How to define a generic comparable array class?

Question

I want to define a generic class ComparableList<> that extend ArrayList and implements Comparable interfaces, such that two objects of type ComparableList can be compared using the compareTo method. The compareTo should perform a lexicographic comparison. Here's my code:

class ComparableList <T extends Comparable<T>> extends ArrayList implements Comparable<ComparableList>{
@Override
    public int compareTo(ComparableList o){
        Iterator citer = this.iterator();
        Iterator oiter = o.iterator();
        while (citer.hasNext() && oiter.hasNext()){
            if (citer.next() > oiter.next()){
                return 1;
            }else if (citer.next() < oiter.next()){
                return -1;
            }else {
                if (!citer.hasNext()){
                    return -1;
                }
                if(!oiter.hasNext()){
                    return 1;
                }
            }
        }
        return 0;
}

}

and I got error messages like this:

TCL.java:11: error: bad operand types for binary operator '>'
            if (citer.next() > oiter.next()){
                             ^
first type:  Object
second type: Object
TCL.java:13: error: bad operand types for binary operator '<'
            }else if (citer.next() < oiter.next()){
                                   ^
first type:  Object 
second type: Object

I thought it should be a ComparableList but not an Object. Can anyone tell me the reason?

Jiri Tousek · Accepted Answer · 2016-12-10 08:29:44Z

1

You need to compare the objects using Comparable.comapreTo() (that's why you have <T extends Comparable<T> there). You need to first check for nulls on either side.

Also, each call to Iterator.next() iterates to next element, you don't want to call it twice in one loop iteration - store the items at the loop start then use the stored values.

answered Dec 10, 2016 at 8:29

Jiri Tousek

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2 Comments

Netjimmy Over a year ago

I modified the code to Iterator i = iter.next(), o = oiter.next(); but the compiler said Object cannot be converted to Iterator Seems like it is still not a ComparableList type

Jiri Tousek Over a year ago

Why would you expect iter.next() to return an Iterator? It returns the next item - that one has type T.

T.J. Crowder · Accepted Answer · 2016-12-10 08:30:14Z

1

Comparable doesn't override the > and < operators (nothing can). Since your T implements Comparable, use compareTo:

int result = citer.next().compareTo(oiter.next());
if (result != 0) {
    return result;
} else {
    if (citer.hasNext()) {
        return -1;
    }
    if (oiter.hasNext()) {
        return 1;
    }
}

Note that that also calls next only once per iteration, since next advanced the iterator.

answered Dec 10, 2016 at 8:30

T.J. Crowder

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1 Comment

Netjimmy Over a year ago

I modified my code but get method compareTo(Object) error. Seems like it is still an Object, not a ComparableList type I want

LeleDumbo · Accepted Answer · 2016-12-10 08:31:19Z

1

Each element in your ComparableList is of type T extends Comparable<T>, for sure the binary operator is not available for it (Java doesn't have operator overloading), but since it extends Comparable, you have compareTo to be used as replacement for < and >. Use it instead.

answered Dec 10, 2016 at 8:31

LeleDumbo

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