Question

Objectives

Make the transition to object oriented programming.

Go over some of the basics concepts:

Classes

Instance variables

Methods

Object state

The keyword "this", what it does

toString method

Introduction

Phase 1

Review the following Java programs as an example of a Java class and client code that uses it: MimicOct.java and UnderTheSea.java.

All of you have been working with classes in Java. One of the most important things to know about classes is that they are actually blueprints for objects. They basically contain all of the information as to how objects are built.

One of the most important features of objects is the fact that they have state and behavior. The way that they hold state is through instance variables. Instance variables are simply variables that "describe" the object and its state. There are several ways to handle the accessibility of an instance variable within an object, but in this lab we are going to work only with public instance variables. Public means that any class within the project that you're working in has permission to access and modify the instance variables. To access or modify an instance variable that is public, such as populationNumber, in an object called species, simply type species.populationNumber = someInteger.

Methods are the ways that objects perform their behavior. Our focus will be on non-static methods; these require an instance of the class in order to operate. Static methods are methods that don't need an instance of

an object to call it.

Phase 2

Please download Species.java and CircleOfLife.java. Use CircleOfLife.java for testing Species.java as you follow these steps:

Phase 3

Create a Species constructor that takes in a String for its name, an int for its population, and an int for its growth rate. Don't let someone initialize a species with a population above 1500 or below 1, or a growth rate outside of the range of 1 to 20 percent. If the population is initialized below 1, set it to 1, and if above 1500, set it to 1500. Similarly for the growth rate.

Phase 4

Return an appropriate String in the toString method that describes the state of a Species object.
Example String to return in the toString() method:

Name of species: cat

Population: 1200

Growth Rate: 12%

Once you have created a toString method, you can use it to display a Species object stored in a variable s as follows:
System.out.println(s);
which is a shortcut for
System.out.println(s.toString());

Phase 5

Create two species objects of whatever animal that you wish (do this in CircleOfLife.java).

Phase 6

Complete the code for mergeSpecies(Species other). This is a non-static method (as all of the rest of them are) that takes another species as a parameter. This method adds the populations of the two species, changes the name of the species to the concatenation of the two names, and the growth rate to the maximum of the two growth rates

Phase 7

Complete the code for grow() and populationInXYears(int x) (hint: don't change the value of the population for the populationInXYears method -- simply return what the population will be in X years given the growth rate of that species.

EXAMPLE

If the species are rabbits and their growth rate is 10%, population is 100, and x is 2, you should see

The projected population for the rabbits in 2 years will be 121.

Answer 1

The updated source code is mentioned below with the inline definitions of previously incomplete functions.

Species.java

public class Species {  

    // Put the instance variable here.

    String name;
    int population;
    int growth_rate;

    // Create a Species constructor that takes in a String for its name, an int for
    // its population, and an int for its growth rate per year as a percent.

    public Species(String name, int population, int growth)
    {
        this.name = name;
        if (population < 1) population = 1;
        if (population > 1500 ) population = 1500;
        this.population = population;

        if (growth < 1) growth = 1;
        if (growth > 20) growth = 20;
        this.growth_rate = growth;
    }
  
    // mergeSpecies adds the populations of the two species, changes the name
    // of the species to the concatenation of the two names, and the growth
    // rate to the maximum of the two growth rates.

    public void mergeSpecies(Species other) {
        // Adding two populations in both the species instances
        this.population += other.population;
        other.population = this.population;

        // changing name of both the species
        this.name += " ";
        this.name += other.name;
        other.name = this.name;

        // changing both the growth rates to max of growth rates
        if (this.growth_rate < other.growth_rate)
            this.growth_rate = other.growth_rate;
        other.growth_rate = this.growth_rate;
        //System.out.println("mergeSpecies NOT IMPLEMENTED YET");
    }

    public String toString(){
        String output = "Name of Species: ";
        output += this.name;
        output += "\n";
        output += "Population: ";
        output += this.population;
        output += "\n";
        output += "Growth Rate: ";
        output += this.growth_rate;
        output += "\n";
        //System.out.println("toString NOT IMPLEMENTED YET");
        return output;
    }

    // Increases the population according to the growth rate of the species, i.e.
    // updates the population instance variable by adding to it the growth rate/100 times the current population.

    public void grow() {
        population += (growth_rate * population)/100;
        //System.out.println("grow NOT IMPLEMENTED YET");
    }

    // Returns the population of the species in x years according to its growth rate.

    public int populationInXYears(int x){
        //System.out.println("getPoplulationInXYears NOT IMPLEMENTED YET");
        int local = population;
        while (x > 0)
        {
            local += (growth_rate* local )/100;
            x--;
        }

        return local;
    }

    public static void main(String[] args) {

        // Put simple code here to test the Species class. It is always a good idea to include
        // a main method in any class you define.

        Species a = new Species("Frog", 100, 10);
        System.out.println(a);

        a.grow();
        System.out.println(a);

        System.out.println("a.populationInXYears(10) returns " + a.populationInXYears(10));

        Species b = new Species("Rabbit", 10, 23);
        a.mergeSpecies(b);
        System.out.println("a.mergeSpecies(b) is " + a);
      
    }
}

CircleOfLife.java

public class CircleOfLife {

    public static void main(String args[]) {

        // Create a new Species object here, passing in the appropriate arguments
        Species a = new Species("Rabbit", 100, 10);
      
        // Print out the species' growth rate
        System.out.println("Growth rate of species Rabbit " + a.growth_rate);

        // Use the species' toString here
        System.out.println(a);
      
        // Call populationInXYears here
        System.out.println("Population of Rabbit species in 2 years " + a.populationInXYears(2));
      
        // Create a new Species object here, passing in the appropriate arguments
        // using a very large number for the population (e.g. 100000000)
        Species b = new Species("Dog", 1000500, 12);

        // Print out the species' population to make sure it is set to 1500
        System.out.println("Population of Dog initialized : " + b.population);

        // Call populationInXYears here, feel free to hardcode in the int to be passed to the method
        System.out.println("Dog population in 3 years " + b.populationInXYears(3));

        // Call mergeSpecies here. Test that mergeSpecies is doing what you expected it to
   a.mergeSpecies(b);  
        System.out.println("After merging total population : " + b.population);

    }
}