Comparing elements in arrays

Question

I am stuck on one component of a java assignment for a data structures class that is preventing me from completing the rest of it. It is a simple concept but I am missing something in the implementation. I have a Dynamic array called list1 and I need to compare it to another list (list2). I have a method: equals(Object a) so the call is list1.equals(list2);

I know how to compare lists with an iterator but how do I reference the list1 Object to compare the two?

I am not asking for you to do my assignment, just help me understand how this would work.

public static void main(String args[]) 
    {
        DynamicArrayOfInts list1 = new DynamicArrayOfInts();        

    }   
    public DynamicArrayOfInts() 
    {
      storage = new int[INITIAL_CAPACITY]; 
      size = 0;
    }
    public boolean equals(Object aThat) 
    { 
        if(aThat.equals(storage))
            return true;
        else    
            return false;
    }

this is answered here: http://stackoverflow.com/questions/2762093/java-compare-two-lists — Lorenz Lo Sauer, Sep 22 '12 at 17:33

Balázs Édes · Accepted Answer · 2012-09-22T17:37:58.940

2

you should do something like this:

public boolean equals(Object obj)
{
    if(obj == null) return false;
    if(obj.getClass() != DynamicArrayOfInts.class) return false;

    DynamicArrayOfInts other = (DynamicArrayOfInts) obj;

    /*compare this.storage to other.storage with the iterator
    or with simple indexing.*/
}

edited Sep 22 '12 at 17:37

answered Sep 22 '12 at 17:32

Balázs Édes

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Thank you, that's what I was missing. It makes sense now. – cHam Sep 22 '12 at 18:12

hvgotcodes · Answer 2 · 2012-09-22T17:36:54.937

I think you are saying that you do the comparison inside the equals method. If that is so, you need to cast the argument a to a list. So

public boolean equals(Object a) {
    MyList list1 = (MyList) a; 
    ...
}

The line MyList list1 = (MyList) a means "treat the reference a as a reference to an instance of MyList, and assign it to the reference list1.

equals typically takes an Object so you can pass (almost) anything in to equals. Depending on the specific case, the first thing you can do is check the type of the argument to equals and return false if it has no chance of being equal.

Look at this example I found on the intertubes. The first thing you see in their example is

if ( this == aThat ) return true;
if ( !(aThat instanceof Car) ) return false;

doing things like this allows you to return quickly if the comparison is trivial. For example, it's a tautology that an instance is equal to itself; there is no way that cannot be true.

score 1 · Answer 3 · answered Sep 22 '12 at 17:34

1

Inside the equals method you use the this operator to access list1 in place of the nae list1.

answered Sep 22 '12 at 17:34

Jessica Brown

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Comparing elements in arrays

3 Answers3