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
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.
null is actually not instanceof anything!
The instanceof operator from the Java Language Specification (§15.20.2):
At run time, the result of the instanceof operator is true if the value of the RelationalExpression is not null and the reference could be cast (§15.16) to the ReferenceType without raising a ClassCastException. Otherwise the result is false.
4.1. The Kinds of Types and Values
There are two kinds of types in the Java programming language: primitive types (§4.2) and reference types (§4.3). There are, correspondingly, two kinds of data values that can be stored in variables, passed as arguments, returned by methods, and operated on: primitive values (§4.2) and reference values (§4.3).
Type: PrimitiveType ReferenceType There is also a special null type, the type of the expression null (§3.10.7, §15.8.1), which has no name.
Because the null type has no name, it is impossible to declare a variable of the null type or to cast to the null type.
The null reference is the only possible value of an expression of null type.
The null reference can always undergo a widening reference conversion to any reference type.
In practice, the programmer can ignore the null type and just pretend that null is merely a special literal that can be of any reference type.
Formally, null is a singleton member of the null type, which is defined to be the subtype of every other Java type.
null is a reference type and its value is the only reference value which doesn't refer to any object. Therefore there is no representation of null in memory. The binary value of a reference-typed variable whose value is null is simply zero (all zero bits). Even though this is not explicitly specified, it follows from the general initialization semantics of objects and any other value would cause major problems to an implementation.