NullPointerException problem
Consider the following method inside a class
mi.NpeString:
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/**
* <p>Calculate the sum of lengths of all strings belonging to a given array.</p>
*
* <p>Example: The array {"a", "string"} consists of two strings having lengths 1 and 6 respectively. The
* overall sum of lenghts is thus 1 + 6 == 7.</p>
*
* @param strings An array of string or null values.
* @return The sum of all string's length values. <code>null</code> entries will be treated
* as empty strings.
*/
static int getLength(final String[] strings) {
int length = 0;
for (final String s: strings) {
length += s.length();
}
return length;
} |
We observe the following run-time error at line 26:
Exception in thread "main" java.lang.NullPointerException
at mi.NpeString.getLength(NpeString.java:26)
at ...What caused this NPE error? Provide a possible sample data array causing this exception. Explain the implementation error with respect to the provided Javadoc promise and propose a solution resolving the flaw.
Solution
The Javadoc™ contract regarding our method
getLength(final String[] strings)'s parameter
strings states:
/**
...
* @param strings An array of string or null values.
...The variable strings may thus contain
null values e.g.:
String[] names = {"Jim", null, "Eve"};
int length = getLength(names); // Will cause a NPEThe local variable s may therefore
become null causing a NullPointerException
when invoking s.length() requiring an if - guard filtering null values beforehand:
static int getLength(final String[] strings) {
int length = 0;
for (final String s: strings) {
if (null != s) {
length += s.length();
}
}
return length;
}