Triggers and Sequences in Java

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Triggers and Sequences
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holds the new values in this case, there is only one new value, the element 100 from the right side of the assignment statement Typically in a sequence trigger, you use the values provided to work out what has changed and act accordinglyThe easiest way to do this is to view every trigger as if it were reporting a replacement operationAs you ll see later, this works for insertions and removals as well as for element replacements like the one in this example In this case, it s easy to see what happened if you read the information printed by the trigger like this:
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newVals "Elements 1 through 1 (ie, low through high) of the sequence for which the value before this change was [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10] (ie the value of the variable oldVals) were replaced by the single value 100 (ie, the value of the variable newVals)"
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Now let s see what happens if we replace one element by a sequence containing more than one value:
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numbers[11] = [200204];
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This time, the trigger prints the following:
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oldVals: [ 0, 100, 2, 3, 4, 5, 6, 7, 8, 9, 10 ] low = 1, high = 1 newVals = [ 200, 201, 202, 203, 204 ] Result = [ 0, 200, 201, 202, 203, 204, 2, 3, 4, 5, 6, 7, 8, 9, 10 ]
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As before, low and high have the same value, so you can see that only element 1 of the original sequence was affected Now, however, newVals contains five elements, which are the values that replace element number 1, in the order in which they appear in the modified sequence
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Removing Elements from a Sequence
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A removal is characterized by the newVals sequence having no elementsThe high and low values indicate how many elements were removed and where those elements were in the sequence (which is always a contiguous range) Our first example removes a single element from the numbers sequence:
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delete numbers[1];
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This gives the following result:
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oldVals: [ 0, 200, 201, 202, 203, 204, 2, 3, 4, 5, 6, 7, 8, 9, 10 ] low = 1, high = 1 newVals = [ ] Result = [ 0, 201, 202, 203, 204, 2, 3, 4, 5, 6, 7, 8, 9, 10 ]
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As you can see, newVals is empty, which indicates a removalThe low and high variables show that only the single element formerly at index 1 was removed, and you can use these bounds together with oldVals to see that the element removed had the value 200 (the value of oldVals[1])
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10 Triggers
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Removing a range of elements gives a similar result:
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delete numbers[8];
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This code removes all elements starting with the one at index 8:
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oldVals: [ 0, 201, 202, 203, 204, 2, 3, 4, 5, 6, 7, 8, 9, 10 ] low = 8, high = 13 newVals = [ ] Result = [ 0, 201, 202, 203, 204, 2, 3, 4 ]
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The fact that more than one element has been removed is evident from the values of the variables low and high, which show that six elements have been deleted from the sequence, starting at index 8 Sometimes, what appears to be a single operation can result in more than one invocation of the trigger For example, consider the following code:
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insert 0 into numbers; delete 0 from numbers;
The first line adds a 0 at the end of the sequenceThis causes the trigger to fire to report an insertion, which we ignore for nowAfter this insertion, the sequence looks like this:
[ 0, 201, 202, 203, 204, 2, 3, 4, 0 ]
The second line of code removes any element in the sequence that has the value 0 Because there are two such elements, the trigger fires twiceThe first trigger reports the following values:
oldVals: [ 0, 201, 202, 203, 204, 2, 3, 4, 0 ] low = 8, high = 8 newVals = [ ] Result = [ 0, 201, 202, 203, 204, 2, 3, 4 ]
This indicates that the element at index 8 has been removed Note that when the trigger is invoked, only this element has been removed the 0 at element 0 is still in place The second trigger invocation reports the subsequent removal of this second 0 element:
oldVals: [ 0, 201, 202, 203, 204, 2, 3, 4 ] low = 0, high = 0 newVals = [ ] Result = [ 201, 202, 203, 204, 2, 3, 4 ]
Finally, let s delete the whole sequence:
delete numbers;
This gives the following result:
oldVals: [ 201, 202, 203, 204, 2, 3, 4 ] low = 0, high = 6 newVals = [ ] Result = [ ]