Gotchas: Difference between revisions
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meaningless sentences are legal, so you do get less immediate feedback on your
mistakes.
=={{header|Java}}==
<syntaxhighlight lang="java">
import java.util.ArrayList;
import java.util.HashSet;
import java.util.List;
import java.util.Set;
public final class Gotchas {
public static void main(String[] aArgs) {
// Gotcha 1: An integer argument to a Collection, such as a List, sets the capacity of the Collection,
// but does not fill the Collection with elements.
List<Integer> numbers = new ArrayList<Integer>(100);
// The above list has the capacity to hold 100 elements, but is currently empty.
// Setting the element with index 3 to a value of 42 will create a runtime exception,
// because the list has length 0, and this element does not yet exist.
numbers.set(3, 42);
// The gotcha is only revealed when a runtime exception is thrown.
System.out.println(numbers);
// java.lang.IndexOutOfBoundsException: Index 3 out of bounds for length 0
// Gotcha 2: Copying a Collection in a simple manner works,
// but means that changes to the original Collection are reflected in the copy,
// which is not normally the desired outcome.
Set<String> letters = new HashSet<String>();
letters.add("a"); letters.add("b"); letters.add("c");
// Create a copy of the set.
Set<String> copy = letters;
// The two sets are identical.
System.out.println(letters + " :: " + copy);
// Add an element to the set 'letters'.
letters.add("d");
// The same letter has been added to the set 'copy'.
// Both sets now contain the same 4 letters.
System.out.println(letters + " :: " + copy);
// In a program this can cause mysterious results which can be difficult to debug.
// The correct way to copy a Collection is to use a copy constructor as shown below.
Set<String> correctCopy = new HashSet<String>(letters);
letters.add("e");
// The set 'correctCopy' only contains its original 4 letters.
System.out.println(letters + " :: " + correctCopy);
}
}
</syntaxhighlight>
=={{header|jq}}==
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