Gracefully avoiding NullPointerException in Java

Question

Consider this line:

if (object.getAttribute(\"someAttr\").equals(\"true\")) { // ....

Obviously this line is a potential bug, the attribute

Answer 1

Here is my approach, needs a PropertyUtil class though, but its only written once:

/**
 * Generic method to encapsulate type casting and preventing nullPointers.
 * 
 * @param <T>          The Type expected from the result value.
 * @param o            The object to cast.
 * @param typedDefault The default value, should be of Type T.
 * 
 * @return Type casted o, of default.
 */
public static <T> T getOrDefault (Object o, T typedDefault) {
    if (null == o) {
        return typedDefault;
    }
    return (T) o;
}

Client code can do this:

PropertyUtil.getOrDefault(obj.getAttribute("someAttr"), "").equals("true");

or, for a list:

PropertyUtil.getOrDefault(
    genericObjectMap.get(MY_LIST_KEY), Collections.EMPTY_LIST
).contains(element);

Or to a consumer of List, that would reject Object:

consumeOnlyList(
    PropertyUtil.getOrDefault(
        enericObjectMap.get(MY_LIST_KEY), Collections.EMPTY_LIST
    )
)

The default might be an impl of the null object pattern https://en.wikipedia.org/wiki/Null_Object_pattern

Answer 2

Util.isEmpty(string) - returns string == null || string.trim().isEmpty() Util.notNull(string) returns "" if string == null, string otherwise. Util.isNotEmpty(string) returns ! Util.isEmpty(string)

And we have a convention that for strings, Util.isEmpty(string) semantically means true and Util.isNotEmpty(string) semantically means false.

Answer 3

In the second option, you can take advantage of short-circuiting &&:

String attr = object.getAttribute("someAttr");
if (attr != null && attr.equals("true")) { // ....

Answer 4

It's a very good question. I usually use the not graceful:

if (object.getAttribute("someAttr") != null && object.getAttribute("someAttr").equals("true")) { // ....

(and I will not use it anymore)

Answer 5

I like option 1 and I would argue that it is readable enough.

Option 3 btw would be to introduce a getAttribute method that takes a default value as a parameter.

Answer 6

Always aspire for shorter code, given that both are functionaly equivalent. Especially in case like this where readability is not sacrificed.