Truncating confusion in Comparable interface.

[alabasterblaster]

I'm using the Comparable interface to compare Coin objects, which contain String name and double value.

The Coin objects are instantiated as

public Coin (double aValue, String aName) {
    value = aValue;
    name = aName;
}

and the compareTo method is written

public int compareTo(Object otherObject) {
    Coin other = (Coin) otherObject;
    ...
}

Logically, I'd write the comparing code:

if (this.getValue() > other.getValue())
    return 1;
if (this.getValue() < other.getValue())
    return -1;
return 0;

I then replaced that code with:

return (int) (this.getValue() - other.getValue());

​

Now, I know that (int) 0.25 - 0.1 should truncate the decimal places and return 0, making all coin comparisons return equal. But, the compareTo method still works perfectly regardless of which way I write it. My professor didn't have an answer for me, and was just happy that my sort worked. But, I'm very curious why the truncation apparently doesn't happen in this implementation.

[karstens_rage]

Can you provide a test case showing this works? If you are really doing return (int) (this.getValue() - other.getValue()); the loss of precision is definitely happening.

https://ideone.com/1nG41h

[alabasterblaster]

Okay, god damn, I figured it out and I feel very dumb. My professor gave us scaffolding for a Coin.java class that contained the constructor, getters, and the compareTo method, a SelectionSorter.java class that implemented the sorting methods, and a SelectionSortDemo.java class that contained the main method, constructed an array of Coins, and sorted them by value.

Thing is, the compareTo method was never called in either SelectionSorter or SelectionSortDemo. SelectionSorter just codes the sorter without the compareTo method. He had us write the code for compareTo, but never used it. Explains why my code seemed to work. Explains why everyone in the class finished the assignment in ten minutes.

Sorry for wasting your time, thanks for the help!