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What is the difference between Optional.ofNullable() and `Optional.of()

First of all, I know the difference between the two methods.
Optional.of : Used to ensure that there is no null, if null is entered, nullPointExcepction
Optional.ofNullable : may or may not be null. Used to respond flexibly.
So, if I add orElseThrow(() -> new NullPointerException("null data")) to this, will it end up being the same?
I want to throw an error with explicit content.
So I get Optional.ofNullable(data).orElseThrow(() -> new NullPointerException("null data")))
use it as Is this pointless behaviour?
Optional.of(data).orElseThrow(() -> new NullPointerException("null data")))
I think this is also possible, but I'm just using ofNullable() to make the code look consistent.
to sum it up,
In the end, if you add orElseThrow(nullPoint)
Are of or ofNullable the same result?
then rather
Is of.orElseThrow better?

to sum it up, In the end, if you add orElseThrow(nullPoint) Are of or ofNullable the same result?
No. To see this, simply look at the types.
public static <T> Optional<T> of(T value);
public <X extends Throwable> T orElseThrow(Supplier<? extends X> exceptionSupplier)
throws X extends Throwable;
Optional.of returns an Optional<T>, where orElseThrow is going to leave you with a T. So Optional.ofNullable(x).orElseThrow(...) is really just a very roundabout
if (x == null) {
throw new NullPointerException(...);
}
You're not actually doing anything with the Optional, just making one and discarding it in a really verbose way. So if that's your intent, just do an explicit null check; there's no need at all for Optional.
Which raises the question of why we would use of or ofNullable. With the introduction of Optional, there are now two ways to represent the concept of "this value might not exist" in Java: null and Optional.empty(). People on the Internet will argue till the end of time about which is better and when you should use which one (I have strong opinions on this which I'll refrain from sharing here, since it's not what you asked), but the point is that there are two different ways to do it.
For the rest of this post, I'll borrow a bit of notation from Kotlin and write T? to mean "a T value which might be null". It's not valid Java notation, but it gets the point across. So if we want to represent "A T which may or may not exist" in Java, we can use either Optional<T> or T?.
If we want to go from T? to Optional<T>, that's what Optional.ofNullable is for. It says "If the thing is null, give me Optional.empty(); otherwise give me the thing in an Optional". To go the other way, we can use Optional.orElse(null), which says "If I have a T, give it to me, otherwise show me null". So now we have a way to convert between the two approaches. So what's Optional.of for?
You should view Optional.of as an assertion of sorts. If Java had nullable types like Kotlin, then the difference would be something like
public static <T> Optional<T> of(T value);
public static <T> Optional<T> ofNullable(T? value);
That is, ofNullable expects that its value might be null. of is already assuming that it's not. Optional.of should be thought of an assertion that the value you're giving it is not null. If that assertion fails, we throw NullPointerException immediately rather than letting errors propagate to other parts of the program. If you're calling Optional.of and recovering from the NullPointerException it throws[1], then you are doing something very wrong. That function is an assertion we were dealing with non-null data to begin with, and if that assertion fails then your program should fail immediately with a good stack trace.
It sounds like, based on your use case, you have a value that might be null. In that case, Optional.ofNullable makes sense; it's prepared to handle the use case. If you want to throw a custom exception, you should do a null check beforehand (since you're the one handling the null, not Optional) and then call Optional.of. Or, of course, you can just do an old-fashioned null check and not use Optional at all, if you're planning to extract it anyway with orElseThrow. Certainly, the pipeline Optional.ofNullable(value).orElseThrow(...) in one line would be a code smell.
[1] Note that I say "recovering from", not "catching". A nice top-level catch (Exception exc) which logs all errors is perfectly acceptable and generally a good idea in larger applications. But if you're doing catch (NullPointerException exc) { return 0; } or something like that then you need to reconsider which Optional method you should be using.

