En hurtig og grundig guide til 'null': hvad det er, og hvordan du skal bruge det

Hvad er meningen med null? Hvordan nullimplementeres? Hvornår skal du bruge nulli din kildekode, og hvornår skal du ikke bruge den?

Introduktion

nuller et grundlæggende koncept i mange programmeringssprog. Det er allestedsnærværende i alle slags kildekoder skrevet på disse sprog. Så det er vigtigt at forstå ideen om null. Vi er nødt til at forstå dets semantik og implementering, og vi har brug for at vide, hvordan vi bruger nulli vores kildekode.

Kommentarer i programmørfora afslører undertiden en smule forvirring med null. Nogle programmører prøver endda helt at undgå null. Fordi de tænker på det som en 'million-dollar-fejl', et udtryk, der er opfundet af Tony Hoare, opfinderen af null.

Her er et simpelt eksempel: Antag at Alice email_addresspeger på null. Hvad betyder det? Betyder det, at Alice ikke har en e-mail-adresse? Eller at hendes e-mail-adresse er ukendt? Eller at det er hemmeligt? Eller betyder det simpelthen, at det email_addresser 'udefineret' eller 'ikke-initialiseret'? Lad os se. Efter at have læst denne artikel skal alle være i stand til at besvare sådanne spørgsmål uden tøven.

Bemærk: Denne artikel er programmeringssprogneutral - så vidt muligt. Forklaringer er generelle og ikke bundet til et bestemt sprog. Se dine manualer til programmeringssprog for specifik rådgivning om null. Denne artikel indeholder dog nogle enkle kildekodeeksempler, der vises i Java. Men det er ikke svært at oversætte dem til dit yndlingssprog.

Kørselstid Implementering

Før vi diskuterer betydningen af null, er vi nødt til at forstå, hvordan nullimplementeres i hukommelsen ved kørselstid.

Bemærk: Vi vil se på en typisk implementering af null. Den faktiske implementering i et givet miljø afhænger af programmeringssprog og målmiljø og kan afvige fra implementeringen vist her.

Antag, at vi har følgende kildekodeinstruktion:

String name = "Bob";

Her erklærer vi en variabel af typen Stringog med identifikatoren, nameder peger på strengen "Bob".

At sige "peger på" er vigtigt i denne sammenhæng, fordi vi antager, at vi arbejder med referencetyper (og ikke med værdityper ). Mere om dette senere.

For at holde tingene enkle vil vi antage følgende antagelser:

  • Ovenstående instruktion udføres på en 16-bit CPU med et 16-bit adresserum.
  • Strenge er kodet som UTF-16. De afsluttes med 0 (som i C eller C ++).

Følgende billede viser et uddrag af hukommelsen efter udførelse af ovenstående instruktion:

Hukommelsesadresserne i ovenstående billede vælges vilkårligt og er irrelevante for vores diskussion.

Som vi kan se, er strengen "Bob"gemt på adresse B000 og optager 4 hukommelsesceller.

Variablen namefindes på adresse A0A1. Indholdet af A0A1 er B000, som er starthukommelsesplaceringen for strengen "Bob". Derfor siger vi: Variablen namepeger på"Bob" .

Så langt så godt.

Antag nu, at efter at have udført ovenstående instruktion, udfører du følgende:

name = null;

Peger nu namenull.

Og dette er den nye tilstand i hukommelsen:

Vi kan se, at intet har ændret sig for strengen, "Bob"der stadig er gemt i hukommelsen.

Bemærk: Den nødvendige hukommelse til lagring af strengen "Bob"frigives muligvis senere, hvis der er en affaldssamler og ingen andre referencepunkter til "Bob", men dette er irrelevant i vores diskussion.

Hvad der er vigtigt er, at indholdet af A0A1 (som repræsenterer værdien af ​​variablen name) nu er 0000. Så variabel namepeger ikke "Bob"længere på. Værdien 0 (alle bits ved nul) er en typisk værdi, der bruges i hukommelsen til at betegne null. Det betyder, at der ikke er nogen værdi forbundet medname . Du kan også tænke på det som fravær af data eller simpelthen ingen data .

Bemærk: Den aktuelle hukommelsesværdi, der bruges til at betegne, nuller implementeringsspecifik. For eksempel angiver Java Virtual Machine Specification i slutningen af ​​afsnit 2.4. " Referencetyper og værdier:"

Specifikationen for Java Virtual Machine kræver ikke en konkret værdikodning null.

