Gracefully avoiding NullPointerException in Java

问题

Consider this line:

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

Obviously this line is a potential bug, the attribute might be null and we will get a NullPointerException. So we need to refactor it to one of two choices:

First option:

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

Second option:

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

The first option is awkward to read but more concise, while the second one is clear in intent, but verbose.

Which option do you prefer in terms of readability?

回答1:

I've always used

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

because although it is a little more difficult to read it's much less verbose and I think it's readable enough so you get used to it very easily

回答2:

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

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

回答3:

There are certain situations where the concise approach feels wrong to start with but effectively becomes idiomatic. This is one of them; the other is something like:

String line;
while ((line = bufferedReader.readLine()) != null) {
  // Use line
}

Side effects in a condition? Unthinkable! Except it's basically nicer than the alternatives, when you recognise the particular pattern.

This pattern is similar - it's so common in Java that I'd expect any reasonably experienced developer to recognise it. The result is pleasantly concise. (Amusingly, I sometimes see C# code which uses the same idiom, unnecessarily - the equality operator works fine with strings in C#.)

Bottom line: use the first version, and become familiar with it.

回答4:

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.

回答5:

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

回答6:

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.

回答7:

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)

回答8:

I have another answer;

List<Map<String, Object>> group = jjDatabase.separateRow(db.Select("SELECT * FROM access_user_group  WHERE user_id=1 ;"));

there is not "group_c80" as column in 'access_user_group' in my database, so in get(0).get("group_c80") null pointer exception accords. But i handled it through below code :

for (int j = 1; j < 100; j++) {
                    String rulId="0";//defult value,to privent null pointer exeption in group_c
                    try {
                        rulId = group.get(0).get("group_c" + j)).toString();
                    } catch (Exception ex) {
                        ServerLog.Print( "Handeled error in database for " + "group_c" + (j < 10 ? "0" + j : j) +"This error handeled and mot efect in program");
                        rulId = "0";
                    }}

回答9:

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

来源：https://stackoverflow.com/questions/963936/gracefully-avoiding-nullpointerexception-in-java

标签

java

coding-style

nullpointerexception

code-readability