
  }

  
  public String getName() {
    return this.name;
  }
  
  public void setName(String name){
    this.name = name;
  }
  
  public int getNumberOfPlanes(){
    return this.numberOfPlanes;
  }
  
  
}

Plane.java:

public class Plane {
private int length;
private int passengerCapacity;
private int crewNeeded;
private String manufacturer;
private String modelname;
private boolean active;
 
public Plane ( String manufacturer, String modelname, 
boolean active, int crewNeeded, int passengerCapacity ){
  this.manufacturer = manufacturer;
  this.modelname = modelname;
  this.active = active;
  this.crewNeeded = crewNeeded;
  this.passengerCapacity = passengerCapacity;
  this.length = length;
}
public Plane (String modelname){
  this.modelname = modelname;
  this.length = 30;
  this.active = true;
  this.crewNeeded = 5;
  this.passengerCapacity = 40;

}
 
 
public int getPassengerCapacity (){
   return this.passengerCapacity;
}
public void setPassengerCapacity (int passengerCapacity){
   this.passengerCapacity = passengerCapacity;
}
public int getCrewNeeded(){
  return  this.crewNeeded;
}
public void setcrewNeeded(int crewNeeded){
  this.crewNeeded = crewNeeded;
}
public String getManufacturer(){
  return this.manufacturer;
}
public void setManufacturer(String manufacturer){
  this.manufacturer = manufacturer;
}
public String getModelName(){
  return this.modelname;
}
public boolean isActive(){
  return this.active;
}
public void setActive (boolean active){
  this.active = active;
}
 
 
}

Array exercises in Java

https://www.codesdope.com/practice/java-array/

Nice option with the olutions

http://www.eecs.qmul.ac.uk/~pc/teaching/introprogramming/week7/exercises7.html

Arrays in c++

https://www.w3resource.com/cpp-exercises/array/index.php