Triggers and Sequences in Java

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Triggers and Sequences
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In the case of replacement or removal, the low and high variables refer to the elements in the sequence that are replaced or removed In the case of an insertion, there is no existing element that is affected, so it is not obvious what the values of these variables should be in this case Bear in mind that it must be possible to unambiguously deduce from these values and from that of the sequence containing the new elements that an insertion has occurredThe values that are actually assigned for an insertion where the first new element appears at index n in the sequence are as follows:
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low = n high = n - 1
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This probably looks a little strange, because the high value is smaller than the low value Only one value is needed to indicate where the insertion occurred, so this index is assigned to the variable low If high were then to be set to the same value as low, it would look as if a replacement of a single element had been made If high were set to a larger value than low, it would indicate the replacement of (high - low + 1) elements Setting high to the value (low - 1) (or any other value that is less than low) makes it easy for the trigger code to determine that an insertion has occurred Let s start again with the numbers sequence initialized to [010] and then insert an element at the start:
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numbers = [010]; insert 0 before numbers[0];
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The insertion on the second line results in the following output from the trigger:
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oldVals: [ 0, low = 0, high newVals = [ 0 Result = [ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 ] = -1 ] 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 ]
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The fact that high is less than low indicates an insertion, and the value of low shows that the first element was inserted at index 0The newVals variable indicates that only one element was inserted Insertion at the end produces a similar result, but with a different value for low:
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insert 10 into numbers;
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The result is as follows:
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oldVals: [ 0, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 ] low = 11, high = 10 newVals = [ 10 ] Result = [ 0, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 ]
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Here, low has the value 11, but the sequence only had 11 elements before the insertion, so the maximum valid index would have been 10The fact that the insert index is greater than this shows that the element was added at the end
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10 Triggers
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Not all insertions involve one new element Here s one that does not:
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insert [100105] into numbers;
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The resulting output shows that six elements have been added to the end of the sequence:
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oldVals: [ 0, 0, low = 12, high = newVals = [ 100, Result = [ 0, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 ] 11 101, 102, 103, 104, 105 ] 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 100, 101, 102, 103, 104, 105 ]
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Finally, when a sequence is assigned its initial value, an insert trigger is firedThis statement
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var numbers = [010];
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produces the following output from the trigger:
oldVals: [ ] low = 0, high = -1 newVals = [ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 ] Result = [ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 ]
Example Use of a Sequence Trigger
You ve seen how triggers work, so let s look at how you might use oneTriggers are typically used to protect the state of a class or to keep otherwise unrelated application state synchronized in circumstances in which it would not be possible to use a bindingYou have already seen an example of the former, so here we take a look at an example of the latter Suppose we have a sequence of numbers and we want to maintain a separate variable whose value is the sum of the numbers in the sequence:
var numbers:Integer[]; var total:Integer;
One way to do this is to declare a function that returns the sum of the values in a sequence and use binding to assign its value to the variable total:
function sum(values:Integer[]):Integer { var sum = 0; for (i in [0<sizeof values]) { sum += values[i]; } return sum; } var total = bind sum(numbers);
This works, but a possible performance problem exists because whenever a change is made to the sequence, the total has to be recomputed from scratch If the sequence has a