Husk:

Hvis en reference peger på null, betyder det simpelthen, at der eringen værdi forbundet med det .

Teknisk set indeholder den hukommelsesplacering, der er tildelt referencen, værdien 0 (alle bits ved nul) eller enhver anden værdi, der angiver nulli det givne miljø.

Ydeevne

Som vi lærte i det foregående afsnit, er involverede operationer nullekstremt hurtige og lette at udføre i løbetid.

Der er kun to slags operationer:

  • Initialiser eller indstil en henvisning til null(f.eks. name = null): Den eneste ting at gøre er at ændre indholdet i en hukommelsescelle (f.eks. Indstille den til 0).
  • Kontroller, om en reference peger på null(f.eks. if name == null): Det eneste, du skal gøre, er at kontrollere, om referencens hukommelsescelle har værdien 0.

Husk:

Drift på nuller ekstremt hurtig og billig.

Reference vs værdityper

Indtil videre antog vi at arbejde med referencetyper . Årsagen til dette er enkel: nullfindes ikke for værdityper .

Hvorfor?

Som vi tidligere har set, er en henvisning en markør til en hukommelsesadresse, der gemmer en værdi (f.eks. En streng, en dato, en kunde, uanset hvad). Hvis en reference peger på null, er der ingen værdi forbundet med den.

På den anden side er en værdi pr. Definition selve værdien. Der er ingen pointer involveret. En værditype gemmes som selve værdien. Derfor eksisterer begrebet nullikke for værdityper.

The following picture demonstrates the difference. On the left side you can see again the memory in case of variable name being a reference pointing to "Bob". The right side shows the memory in case of variable name being a value type.

As we can see, in case of a value type, the value itself is directly stored at the address A0A1 which is associated with variable name.

There would be much more to say about reference versus value types, but this is out of the scope of this article. Please note also that some programming languages support only reference types, others support only value types, and some (e.g. C# and Java) support both of them.

Remember:

The concept of null exists only for reference types. It doesn't exist for value types.

Meaning

Suppose we have a type person with a field emailAddress. Suppose also that, for a given person which we will call Alice, emailAddress points to null.

What does this mean? Does it mean that Alice doesn’t have an email address? Not necessarily.

As we have seen already, what we can assert is that no value is associated with emailAddress.

But why is there no value? What is the reason of emailAddress pointing to null? If we don't know the context and history, then we can only speculate. The reason for nullcould be:

Alice doesn’t have an email address. Or…

Alice has an email address, but:

  • it has not yet been entered in the database
  • it is secret (unrevealed for security reasons)
  • there is a bug in a routine that creates a person object without setting field emailAddress
  • and so on.

In practice we often know the application and context. We intuitively associate a precise meaning to null. In a simple and flawless world, null would simply mean that Alice actually doesn't have an email address.

When we write code, the reason why a reference points to null is often irrelevant. We just check for null and take appropriate actions. For example, suppose that we have to write a loop that sends emails for a list of persons. The code (in Java) could look like this:

for ( Person person: persons ) { if ( person.getEmailAddress() != null ) { // code to send email } else { logger.warning("No email address for " + person.getName()); }}

In the above loop we don’t care about the reason for null. We just acknowledge the fact that there is no email address, log a warning, and continue.

Remember:

If a reference points to null then it always means that there isno value associated with it.

In most cases, null has a more specific meaning that depends on the context.

Why is it null?

Sometimes it is important to know why a reference points to null.

Consider the following function signature in a medical application:

List getAllergiesOfPatient ( String patientId )

In this case, returning null (or an empty list) is ambiguous. Does it mean that the patient doesn't have allergies, or does it mean that an allergy test has not yet been performed? These are two semantically very different cases that must be handled differently. Or else the outcome might be life-threatening.

Just suppose that the patient has allergies, but an allergy test has not yet been done and the software tells the doctor that 'there are no allergies'. Hence we need additional information. We need to know why the function returns null.

It would be tempting to say: Well, to differentiate, we return null if an allergy test has not yet been performed, and we return an empty list if there are no allergies.

DON’T DO THIS!

This is bad data design for multiple reasons.

The different semantics for returning null versus returning an empty list would need to be well documented. And as we all know, comments can be wrong (i.e. inconsistent with the code), outdated, or they might even be inaccessible.