First of all, I know the difference between the two methods.
Optional.of : Used to ensure that there is no null, if null is
entered, nullPointExcepction
Optional.ofNullable : may or may not be null. Used to respond
flexibly.
There's a clear point of misunderstanding.
The purpose of Optional.of() is not "to ensure that there is no null". It is not meant to be used as an assertion that a value that was passed into it is non-null. For such a validation you can use Objects.requireNonNull(), it'll either throw an NPE, or will return you a non-null value.
In order to be on the same page, the first important thing you have to keep in mind is that optionals were introduced in the JDK for only one particular purpose - to serve as a return type. Any other cases when optional is utilized as a parameter-type, or a field-type, or when optional objects are being stored in a collection isn't considered to be a good practice. As well, as creating an optional just in order to chain methods on it or to hide a null-check is considered to be an antipattern.
Here is a couple of quotes from the answer by #StuartMarks, developer of the JDK:
The primary use of Optional is as follows:
Optional is intended to
provide a limited mechanism for library method return types where
there is a clear need to represent "no result," and where using null
for that is overwhelmingly likely to cause errors.
A typical code smell is, instead of the code using method chaining to
handle an Optional returned from some method, it creates an Optional
from something that's nullable, in order to chain methods and avoid
conditionals.
I also suggest you to have a look at this answer to the question "Should Optional.ofNullable() be used for null check?", also by Stuart Marks.
With all that being said, combination Optional.of().orElseThrow() is both wrong and pointless:
If provided data is null method of() will raise an NPE and orElseThrow() will not be executed (i.e. its exception will get never be fired).
You're abusing the optional by creating an optional object not in order to return a nullable variable wrapped by it, but to hide the null-check (take a look at the quote above). That obscures the purpose of your code. You can use Objects.requireNonNull() instead to throw an exception if the given value must not be null or requireNonNullElse() to provide a default value.
For the same reason, you shouldn't use Optional.ofNullable().orElseThrow() at the first place.
Optional is like a Box
You might think of optional is if it is a parcel. When you need to send something, you go to the post office (i.e. returning from the method), where the thing that has to be sent is being placed into a box. When somebody (i.e. the caller) receives the parcel, it is being immediately unpacked. That is the whole lifecycle of the box called Optional.
When, according to the logic of your application, an object required to be returned from the method should not be null - use Optional.of(). It'll either send a parcel successfully or will emphasize that there's a problem by raising a NullPointerException.
If the given object is nullable by its nature, i.e. null isn't an abnormal case, then use Optional.ofNullable(), it'll fire either a box containing the object or an empty box.
And the caller (i.e. method that invokes the method returning an optional) is the one who has to unpack the box using a variety of tools that optional provides like orElseThrow(), orElseGet(), ifPresent(), etc.

Java 8 Optional - should I use it in this example [duplicate]

Having been using Java 8 now for 6+ months or so, I'm pretty happy with the new API changes. One area I'm still not confident in is when to use Optional. I seem to swing between wanting to use it everywhere something may be null, and nowhere at all.
There seem to be a lot of situations when I could use it, and I'm never sure if it adds benefits (readability / null safety) or just causes additional overhead.
So, I have a few examples, and I'd be interested in the community's thoughts on whether Optional is beneficial.
1 - As a public method return type when the method could return null:
public Optional<Foo> findFoo(String id);
2 - As a method parameter when the param may be null:
public Foo doSomething(String id, Optional<Bar> barOptional);
3 - As an optional member of a bean:
public class Book {
private List<Pages> pages;
private Optional<Index> index;
}
4 - In Collections:
In general I don't think:
List<Optional<Foo>>
adds anything - especially since one can use filter() to remove null values etc, but are there any good uses for Optional in collections?
Any cases I've missed?

