Question

I’m having to create a Bag class by using a collection (not Java’s built in Collection). I’m having trouble figuring out the equal() method for this. Basically it needs to check if both bags are the same size, create copies for them, use a union to join them, and in a for loop check whether the current value is in each bag; if so, remove them. If both bags are empty then they are equal. For some reason the code keeps spitting out false?

I apologize for all the code, but it’s hard to pinpoint what to leave out or not, since most of the code coincides.

Thanks for all of the help!!

EDIT: This also goes along with this question: Building a bag class in Java

public class Bag<t> implements Plan<t>{
private final int MAX = 10;
private final int DEFAULT = 6;

private static Random random = new Random();

private t[] content; 
private int count;


//Constructors
public Bag(){
    count = 0;
    content = (t[]) (new Object[DEFAULT]);
}
public Bag(int capacity){
    count = 0;
    if(capacity<MAX)
        content = (t[])(new Object[capacity]);
    else System.out.println("Capacity must be less then 10");
}


//Implemented Methods
public void add(t e) {
    try{
        if(!contains(e) && (!(size() == content.length))){
            content[count] = e;
            count++;
        }
        }catch (ArrayIndexOutOfBoundsException exception){
            System.out.println("Bag is Full");
        }
}


public boolean isEmpty() {

    return count==0;        
}


public boolean contains(t e) {
    Object location = null;

    for(int i=0;i<count;i++)
        if(content[i].equals(e)) location=i;

    return (location!=null);
}


public int size() {

    return count;
}


public void addAll(Bag<t> b) {
    for (int i=0;i<b.size();i++)
        add(b.content[i]);
}


public Bag<t> union(Bag<t> a, Bag<t> b) {
    Bag<t> bigBag = new Bag<t>();

    for(int i=0; i<a.size();i++)
        bigBag.add(a.content[i]);
    for(int k=0; k<b.size();k++)
        bigBag.add(b.content[k]);

    return bigBag;
}


public boolean equals(Bag<t> e) {
    Bag<t> bag1 = new Bag<t>();
    Bag<t> bag2 = new Bag<t>();
    Bag<t> bag3 = new Bag<t>();
    t object;

    if(size() == e.size()){
        bag1.addAll(this);
        bag2.addAll(e);

        bag3.union(bag1, bag2);

        for(int i=0; i<bag3.size();i++){
            object = bag3.content[i];
            if((bag1.contains(object)) &&(bag2.contains(object))){
                bag1.remove(object);
                bag2.remove(object);

            } 

        }
    }


    return (bag1.isEmpty()&&(bag2.isEmpty()));

}

Answer 1

The problem appears to be in your union() method, or with how you’re using it. Its actually returning a new Bag rather than adding bags bag1 and bag2 to bag3 which is how equals() is expecting it to behave.

Do you know about class methods yet? This is what union() really is. Add the static keyword in the signature, like so.

public static Bag<t> union(Bag<t> bag1, Bag<t> bag2)

Now, let’s fix the definition of the equals method. The equals() in the sample code has a signature of

public boolean equals(Bag<t> e)

But this should be

public boolean equals(Object o)

So we have to make sure o is a Bag first.

public boolean equals(Object o) {
    if (this == obj) {
        return true;
    } else if (obj == null) {
        return false;
    } else if (!(obj instanceof Bag))
        return false;
    }

    Bag<?> other = (Bag<?>) obj;
    if (this.count != other.count) {
        return false;
    }

    Bag<t> bag1 = new Bag<t>();
    Bag<t> bag2 = new Bag<t>();
    bag1.addAll(this);
    bag2.addAll(other);

    // If you don't know about `static` yet, just use this instead of the following I guess 
    //Bag<t> bag3 = new Bag<t>();
    //bag3 = bag3.union(bag1, bag2);
    Bag<t> bag3 = Bag.union(bag1, bag2);

    t object;
    for(int i=0; i<bag3.size();i++) {
        object = bag3.content[i];
        if ((bag1.contains(object)) && (bag2.contains(object))){
            bag1.remove(object);
            bag2.remove(object);
        }
    }

    return (bag1.isEmpty() && (bag2.isEmpty()));
}

Fix a couple brace/compiler errors and I think this might work. I can’t test it without the missing remove() method though.