int is a primitive, null is not a value that it can take on. You could change the method return type to return java.lang.Integer and then you can return null, and existing code that returns int will get autoboxed.
Nulls are assigned only to reference types, it means the reference doesn't point to anything. Primitives are not reference types, they are values, so they are never set to null.
Using the object wrapper java.lang.Integer as the return value means you are passing back an Object and the object reference can be null.
Answer from Nathan Hughes on Stack Overflowint is a primitive, null is not a value that it can take on. You could change the method return type to return java.lang.Integer and then you can return null, and existing code that returns int will get autoboxed.
Nulls are assigned only to reference types, it means the reference doesn't point to anything. Primitives are not reference types, they are values, so they are never set to null.
Using the object wrapper java.lang.Integer as the return value means you are passing back an Object and the object reference can be null.
Change your return type to java.lang.Integer . This way you can safely return null
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I am a learning java, btw why is used the return null;?
StackOverflow has a good discussion about this exact topic in this Q&A. In the top rated question, kronoz notes:
Returning null is usually the best idea if you intend to indicate that no data is available.
An empty object implies data has been returned, whereas returning null clearly indicates that nothing has been returned.
Additionally, returning a null will result in a null exception if you attempt to access members in the object, which can be useful for highlighting buggy code - attempting to access a member of nothing makes no sense. Accessing members of an empty object will not fail meaning bugs can go undiscovered.
Personally, I like to return empty strings for functions that return strings to minimize the amount of error handling that needs to be put in place. However, you'll need to make sure that the group that your working with will follow the same convention - otherwise the benefits of this decision won't be achieved.
However, as the poster in the SO answer noted, nulls should probably be returned if an object is expected so that there is no doubt about whether data is being returned.
In the end, there's no single best way of doing things. Building a team consensus will ultimately drive your team's best practices.
In all the code I write, I avoid returning null from a function. I read that in Clean Code.
The problem with using null is that the person using the interface doesn't know if null is a possible outcome, and whether they have to check for it, because there's no not null reference type.
In F# you can return an option type, which can be some(Person) or none, so it's obvious to the caller that they have to check.
The analogous C# (anti-)pattern is the Try... method:
public bool TryFindPerson(int personId, out Person result);
Now I know people have said they hate the Try... pattern because having an output parameter breaks the ideas of a pure function, but it's really no different than:
class FindResult<T>
{
public FindResult(bool found, T result)
{
this.Found = found;
this.Result = result;
}
public bool Found { get; private set; }
// Only valid if Found is true
public T Result { get; private set;
}
public FindResult<Person> FindPerson(int personId);
...and to be honest you can assume that every .NET programmer knows about the Try... pattern because it's used internally by the .NET framework. That means they don't have to read the documentation to understand what it does, which is more important to me than sticking to some purist's view of functions (understanding that result is an out parameter, not a ref parameter).
So I'd go with TryFindPerson because you seem to indicate it's perfectly normal to be unable to find it.
If, on the other hand, there's no logical reason that the caller would ever provide a personId that didn't exist, I would probably do this:
public Person GetPerson(int personId);
...and then I'd throw an exception if it was invalid. The Get... prefix implies that the caller knows it should succeed.
Your code as it stands try's to index into the Object array without checking for null with the code things.length, which will throw a NullPointerException because there is no array object in existence, so the length field also does not exist.
To fix this use an if statement!
static String concatAll(Object[] things) {
if(things == null) {
return null;
}
...continue code....
This will first check things for a null value BEFORE executing code that could throw an exception.
Calling a method on a null object causes a NullPointerException.
In your case if obj = null, then when you call concatAll, things will be null as well. You then try and call things.length which is not valid on a null object.
You can make this method safer by checking the input of strings first. What you want to do exactly will depend on your situation, but here is one example.
static String concatAll(Object[] things) {
if (things == null)
{
return "";
}
StringBuffer result = new StringBuffer();
for (int i = 0; i < things.length; i++) {
result.append( things[i] );
}
String x = result.toString();
return x;
}
If you're still getting an NPE, then the problem is in getNeighbours and not the second snippet.
this.adjacencyListis null, -OR-this.adjacencyList.get(v)returns null.
Given that you're passing a name to a method that will then do a lookup by node, and that you can't call .get(someNodeRef) on a list, adjacencyList is probably some sort of hashmap, so your names are off and you should rename some things. Map's .get(x) method returns null if an entry is not found, so most likely the culprit is that v isn't in the map at all, and thus .get(v).isEmpty() throws NPE.
The fixes are as follows:
You should NEVER return null when a valid sentinel value that carries the intended semantic meaning is available. A mouthful, but it means here: Why are you returning null when you intend to treat that the exact same way as 'zero nodes'? There is an instance of
Iterable<Node>that properly represents the concept of zero nodes, and it isn'tnull. It'sList.of()or equivalent: An empty list has no nodes. Great. That's what you intended. So return that..get(v).isEmpty()is bad code here, as it would mean an NPE occurs if you ask for a non-existent node. Unless, of course, you want it to work that way. An easy way out is the defaulting mechanism: Call.getOrDefaultinstead:
if (!this.adjacencyList.getOrDefault(v, List.of()).isEmpty()) ....
except, of course, you should never be returning null when you can return an empty list instead, so your getNeighbours method becomes simply:
return adjacencyMap.getOrDefault(v, List.of());
that one-liner will fix all things.
In general, if you are writing code where null is dealt with in some way, and some sentinel value (such as a blank string or an empty list) is dealt with in the same way, your code is badly styled; however you got that null should have gotten you that empty value instead. e.g. if you ever write this:
if (x == null || x.isEmpty()) ...
you messed up. Figure out where you got x from. Update it there, make x the blank sentinel ("" for strings, List.of for lists, etcetera).
That, and use .getOrDefault and other such methods more: Methods that let you provide what should happen when e.g. a key is not found.
You should probably avoid returning null from your getNeighbors method. It's not good practice to return null for Iterables, Iterators and Collections, since an empty iterable would represent the same concept (there is nothing in that adjacency list) without all the dangers of null. And your code would be simpler. You can check if the iterable contains anything and if not then default to the full iterator.
I was trying to test it with this code:
import java.util.Objects;
public class Main {
public static void main (String args[]) {
String a = myMethod();
System.out.println(Objects.isNull(a));
}
public static String myMethod() {
}
}But a compilation error is raised before I can see the outcome.
Just out of curiosity