The main design goal of Optional is to provide a means for a function returning a value to indicate the absence of a return value. See this discussion. This allows the caller to continue a chain of fluent method calls.
This most closely matches use case #1 in the OP's question. Although, absence of a value is a more precise formulation than null since something like IntStream.findFirst could never return null.
For use case #2, passing an optional argument to a method, this could be made to work, but it's rather clumsy. Suppose you have a method that takes a string followed by an optional second string. Accepting an Optional as the second arg would result in code like this:
foo("bar", Optional.of("baz"));
foo("bar", Optional.empty());
Even accepting null is nicer:
foo("bar", "baz");
foo("bar", null);
Probably the best is to have an overloaded method that accepts a single string argument and provides a default for the second:
foo("bar", "baz");
foo("bar");
This does have limitations, but it's much nicer than either of the above.
Use cases #3 and #4, having an Optional in a class field or in a data structure, is considered a misuse of the API. First, it goes against the main design goal of Optional as stated at the top. Second, it doesn't add any value.
There are three ways to deal with the absence of a value in an Optional: to provide a substitute value, to call a function to provide a substitute value, or to throw an exception. If you're storing into a field, you'd do this at initialization or assignment time. If you're adding values into a list, as the OP mentioned, you have the additional choice of simply not adding the value, thereby "flattening" out absent values.
I'm sure somebody could come up with some contrived cases where they really want to store an Optional in a field or a collection, but in general, it is best to avoid doing this.

I'm late to the game but for what it's worth, I want to add my 2 Cents. They go against the design goal of Optional, which is well summarized by Stuart Marks's answer, but I'm still convinced of their validity (obviously).
Use Optional Everywhere
In General
I wrote an entire blog post about using Optional but it basically comes down to this:
design your classes to avoid optionality wherever feasibly possible
in all remaining cases, the default should be to use Optional instead of null
possibly make exceptions for:
local variables
return values and arguments to private methods
performance critical code blocks (no guesses, use a profiler)
The first two exceptions can reduce the perceived overhead of wrapping and unwrapping references in Optional. They are chosen such that a null can never legally pass a boundary from one instance into another.
Note that this will almost never allow Optionals in collections which is almost as bad as nulls. Just don't do it. ;)
Regarding your questions
Yes.
If overloading is no option, yes.
If other approaches (subclassing, decorating, ...) are no option, yes.
Please no!
Advantages
Doing this reduces the presence of nulls in your code base, although it does not eradicate them. But that is not even the main point. There are other important advantages:
Clarifies Intent
Using Optional clearly expresses that the variable is, well, optional. Any reader of your code or consumer of your API will be beaten over the head with the fact that there might be nothing there and that a check is necessary before accessing the value.
Removes Uncertainty
Without Optional the meaning of a null occurrence is unclear. It could be a legal representation of a state (see Map.get) or an implementation error like a missing or failed initialization.
This changes dramatically with the persistent use of Optional. Here, already the occurrence of null signifies the presence of a bug. (Because if the value were allowed to be missing, an Optional would have been used.) This makes debugging a null pointer exception much easier as the question of the meaning of this null is already answered.
More Null Checks
Now that nothing can be null anymore, this can be enforced everywhere. Whether with annotations, assertions or plain checks, you never have to think about whether this argument or that return type can be null. It can't!
Disadvantages
Of course, there is no silver bullet...
Performance
Wrapping values (especially primitives) into an extra instance can degrade performance. In tight loops this might become noticeable or even worse.
Note that the compiler might be able to circumvent the extra reference for short lived lifetimes of Optionals. In Java 10 value types might further reduce or remove the penalty.
Serialization
Optional is not serializable but a workaround is not overly complicated.
Invariance
Due to the invariance of generic types in Java, certain operations become cumbersome when the actual value type is pushed into a generic type argument. An example is given here (see "Parametric polymorphism").

