A List Implementation That Links Data - Data Structures and Abstractions with Java - page 347

use assert statements involving the second criterion to help us catch an error, as in the following
version of isEmpty :
public boolean isEmpty()
{

boolean result;
if (numberOfEntries == 0) // or getLength() == 0
{

assert firstNode == null ;
result = true ;

}
else
{

assert firstNode != null ;
result = false ;
} // end if
return result;
} // end isEmpty

14.13

Example. Let's look at an example of how the previous implementation of isEmpty can help us
find an error in logic. Consider the definition of the first add method given in Segment 14.9. If
we had been concerned that we might forget to increment the data field numberOfEntries , we
might have written numberOfEntries++ as the method's first action instead of as one of its last.
This change would have caused an error. If the list was empty when the method was called,
numberOfEntries would have been given the value 1, and isEmpty would have been invoked.
Since firstNode would have been null , the second assertion within isEmpty would have pro-
duced an error message like the following one, assuming that we had enabled assertions:
Exception in thread “main” java.lang.AssertionError
at LList.isEmpty(LList.java:167)
at LList.add(LList.java:36)
at Driver.testList(driver.java:16)
at Driver.main(driver.java:5);
This message indicates that the method add called isEmpty , which produced the assertion
error. We could clarify this message by adding to the assert statements in isEmpty . For example, if
the second assert statement is
assert firstNode != null : "numberOfEntries is not 0 but firstNode is null";
the previous error message would begin as follows:
Exception in thread “main” java.lang.AssertionError:
numberOfEntries is not 0 but firstNode is null
If we ran our program without enabling assertions, isEmpty would simply test numberOfEntries .
Since numberOfEntries would not be zero, isEmpty would return false, so add 's else clause would
execute. When add invoked getNodeAt(1) , null would be returned—since firstNode would be
null —and assigned to lastNode . As a result, lastNode.setNextNode(newNode) would cause an
exception and would produce an error message such as

Exception in thread “main” java.lang.NullPointerException
at LList$Node.access$102(LList.java:212)
at LList.add(LList.java:41)
at Driver.testList(driver.java:16)
at Driver.main(driver.java:5);
This message is not as clear as the previous one, so more effort would be needed to discover
the problem.