There is no protection for misuses in client code that calls the function. For example, the following code is wrong, but it compiles without errors. Moreover, the error is difficult to spot for a human reader. We can’t see the error by just looking at the code without considering the comment of getAllergiesOfPatient:

List allergies = getAllergiesOfPatient ( "123" ); if ( allergies == null ) { System.out.println ( "No allergies" ); // <-- WRONG!} else if ( allergies.isEmpty() ) { System.out.println ( "Test not done yet" ); // <-- WRONG!} else { System.out.println ( "There are allergies" );}

The following code would be wrong too:

List allergies = getAllergiesOfPatient ( "123" );if ( allergies == null || allergies.isEmpty() ) { System.out.println ( "No allergies" ); // <-- WRONG!} else { System.out.println ( "There are allergies" );}

If the null/empty-logic of getAllergiesOfPatient changes in the future, then the comment needs to be updated, as well as all client code. And there is no protection against forgetting any one of these changes.

If, later on, there is another case to be distinguished (e.g. an allergy test is pending — the results are not yet available), or if we want to add specific data for each case, then we are stuck.

So the function needs to return more information than just a list.

There are different ways to do this, depending on the programming language we use. Let’s have a look at a possible solution in Java.

In order to differentiate the cases, we define a parent type AllergyTestResult, as well as three sub-types that represent the three cases (NotDone, Pending, and Done):

interface AllergyTestResult {}
interface NotDoneAllergyTestResult extends AllergyTestResult {}
interface PendingAllergyTestResult extends AllergyTestResult { public Date getDateStarted();}
interface DoneAllergyTestResult extends AllergyTestResult { public Date getDateDone(); public List getAllergies(); // null if no allergies // non-empty if there are // allergies}

As we can see, for each case we can have specific data associated with it.

Instead of simply returning a list, getAllergiesOfPatient now returns an AllergyTestResult object:

AllergyTestResult getAllergiesOfPatient ( String patientId )

Client code is now less error-prone and looks like this:

AllergyTestResult allergyTestResult = getAllergiesOfPatient("123");
if (allergyTestResult instanceof NotDoneAllergyTestResult) { System.out.println ( "Test not done yet" ); } else if (allergyTestResult instanceof PendingAllergyTestResult) { System.out.println ( "Test pending" ); } else if (allergyTestResult instanceof DoneAllergyTestResult) { List list = ((DoneAllergyTestResult) allergyTestResult).getAllergies(); if (list == null) { System.out.println ( "No allergies" ); } else if (list.isEmpty()) { assert false; } else { System.out.println ( "There are allergies" ); }} else { assert false;}

Note: If you think that the above code is quite verbose and a bit hard to write, then you are not alone. Some modern languages allow us to write conceptually similar code much more succinctly. And null-safe languages distinguish between nullable and non-nullable values in a reliable way at compile-time — there is no need to comment the nullability of a reference or to check whether a reference declared to be non-null has accidentally been set to null.

Remember:

If we need to know why there is no value associated with a reference, then additional data must be provided to differentiate the possible cases.

Initialization

Consider the following instructions:

String s1 = "foo";String s2 = null;String s3;

The first instruction declares a String variable s1 and assigns it the value "foo".

The second instruction assigns null to s2.

The more interesting instruction is the last one. No value is explicitly assigned to s3. Hence, it is reasonable to ask: What is the state of s3 after its declaration? What will happen if we write s3 to the OS output device?

It turns out that the state of a variable (or class field) declared without assigning a value depends on the programming language. Moreover, each programming language might have specific rules for different cases. For example, different rules apply for reference types and value types, static and non-static members of a class, global and local variables, and so on.

As far as I know, the following rules are typical variations encountered:

  • It is illegal to declare a variable without also assigning a value
  • There is an arbitrary value stored in s3, depending on the memory content at the time of execution - there is no default value
  • A default value is automatically assigned to s3. In case of a reference type, the default value is null. In case of a value type, the default value depends on the variable’s type. For example 0 for integer numbers, false for a boolean, and so on.
  • the state of s3 is 'undefined'
  • the state of s3 is 'uninitialized', and any attempt to use s3 results in a compile-time error.

The best option is the last one. All other options are error-prone and/or impractical — for reasons we will not discuss here, because this article focuses on null.

As an example, Java applies the last option for local variables. Hence, the following code results in a compile-time error at the second line:

String s3;System.out.println ( s3 );

Compiler output:

error: variable s3 might not have been initialized

Remember:

If a variable is declared, but no explicit value is assigned to it, then it’s state depends on several factors which are different in different programming languages.

In some languages, null is the default value for reference types.