Personally, I prefer to use IntelliJ's Code Inspection Tool to use #NotNull and #Nullable checks as these are largely compile time (can have some runtime checks) This has lower overhead in terms of code readability and runtime performance. It is not as rigorous as using Optional, however this lack of rigour should be backed by decent unit tests.
public #Nullable Foo findFoo(#NotNull String id);
public #NotNull Foo doSomething(#NotNull String id, #Nullable Bar barOptional);
public class Book {
private List<Pages> pages;
private #Nullable Index index;
}
List<#Nullable Foo> list = ..
This works with Java 5 and no need to wrap and unwrap values. (or create wrapper objects)

I think the Guava Optional and their wiki page puts it quite well:
Besides the increase in readability that comes from giving null a name, the biggest advantage of Optional is its idiot-proof-ness. It forces you to actively think about the absent case if you want your program to compile at all, since you have to actively unwrap the Optional and address that case. Null makes it disturbingly easy to simply forget things, and though FindBugs helps, we don't think it addresses the issue nearly as well.
This is especially relevant when you're returning values that may or may not be "present." You (and others) are far more likely to forget that other.method(a, b) could return a null value than you're likely to forget that a could be null when you're implementing other.method. Returning Optional makes it impossible for callers to forget that case, since they have to unwrap the object themselves for their code to compile.
-- (Source: Guava Wiki - Using and Avoiding null - What's the point?)
Optional adds some overhead, but I think its clear advantage is to make it explicit
that an object might be absent and it enforces that programmers handle the situation. It prevents that someone forgets the beloved != null check.
Taking the example of 2, I think it is far more explicit code to write:
if(soundcard.isPresent()){
System.out.println(soundcard.get());
}
than
if(soundcard != null){
System.out.println(soundcard);
}
For me, the Optional better captures the fact that there is no soundcard present.
My 2¢ about your points:
public Optional<Foo> findFoo(String id); - I am not sure about this. Maybe I would return a Result<Foo> which might be empty or contain a Foo. It is a similar concept, but not really an Optional.
public Foo doSomething(String id, Optional<Bar> barOptional); - I would prefer #Nullable and a findbugs check, as in Peter Lawrey's answer - see also this discussion.
Your book example - I am not sure if I would use the Optional internally, that might depend on the complexity. For the "API" of a book, I would use an Optional<Index> getIndex() to explicitly indicate that the book might not have an index.
I would not use it in collections, rather not allowing null values in collections
In general, I would try to minimize passing around nulls. (Once burnt...)
I think it is worth to find the appropriate abstractions and indicate to the fellow programmers what a certain return value actually represents.

From Oracle tutorial:
The purpose of Optional is not to replace every single null reference in your codebase but rather to help design better APIs in which—just by reading the signature of a method—users can tell whether to expect an optional value. In addition, Optional forces you to actively unwrap an Optional to deal with the absence of a value; as a result, you protect your code against unintended null pointer exceptions.

In java, just don't use them unless you are addicted to functional programming.
They have no place as method arguments (I promess someone one day will pass you a null optional, not just an optional that is empty).
They make sense for return values but they invite the client class to keep on stretching the behavior-building chain.
FP and chains have little place in an imperative language like java because it makes it very hard to debug, not just to read. When you step to the line, you can't know the state nor intent of the program; you have to step into to figure it out (into code that often isn't yours and many stack frames deep despite step filters) and you have to add lots of breakpoints down to make sure it can stop in the code/lambda you added, instead of simply walking the if/else/call trivial lines.
If you want functional programming, pick something else than java and hope you have the tools for debugging that.

1 - As a public method return type when the method could return null:
Here is a good article that shows usefulness of usecase #1. There this code
...
if (user != null) {
Address address = user.getAddress();
if (address != null) {
Country country = address.getCountry();
if (country != null) {
String isocode = country.getIsocode();
isocode = isocode.toUpperCase();
}
}
}
...
is transformed to this
String result = Optional.ofNullable(user)
.flatMap(User::getAddress)
.flatMap(Address::getCountry)
.map(Country::getIsocode)
.orElse("default");
by using Optional as a return value of respective getter methods.

