为什么set1为null时set1为什么为null？

Question

In the following snippet I execute a query and pass the result set to set . Then the set is assigned to set1 . After that there is a while loop which goes on until set.next returns false . Is it that if set.next returns false after the loop, set1 will also return false ?

ResultSet set = statement.executeQuery();
ResultSet set1 = set;
while (set.next()) {
    if (set.getString(1).equalsIgnoreCase(keyword)) { 
        // If the search matches the exact keyword
        list.put(set.getString(2), set.getString(1));
        // Where key is the name of the node and value is, name of the file
    }
}

I asked this because:

while (set1.next()) {
    System.out.println("@@@Inside the while statement@@@");
    if (set1.getString(1).contains(keyword) 
    && set1.getString(1).compareToIgnoreCase(keyword) !=0) { 
        // If the search contains the keyword but does not exactly match
        list.put(set.getString(2), set.getString(1));
        // Where key is the name of the node and value is, name of the file
        System.out.println("@@@Inside the if statement of second while loop !@@@");
    }
}

This while construct never works. Is that the reason ? If that is, what do I do ?

Answer 1

You have two major bugs:

1. Assigning set to set1 doesn't make a copy - it's the same set
2. String.contains() is case sensitive (your code doesn't match your comments)

The fix is:

1. Don't use two loops - use just one loop
2. Use `toLowerCase() with contains() to achieve a "case insensitive contains" test
3. Also, if your first test is true, so is your second, so you don't need two tests/loops anyway

Try this code:

   ResultSet set = statement.executeQuery();
   while(set.next()) {
       if(set.getString(1).toLowerCase()
              .contains(keyword.toLowerCase)) {
           list.put(set.getString(2), set.getString(1));
       }
   }

Also, don't call a map "list" - call it "map", otherwise it's just going to confuse readers of your code.

Answer 2

您可以尝试这样做。

   ResultSet set1 = statement.executeQuery();

Answer 3

set1 is a reference to the same object that set points to. If you want to iterate over the ResultSet again, you need to move the iterator cursor to the beginning again using something like:

set.beforeFirst();

In Java, assigning a variable to an object doesn't make a copy of the object, it only references the object in memory. You would have to explicitly make a copy of the object if you wanted set and set1 to work independently.

In your particular use case, I don't think that's what you want, it seems better to just move the iteration cursor.

Answer 4

Is it that if set.next returns false after the loop, set1 will also return false ?

YES . Because set1 is referring the same object of Resultset which set is referring. So whatever behavior is exhibited by the ResultSet object referenced by set will also be reflected by set1 .In your first while loop the ResultSet object referenced by set is exhausted completely after iterating to all records (using set.next()) . Thereafter no matter which variable (set or set1) tries to read more both will get .next() as false .

If in Your first while loop construct I check .next() using set variable and get the result using set1 variable it still will work. Try this code instead of your first construct and you will see that output is same as what you have written in your first while construct:

ResultSet set = statement.executeQuery();
       ResultSet set1 = set;
       while(set.next()) {
           if(set1.getString(1).equalsIgnoreCase(keyword)) { // If the search matches the exact keyword
               list.put(set1.getString(2), set1.getString(1));
               // Where key is the name of the node and value is, name of the file
           }
       }

If you still want set1 to return results you should reconstruct a new ResultSet object using :

set1 = stat.executeQuery()

where, stat is the Statement/PreparedStatement whatever you are using in your code.