When to Use null (And When Not to Use It)

The basic rule is simple: null should only be allowed when it makes sense for an object reference to have 'no value associated with it'. (Note: an object reference can be a variable, constant, property (class field), input/output argument, and so on.)

For example, suppose type person with fields name and dateOfFirstMarriage:

interface Person { public String getName(); public Date getDateOfFirstMarriage();}

Every person has a name. Hence it doesn’t make sense for field name to have 'no value associated with it'. Field name is non-nullable. It is illegal to assign null to it.

On the other hand, field dateOfFirstMarriage doesn't represent a required value. Not everyone is married. Hence it makes sense for dateOfFirstMarriage to have 'no value associated with it'. Therefore dateOfFirstMarriage is a nullable field. If a person's dateOfFirstMarriage field points to null then it simply means that this person has never been married.

Note: Unfortunately most popular programming languages don’t distinguish between nullable and non-nullable types. There is no way to reliably state that null can never be assigned to a given object reference. In some languages it is possible to use annotations, such as the non-standard annotations @Nullable and @NonNullable in Java. Here is an example:

interface Person { public @Nonnull String getName(); public @Nullable Date getDateOfFirstMarriage();}

However, such annotations are not used by the compiler to ensure null-safety. Still, they are useful for the human reader, and they can be used by IDEs and tools such as static code analyzers.

It is important to note that null should not be used to denote error conditions.

Consider a function that reads configuration data from a file. If the file doesn’t exist or is empty, then a default configuration should be returned. Here is the function’s signature:

public Config readConfigFromFile ( File file )

What should happen in case of a file read error?

Simply return null?

NO!

Each language has it’s own standard way to signal error conditions and provide data about the error, such as a description, type, stack trace, and so on. Many languages (C#, Java, etc.) use an exception mechanism, and exceptions should be used in these languages to signal run-time errors. readConfigFromFile should not return null to denote an error. Instead, the function's signature should be changed in order to make it clear that the function might fail:

public Config readConfigFromFile ( File file ) throws IOException

Remember:

Allow null only if it makes sense for an object reference to have 'no value associated with it'.

Don’t use null to signal error conditions.

Null-safety

Consider the following code:

String name = null;int l = name.length();

Ved kørsel resulterer ovenstående kode i den berygtede null-pointerfejl , fordi vi forsøger at udføre en metode til en reference, der peger på null. I C # NullReferenceExceptionkastes f.eks. A, i Java er det a NullPointerException.

Nulmarkeringsfejlen er ubehagelig.

Det er den hyppigste fejl i mange softwareapplikationer og har været årsagen til utallige problemer i historien om softwareudvikling. Tony Hoare, opfinderen af null, kalder det 'en milliard-dollar-fejl'.

Men Tony Hoare (Turing Award-vinder i 1980 og opfinder af Quicksort-algoritmen) giver også et tip til en løsning i sin tale:

Senere programmeringssprog ... har indført erklæringer for ikke-nul-referencer. Dette er løsningen, som jeg afviste i 1965.

Contrary to some common belief, the culprit is not null per se. The problem is the lack of support for null handling in many programming languages. For example, at the time of writing (May 2018), none of the top ten languages in the Tiobe index natively differentiates between nullable and non-nullable types.

Therefore, some new languages provide compile-time null-safety and specific syntax for conveniently handling null in source code. In these languages, the above code would result in a compile-time error. Software quality and reliability increases considerably, because the null pointer error delightfully disappears.

Null-safety is a fascinating topic that deserves its own article.

Remember:

Whenever possible, use a language that supports compile-time null-safety.

Note: Some programming languages (mostly functional programming languages like Haskell) don’t support the concept of null. Instead, they use the Maybe/Optional Patternto represent the ‘absence of a value’. The compiler ensures that the ‘no value’ case is handled explicitly. Hence, null pointer errors cannot occur.

Summary

Here is a summary of key points to remember:

  • If a reference points to null, it always means that there is no value associated with it.
  • In most cases, null has a more specific meaning that depends on the context.
  • If we need to know why there is no value associated with a reference, then additional data must be provided to differentiate the possible cases.
  • Tillad nullkun, hvis det giver mening, at en objektreference har 'ingen værdi forbundet med det'.
  • Brug ikke nulltil at signalere fejlforhold.
  • Begrebet nulleksisterer kun for referencetyper. Det findes ikke for værdityper.
  • På nogle sprog nuller standardværdien for referencetyper.
  • null operationer er ekstremt hurtige og billige.
  • Når det er muligt, skal du bruge et sprog, der understøtter kompileringstid-nul-sikkerhed.