Here is an interesting usage (I believe) for... Tests.
I intend to heavily test one of my projects and I therefore build assertions; only there are things I have to verify and others I don't.
I therefore build things to assert and use an assert to verify them, like this:
public final class NodeDescriptor<V>
{
private final Optional<String> label;
private final List<NodeDescriptor<V>> children;
private NodeDescriptor(final Builder<V> builder)
{
label = Optional.fromNullable(builder.label);
final ImmutableList.Builder<NodeDescriptor<V>> listBuilder
= ImmutableList.builder();
for (final Builder<V> element: builder.children)
listBuilder.add(element.build());
children = listBuilder.build();
}
public static <E> Builder<E> newBuilder()
{
return new Builder<E>();
}
public void verify(#Nonnull final Node<V> node)
{
final NodeAssert<V> nodeAssert = new NodeAssert<V>(node);
nodeAssert.hasLabel(label);
}
public static final class Builder<V>
{
private String label;
private final List<Builder<V>> children = Lists.newArrayList();
private Builder()
{
}
public Builder<V> withLabel(#Nonnull final String label)
{
this.label = Preconditions.checkNotNull(label);
return this;
}
public Builder<V> withChildNode(#Nonnull final Builder<V> child)
{
Preconditions.checkNotNull(child);
children.add(child);
return this;
}
public NodeDescriptor<V> build()
{
return new NodeDescriptor<V>(this);
}
}
}
In the NodeAssert class, I do this:
public final class NodeAssert<V>
extends AbstractAssert<NodeAssert<V>, Node<V>>
{
NodeAssert(final Node<V> actual)
{
super(Preconditions.checkNotNull(actual), NodeAssert.class);
}
private NodeAssert<V> hasLabel(final String label)
{
final String thisLabel = actual.getLabel();
assertThat(thisLabel).overridingErrorMessage(
"node's label is null! I didn't expect it to be"
).isNotNull();
assertThat(thisLabel).overridingErrorMessage(
"node's label is not what was expected!\n"
+ "Expected: '%s'\nActual : '%s'\n", label, thisLabel
).isEqualTo(label);
return this;
}
NodeAssert<V> hasLabel(#Nonnull final Optional<String> label)
{
return label.isPresent() ? hasLabel(label.get()) : this;
}
}
Which means the assert really only triggers if I want to check the label!

Optional class lets you avoid to use null and provide a better alternative:
This encourages the developer to make checks for presence in order to avoid uncaught NullPointerException's.
API becomes better documented because it's possible to see, where to expect the values which can be absent.
Optional provides convenient API for further work with the object:
isPresent(); get(); orElse(); orElseGet(); orElseThrow(); map(); filter(); flatmap().
In addition, many frameworks actively use this data type and return it from their API.

An Optional has similar semantics to an unmodifiable instance of the Iterator design pattern:
it might or might not refer to an object (as given by isPresent())
it can be dereferenced (using get()) if it does refer to an object
but it can not be advanced to the next position in the sequence (it has no next() method).
Therefore consider returning or passing an Optional in contexts where you might previously have considered using a Java Iterator.

Here are some of the methods that you can perform on an instance of Optional<T>:
map
flatMap
orElse
orElseThrow
ifPresentOrElse
get
Here are all the methods that you can perform on null:
(there are none)
This is really an apples to oranges comparison: Optional<T> is an actual instance of an object (unless it is null… but that would probably be a bug) while null is an aborted object. All you can do with null is check whether it is in fact null, or not. So if you like to use methods on objects, Optional<T> is for you; if you like to branch on special literals, null is for you.
null does not compose. You simply can’t compose a value which you can only branch on. But Optional<T> does compose.
You can, for instance, make arbitrary long chains of “apply this function if non-empty” by using map. Or you can effectively make an imperative block of code which consumes the optional if it is non-empty by using ifPresent. Or you can make an “if/else” by using ifPresentOrElse, which consumes the non-empty optional if it is non-empty or else executes some other code.
…And it is at this point that we run into the true limitations of the language in my opinion: for very imperative code you have to wrap them in lambdas and pass them to methods:
opt.ifPresentOrElse(
string -> { // if present...
// ...
}, () -> { // or else...
// ...
}
);
That might not be good enough for some people, style-wise.
It would be more seamless if Optional<T> was an algebraic data type that we could pattern match on (this is obviously pseudo-code:
match (opt) {
Present(str) => {
// ...
}
Empty =>{
// ...
}
}
But anyway, in summary: Optional<T> is a pretty robust empty-or-present object. null is just a sentinel value.
Subjectively disregarded reasons
There seems to be a few people who effectively argue that efficiency should determine whether one should use Optional<T> or branch on the null sentinel value. That seems a bit like making hard and fast rules on when to make objects rather than primitives in the general case. I think it’s a bit ridiculous to use that as the starting point for this discussion when you’re already working in a language where it’s idiomatic to make objects left-and-right, top to bottom, all the time (in my opinion).

