遇到Eclipse中的“错误”或某些我不理解的逻辑

Question

public SortedArrayList<T> incrementItems(int increment) {
for(T sh: this) {
    Integer newValue = (Integer) sh + (Integer) increment;
    this.set(this.indexOf(sh), (T) newValue);
    }
return this;
}

This is my method, it simply goes through every element of the list and increments the value, it runs fine 80% of the time. However, if 1 is the "element" it increments it by double (again only sometimes).

I've given some examples below:

    SortedArrayList<Integer> L1 = new SortedArrayList<Integer>();
    L1.addAll(Arrays.asList(1,3,6,1,6,6,7,8,11,11,14,15,20,20));
    System.out.println(L1.incrementItems(10));

Output is: [21, 13, 16, 21, 16, 16, 17, 18, 11, 11, 24, 25, 30, 30]

    SortedArrayList<Integer> L1 = new SortedArrayList<Integer>();
    L1.addAll(Arrays.asList(1,3,6,1,6,6,7,8,11,11,14,15,20,20));
    System.out.println(L1.incrementItems(9));

Output is: [10, 12, 24, 10, 15, 15, 16, 17, 29, 29, 23, 15, 20, 20]

    SortedArrayList<Integer> L1 = new SortedArrayList<Integer>();
    L1.addAll(Arrays.asList(1,3,6,1,6,6,7,8,11,11,14,15,20,20));
    System.out.println(L1.incrementItems(4));

Output is: [5, 19, 10, 5, 10, 10, 7, 12, 15, 11, 18, 15, 24, 24]

Some numbers trigger this to happen, others don't. So again I'd appreciate any feedback.

Answer 1

This has nothing to do with the Eclipse IDE.

You are invoking indexOf on your List , which will retrieve the first element matching.

Every time.

See docs :

Returns the index of the first occurrence of the specified element in this list[...]

So if you are looping and 1 comes across twice, indexOf will return the first position of 1 , and the element will be incremented.

What happens next is based on the further items in your List : if a match is found for the incremented item later in your iteration, the same item will be incremeneted again while the latter occurrences will be left untouched.

As an off-topic issue, you seem to be misusing generics here: your SortedArrayList class accepts any kind of type parameters, yet its incrementItems only assumes values will be Integer .

Note

If you're using Java 8, you can leverage the map functionality of streams to easily project all your List elements to their incremented value.

For instance:

// this will generate a new `List` with incremented elements
List<Integer> incremented = originalList
    .stream()
    .map(x -> Integer.sum(x, increment))
    .collect(Collectors.toList());

If you are stuck with pre-Java 8 idioms, you can create a new List with the code such as:

List<Integer> incremented = new ArrayList<>();
for (Integer i: originalList) {
    incremented.add(i + increment);
}