Cannot Find Symbol Method Error

Question

I have an assignment from my Java 1 class (I'm a beginner) and the question instructs us to make some code more object-oriented. I've done what I can for the assignment, but one of my files consistently gives me a Cannot Find Symbol Method error even though the files are in the same project. I know the methods are there, so what's going on? The error only occurs in AlienPack, which doesn't seem to recognize the other files, all of which are in the same project (including AlienPack). The getDamage() method that's being called in AlienPack isn't being found (it's in SnakeAlien, OgreAlien, etc).

EDIT: The new error for the getDamage() methods I'm trying to invoke in AlienPack is that the methods still aren't being found. AlienDriver can't find calculateDamage() either.

Here's the code I've got so far:

Alien:

public class Alien {
// instance variables
private String name;
private int health;

// setters
public void setName(String n) {
    name = n; }

public void setHealth(int h) {
    if(h>0&&h<=100) {
        health = h;
    } else {
        System.out.println("Error! Invalid health value!");
        System.exit(0); } }

// getters
public String getName() {
    return name; }

public int getHealth() {
    return health; }

// constructors
public Alien() {
    setName("No name");
    setHealth(100); }

public Alien(String n, int h) {
    setName(n);
    setHealth(h); }

public Alien(Alien anAlien) {
    setName(anAlien.getName());
    setHealth(anAlien.getHealth()); }

public Alien clone() {
    return new Alien(this);
} }

SnakeAlien:

public class SnakeAlien extends Alien { // new file
// instance variables
private int damage;

// setters
public void setDamage(int d) {
    if(d>0) {
        damage = d;
    } else {
        System.out.println("Error! Invalid damage value!");
        System.exit(0); } }

// getters
public int getDamage() {
    return damage; }

// constructors
public SnakeAlien() {
    super();
    setDamage(0); }

public SnakeAlien(String n, int h, int d) {
    super(n, h);
    setDamage(d); }

public SnakeAlien(SnakeAlien anAlien) {
    super(anAlien);
    setDamage(anAlien.getDamage()); }

public SnakeAlien clone() {
    return new SnakeAlien(this);
} }

OgreAlien:

public class OgreAlien extends Alien { // new file
// instance variables
private int damage;

// setters
public void setDamage(int d) {
    if(d>0) {
        damage = d;
    } else {
        System.out.println("Error! Invalid damage value!");
        System.exit(0); } } 

// getters
public int getDamage() {
    return damage; }

// constructors
public OgreAlien() {
    super(); 
    setDamage(0); }

public OgreAlien(String n, int h, int d) {
    super(n, h);
    setDamage(d); }

public OgreAlien(OgreAlien anAlien) {
    super(anAlien);
    setDamage(anAlien.getDamage()); }

public OgreAlien clone() {
    return new OgreAlien(this);
} }

MarshmallwManAlien:

public class MarshmallowManAlien extends Alien { // new file
// instance variables
private int damage;

// setters
public void setDamage(int d) {
    if(d>0) {
        damage = d;
    } else {
        System.out.println("Error! Invalid damage value!");
        System.exit(0); } }

// getters
public int getDamage() {
    return damage; }

// constructors
public MarshmallowManAlien() {
    super();
    setDamage(0); }

public MarshmallowManAlien(String n, int h, int d) {
    super(n, h);
    setDamage(d); }

public MarshmallowManAlien(MarshmallowManAlien anAlien) {
    super(anAlien);
    setDamage(anAlien.getDamage()); }

public MarshmallowManAlien clone() {
    return new MarshmallowManAlien(this);
} }

AlienPack:

public class AlienPack { // new file, this one isn't recognizing the others
// instance variables
private Alien[] pack;

// setters
public void setPack(Alien[] aliens) {
    pack = new Alien[aliens.length];
    for(int i = 0; i<aliens.length; i++) {
        pack[i]=aliens[i].clone(); } }

// getters
public Alien[] getPack() {
    Alien[] temp = new Alien[pack.length];
    for(int i = 0; i<pack.length; i++) {
        temp[i]=pack[i].clone(); }
    return temp; }

// constructors
public AlienPack() {
    Alien[] nothing = new Alien[1];
    nothing[0]=null;
    setPack(nothing); }

public AlienPack(Alien[] aliens) {
    setPack(aliens);}

// other methods
public int calculateDamage() {
    int damage = 0;
    for(int i = 0; i<pack.length; i++) {
        if((new SnakeAlien()).getClass()==pack[i].getClass()) {
            pack[i].getDamage() +=damage;
        } else if((new OgreAlien()).getClass()==pack[i].getClass()) {
            pack[i].getDamage() +=damage;
        } else if((new MarshmallowManAlien()).getClass()==pack[i].getClass()) {
            pack[i].getDamage() +=damage;
        } else {
            System.out.println("Error! Invalid object!");
            System.exit(0); } }     
    return damage; } }

AlienDriver:

public class AlienDriver { // driver class
public static void main(String[] args) {

    Alien[] group = new Alien[5];
    group[0]= new SnakeAlien("Bobby", 100, 10);
    group[1]= new OgreAlien("Timmy", 100, 6);
    group[2]= new MarshmallowManAlien("Tommy", 100, 1);
    group[3]= new OgreAlien("Ricky", 100, 6);
    group[4]= new SnakeAlien("Mike", 100, 10);

    System.out.println(group.calculateDamage());

} }

Answer 1

Two problems:

pack[i].getClass().getDamage() ...

should be just

pack[i].getDamage() ...

You seem to be confused about what the getClass() method does. It returns an object which represents the class (i.e. java.lang.Class) of another object. It is used for reflection. To invoke getDamage() you would just invoke it directly on pack[i] as shown above.

However...

You are attempting to invoke the method getDamage() using a reference of type Alien, which is a base class of all the concrete alien types. If you want to do it that way, getDamage() needs to be declared abstract in the base class so it can be found and dispatched to the correct subclass when invoking it via an Alien reference.

In Alien:

public abstract class Alien {
    ...
    public abstract int getDamage();

An alternative is to cast to the appropriate subclass at each point since you know what it is:

((SnakeAlien)pack[i]).getDamage() +=damage;

However (again) even that is wrong. You can't apply += to an "rvalue". What you need to do here is either:

1. Also declare setDamage() abstract in Alien and do pack[i].setDamage(pack[i].getDamage()+damage);
2. If casting, ((SnakeAlien)pack[i]).setDamage( ((SnakeAlien)pack[i].getDamage()) + damage);

My Recommendation:

In class Alien:

public abstract class Alien {
    ...
    private int damage = 0; // Move damage up to the abstract base class
    public int addToDamage(int n) { this.damage += n; }
    ...
}

In your driver, no need to test the class. Invoke the addToDamage() method on the Alien reference.

Answer 2

I think that at least part of your problem is the getClass() method. You are expecting it to return an object but it does not. Just call directly to the array.

pack[I].getDamage()

should work assuming that the correct type of object is stored in pack()