I do not think that Optional is a general substitute for methods that potentially return null values.
The basic idea is: The absence of a value does not mean that it potentially is available in the future. It's a difference between findById(-1) and findById(67).
The main information of Optionals for the caller is that he may not count on the value given but it may be available at some time. Maybe it will disappear again and comes back later one more time. It's like an on/off switch. You have the "option" to switch the light on or off. But you have no option if you do not have a light to switch on.
So I find it too messy to introduce Optionals everywhere where previously null was potentially returned. I will still use null, but only in restricted areas like the root of a tree, lazy initialization and explicit find-methods.

Seems Optional is only useful if the type T in Optional is a primitive type like int, long, char, etc. For "real" classes, it does not make sense to me as you can use a null value anyway.
I think it was taken from here (or from another similar language concept).
Nullable<T>
In C# this Nullable<T> was introduced long ago to wrap value types.

1 - As a public method return type when the method could return null:
This is the intended use case for Optional, as seen in the JDK API docs:
Optional is primarily intended for use as a method return type where
there is a clear need to represent "no result," and where using null
is likely to cause errors.
Optional represents one of two states:
it has a value (isPresent returns true)
it doesn't have a value (isEmpty returns true)
So if you have a method that returns either something or nothing, this is the ideal use case for Optional.
Here's an example:
Optional<Guitarist> findByLastName(String lastName);
This method takes a parameter used to search for an entity in the database. It's possible that no such entity exists, so using an Optional return type is a good idea since it forces whoever is calling the method to consider the empty scenario. This reduces chances of a NullPointerException.
2 - As a method parameter when the param may be null:
Although technically possible, this is not the intended use case of Optional.
Let's consider your proposed method signature:
public Foo doSomething(String id, Optional<Bar> barOptional);
The main problem is that we could call doSomething where barOptional has one of 3 states:
an Optional with a value e.g. doSomething("123", Optional.of(new Bar())
an empty Optional e.g. doSomething("123", Optional.empty())
null e.g. doSomething("123", null)
These 3 states would need to be handled in the method implementation appropriately.
A better solution is to implement an overloaded method.
public Foo doSomething(String id);
public Foo doSomething(String id, Bar bar);
This makes it very clear to the consumer of the API which method to call, and null does not need to be passed.
3 - As an optional member of a bean:
Given your example Book class:
public class Book {
private List<Pages> pages;
private Optional<Index> index;
}
The Optional class variable suffers from the same issue as the Optional method parameter discussed above. It can have one of 3 states: present, empty, or null.
Other possible issues include:
serialization: if you implement Serializable and try to serialize an object of this class, you will encounter a java.io.NotSerializableException since Optional was not designed for this use case
transforming to JSON: when serializing to JSON an Optional field may get mapped in an undesirable way e.g. {"empty":false,"present":true}.
Although if you use the popular Jackson library, it does provide a solution to this problem.
Despite these issues, Oracle themselves published this blog post at the time of the Java 8 Optional release in 2014. It contains code examples using Optional for class variables.
public class Computer {
private Optional<Soundcard> soundcard;
public Optional<Soundcard> getSoundcard() { ... }
...
}
In the following years though, developers have found better alternatives such as implementing a getter method to create the Optional object.
public class Book {
private List<Pages> pages;
private Index index;
public Optional<Index> getIndex() {
return Optional.ofNullable(index);
}
}
Here we use the ofNullable method to return an Optional with a value if index is non-null, or otherwise an empty Optional.
4 - In Collections:
I agree that creating a List of Optional (e.g. List<Optional<Foo>>) doesn't add anything.
Instead, just don't include the item in the List if it's not present.

