I am a learning java, btw why is used the return null;?
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.
Change your return type to java.lang.Integer . This way you can safely return null
java - What happens when a method returns null - Stack Overflow
How can I return null in Java when the method is defined as int data type? - Stack Overflow
What does '\0' mean in java?
Why we return null on java?
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.
null is valid value for any Object in Java. Since Lot is also a Java object. null is valid.
But
If you are not careful you may end up with NullPointerException.
Eg:
Lot lot=someInstance.getLot(2); // say lot is null
Then
String something=lot.getThis(); // here is null.getThis()
You will end up NullPointerException here.
You need to handle with care these cases to avoid NullPointerException.
Eg:
Lot lot=someInstance.getLot(2);
if(lot!=null){
String something=lot.getThis();
}
Null in java means, that your instance (variable) contains no object. You can use it, but you must not call any method on that object, because If you did so, you'd get a NullPointerException.
When null is returned from a method, it usually means that the method was not able to create a meaningful result. For example the method, that reads data from database was not able to find the specified object or some error occurred during the method run.
If a method can return null, then you should check the result before further processing like. See the example of raising a sallary of an employee:
Employee e = database.getEmployeeById(1);
if (e==null) //this is the check
{
System.out.println('There is no such employee');
}
else
{
e.setSallary(e.getSallary() * 1.1);
}