Java覆盖Arrays.sort（）的compareTo方法

Question

I am trying to override the compareTo method to be able to use Arrays.sort() But when i do this it seems like it is not working.

I dont get any error back but i have a tester that tests and gives an error back. I am supposed to compare the 'totalsales' from the items being compared

Here is the compare to method comparing two ItemSales Items

    @Override
    public int compareTo(ItemSales item) {

        return Double.compare(item.totalSales, this.totalSales);
    }

Here is the tester testing this, ofcourse theres much more besides this, but everything else works even clone().

ItemSales item = new ItemSales();
ItemSales newItem = new ItemSales(10.0, 50, 50.0);
ItemSales secondItem = (ItemSales) newItem.clone(); 
ItemSales[] myItems = { secondItem, item, newItem };

ItemSales[] myItems = { secondItem, item, newItem };

Arrays.sort(myItems);
if ( myItems[0].getTotalSales() == 860.0 && myItems[1].getTotalSales() == 870.0) {
    System.out.println("ItemSalesDemo.main()             - sorted, this is correct");
} else {
    System.out.println("ItemSalesDemo.main()             - ");
    System.out.println("ItemSalesDemo.main()             ------------------------");
    System.out.println("ItemSalesDemo.main()             -         Error        -");
    System.out.println("ItemSalesDemo.main()             ------------------------");
    System.out.println("ItemSalesDemo.main()             - item and newItem should have been equal");
    System.out.println("ItemSalesDemo.main()             - ");
    System.exit(-1);
} 

As you may know i get the "item and newItem should have been equal" error

Answer 1

I think the code would work better if you changed it:

@Override
public int compareTo(ItemSales item) {
    if (this.totalSales < item.totalSales) {
        return -1;
    }
    else if(this.totalSales > item.totalSales){
        return 1;
    }

    return 0;
}

The problem with your earlier Double.compare() was that you had the parameters in wrong order, it should have been compare(this, item) and not the other way around

Answer 2

I see two possible difficulties in the compareTo method:

    if (this.totalSales != item.totalSales) {
        return 0;
    }

Normally, the compareTo method should return 0 if the two objects are equal. You often hear the phrase, "compareTo should be consistent with equals()", which means that equals() should return true when compareTo() returns 0 , and equals() should return false when compareTo() returns something other than 0 .

As a general rule, compareTo should return -1 if this is less than the item (the other ItemSales you're comparing against). compareTo should return 1 if 'this' is greater than the other item .

The second problem is what looks like a potential infinite loop:

    return item.compareTo(this);

I like what you have commented out better:

    return Double.compare(item.totalSales, this.totalSales);

That's more what I would expect from a compare method.

Answer 3

Change your code as

 @Override
    public int compareTo(ItemSales item) {

        return Double.compare( this.totalSales ,item.totalSales);
    }

As per your if condition , you are excepting in ascending order. So change the parameters in Double.compare. As others told please read the Comparable spec first

myItems[0].getTotalSales() == 860.0