What to do in findXY method if no such object exists?

Assume I have a Java method
public X findX(...)
which tries to find an Object of type X fulfilling some conditions (given in the parameters). Often such functions cannot guarantee to find such an object. I can think of different ways to deal with this:
One could write an public boolean existsX(...) method with the same signature which should be called first. This avoids any kind of exceptions and null handling, but probably you get some duplicate logic.
One could just return null (and explain this in javadoc). The caller has to handle it.
One could throw a checked exception (which one would fit for this?).
What would you suggest?

The new Java 8 Optional class was made for this purpose.
If the object exists then you return Optional.of(x) where x is the object, if it doesn't then return Optional.empty(). You can check if an Optional has an object present by using the isPresent() method and you can get the object using get().
https://docs.oracle.com/javase/8/docs/api/java/util/Optional.html

If you can't use Optional I would go with option 2.
In the case of 1. You'd be doing double work, first you have to check if X exists, but if it does, you're basically discarding the result, and you have to do the work again in findX. Although the result of existsX could be cached and checked first when calling findX, this would still be an extra step over just returning X.
In the case of 3. To me this comes down to usability. Sometimes you just know that findX will return a result (and if it doesn't, there is a mistake somewhere else), but with a checked exception, you would still have to write the try and (most likely empty) catch block.
So option 2 is the winner to me. It doesn't do extra work, and checking the result is optional. As long as you document the null return, there should be no problems.

Guava (potentially other libraries, too) also offers an Optional class that might be worth exploring if your project uses Guava since you seem to not use Java 8.

If you
don't/can't/won't use Java 8 and its accompanying Optional type
don't want another library dependency (like Guava) for a single class implementation
but
you want something more robust than methods that can return null for which you have to have a check in all consumers (which was the standard before Java 8)
then writing your own Optional as an util class is the easiest option.

java.lang.String(String original) is inadequately documented

Specifically, it throws a NullPointerException if original is null. Yet the javadocs do not mention it.
I know, it says this constructor should not be necessary unless an explicit copy is needed, but I am writing copy constructors for some larger objects that contain Strings, and while it probably isn't strictly necessary to make an explicit copy, since everything else is getting an explicit copy in this case, I'm willing to pay a small price in inefficiency.
But shouldn't there be a throws in this javadoc?

From JavaTM 2 Platform Std. Ed. v1.4.2:
Unless otherwise noted, passing a null argument to a constructor or method in this class will cause a NullPointerException to be thrown.

Strings are immutable. You do not want to create a String myString = new String("blah"); since this would result in a new object in heap. It is better to do String myString = "blah"; and link to the strings pool.
String myString = "blah";
String myOtherString = myString;
myOtherString = "";
// at this point myString == "blah"

NullPointerException is an unchecked exception so although if you're very pedantic, you can add a #throws line to the javadoc, it's better not to. It's assumed that if you pass null where it doesn't make sense, you get an NPE. (Unchecked exceptions are more often than not signal a coding error, rather than an exceptional but valid scenario.)
And new String( null ) doesn't make sense, as a constructor couldn't result in an exact "copy" of null.
Copying strings is totally unnecessary too as they're immutable.

The javadoc for this constructor says:
Unless an explicit copy of original is needed, use of this constructor is unnecessary since Strings are immutable
So you really shouldn't be calling it.
Also, I think it's reasonable to throw an NPE if you pass a null in

RuntimeException is reserved for exceptions that indicate incorrect use of an API (using a null object), which can occur anywhere in a Java program, so it's not particular to the String class (or any specific class)

The issue of the documentation has been addressed by the accepted answer.
I just want to respond to this comment ... which is probably the root of your problem.
But that assumes it doesn't make sense to pass null. Suppose you are copying some Object that has many data member fields, some of which are Strings, and which may or may not be null. In that case I want String(String) to return null, specifically, I want it to copy the null value.
That is not possible. The JLS specifically states that the new operation yields a new object and not a null.
If you want to be able to "create and return an object or null" you have to embed this logic in a factory method of some kind; e.g.
String myNewString(String s) {
return s == null ? s : new String(s);
}
But it is also worth noting that in most circumstances copying a String in Java is unnecessary and wasteful. You should only do this if your application is specifically making use of the object identity of the strings; i.e. that it uses == to compare string objects and depends on strings with the same characters comparing as false. (Applications that require that are pretty rare ...)

Is NULL arguments a bad practice?

Is it a bad practice to pass NULL argument to methods or in other words should we have method definitions which allow NULL argument as valid argument.
Suppose i want two method
1. to retrieve list of all of companies
2. to retrieve list of companies based upon filter.
We can either have two methods like as below
List<Company> getAllCompaniesList();
List<Company> getCompaniesList(Company companyFilter);
or we can have one single method
List<Company> getCompaniesList(Company companyFilter);
here in second case, if argument is NULL then method return list of all of companies.
Beside question of good practice practically i see one more issue with later approach which is explained below.
I am implementing Spring AOP, in which i want to have some checks on arguments like
1. Is argument NULL ?
2. is size of collection 0?
There are some scenarios where we can not have null argument at all like for method
void addBranches(int companyId, List<Branch>);
This check can be performed very well by using Spring AOP by defining method like following
#Before(argNames="args", value="execution(* *)")
void beforeCall(JoinPoint joinPoint ,Object[] args )
{
foreach(Object obj in args)
{
if(obj == NULL)
{
throw new Exception("Argument NULL");
}
}
}
But problem i am facing is since i have defined some of methods which should accept NULL argument for multiple functionality of one single method as mentioned above for method List getCompaniesList(Company companyFilter);
So i can not apply AOP uniformly for all of methods and neither some expression for methods name match will be useful here.
Please let me know if more information is required or problem is not descriptive enough.
Thanks for reading my problem and giving thought upon it.

It's fine, in cases when there are too many overloaded methods. So instead of having all combinations of parameters, you allow some of them to be null. But if you do so, document this explicitly with
#param foo foo description. Can be null
In your case I'd have the two methods, where the first one invokes the second with a null argument. It makes the API more usable.
There is no strict line where to stop overloading and where to start relying on nullable parameters. It's a matter of preference. But note that thus your method with the most params will allow some of them to be nullable, so document this as well.
Also note that a preferred way to cope with multiple constructor parameters is via a Builder. So instead of:
public Foo(String bar, String baz, int fooo, double barr, String asd);
where each of the arguments is optional, you can have:
Foo foo = new FooBuilder().setBar(bar).setFooo(fooo).build();

I use a very simple rule:
Never allow null as an argument or return value on a public method.
I make use of Optional and Preconditions or AOP to enforce that rule.
This decision already saved me tons of hours bugfixing after NPE's or strange behaviour.

It's common practice, but there are ways of making your code clearer - avoiding null checks in sometimes, or moving them elsewhere. Look up the null object pattern - it may well be exactly what you need: http://en.m.wikipedia.org/wiki/Null_Object_pattern?wasRedirected=true

The rule is: simple interface, complicated implementation.
Design decisions about your API should be made by considering how the client code is likely to use it. If you expect to see either
getAllCompaniesList(null);
or
if (companyFilter == null) {
getAllCompaniesList();
} else {
getAllCompaniesList(companyFilter);
}
then you're doing it wrong. If the likely use-case is that the client code will, at the time it is written, either have or not have a filter, you should supply two entry points; if that decision is likely not made until run-time, allow a null argument.

Another approach that may be workable may be to have a CompanyFilter interface with an companyIsIncluded(Company) method that accepts a Company and returns true or false to say whether any company should be included. Company could implement the interface so that companyIsIncluded method's behavior mirrored equals(), but one could easily have a singleton CompanyFilter.AllCompanies whose companyIsIncluded() method would always return true. Using that approach, there's no need to pass a null value--just pass a reference to the AllComapnies singleton.