Tipi di corde in Groovy

1. Panoramica

In questo tutorial, daremo uno sguardo più da vicino ai diversi tipi di corde in Groovy, comprese le stringhe con virgolette singole, doppie virgolette, triple e slashy.

Esploreremo anche il supporto delle stringhe di Groovy per caratteri speciali, multi-riga, regex, escape e interpolazione variabile.

2. Miglioramento di java.lang.String

Probabilmente è bene iniziare affermando che poiché Groovy è basato su Java, ha tutte le funzionalità String di Java come la concatenazione, l'API String e i vantaggi intrinseci del pool di costanti String a causa di ciò.

Vediamo prima come Groovy estende alcune di queste basi.

2.1. Concatenazione di stringhe

La concatenazione di stringhe è solo una combinazione di due stringhe:

def first = 'first' def second = "second" def concatenation = first + second assertEquals('firstsecond', concatenation)

Dove Groovy si basa su questo è con i suoi molti altri tipi di corde, che daremo un'occhiata tra un momento. Nota che possiamo concatenare ogni tipo in modo intercambiabile.

2.2. Interpolazione di stringhe

Ora, Java offre alcuni modelli di base tramite printf , ma Groovy va più in profondità, offrendo l'interpolazione di stringhe, il processo di creazione di modelli di stringhe con variabili :

def name = "Kacper" def result = "Hello ${name}!" assertEquals("Hello Kacper!", result.toString())

Sebbene Groovy supporti la concatenazione per tutti i suoi tipi di stringa, fornisce solo l'interpolazione per alcuni tipi.

2.3. GString

Ma nascosto in questo esempio c'è una piccola ruga: perché stiamo chiamando toString () ?

In realtà, il risultato non è di tipo String , anche se sembra.

Poiché la classe String è finale , la classe String di Groovy che supporta l'interpolazione, GString , non la sottoclasse. In altre parole, affinché Groovy fornisca questo miglioramento, ha la sua classe di stringhe, GString , che non può estendersi da String.

In poche parole, se lo facessimo:

assertEquals("Hello Kacper!", result)

questo invoca assertEquals (Object, Object) e otteniamo:

java.lang.AssertionError: expected: java.lang.String but was: org.codehaus.groovy.runtime.GStringImpl Expected :java.lang.String Actual :org.codehaus.groovy.runtime.GStringImpl

3. Stringa con virgolette singole

Probabilmente la stringa più semplice in Groovy è quella con virgolette singole:

def example = 'Hello world'

Sotto il cofano, queste sono semplicemente vecchie stringhe Java e sono utili quando abbiamo bisogno di virgolette all'interno della nostra stringa.

Invece di:

def hardToRead = "Kacper loves \"Lord of the Rings\""

Possiamo facilmente concatenare una stringa con un'altra:

def easyToRead = 'Kacper loves "Lord of the Rings"'

Poiché possiamo scambiare tipi di virgolette come questo, riduce la necessità di sfuggire alle virgolette.

4. Stringa tripla virgoletta singola

Una stringa di virgolette singole tripla è utile nel contesto della definizione di contenuti su più righe.

Ad esempio, supponiamo di avere alcuni JSON da rappresentare come una stringa:

{ "name": "John", "age": 20, "birthDate": null }

Non abbiamo bisogno di ricorrere alla concatenazione e ai caratteri di nuova riga espliciti per rappresentarlo.

Usiamo invece una stringa tripla con virgolette singole:

def jsonContent = ''' { "name": "John", "age": 20, "birthDate": null } '''

Groovy lo memorizza come una semplice stringa Java e aggiunge la concatenazione e le nuove righe necessarie per noi.

Tuttavia, c'è ancora una sfida da superare.

In genere per la leggibilità del codice, indentiamo il nostro codice:

def triple = ''' firstline secondline '''

Ma le stringhe di virgolette singole triple conservano gli spazi . Ciò significa che la stringa sopra è in realtà:

(newline) firstline(newline) secondline(newline)

non:

1 2 firstline(newline)secondline(newline)

come forse intendevamo.

Stay tuned to see how we get rid of them.

4.1. Newline Character

Let's confirm that our previous string starts with a newline character:

assertTrue(triple.startsWith("\n"))

It's possible to strip that character. To prevent this, we need to put a single backslash \ as a first and last character:

def triple = '''\ firstline secondline '''

Now, we at least have:

1 2 firstline(newline)secondline(newline)

One problem down, one more to go.

4.2. Strip the Code Indentation

Next, let's take care of the indentation. We want to keep our formatting, but remove unnecessary whitespace characters.

The Groovy String API comes to the rescue!

To remove leading spaces on every line of our string, we can use one of the Groovy default methods, String#stripIndent():

def triple = '''\ firstline secondline'''.stripIndent() assertEquals("firstline\nsecondline", triple)

Please note, that by moving the ticks up a line, we've also removed a trailing newline character.

4.3. Relative Indentation

We should remember that stripIndent is not called stripWhitespace.

stripIndent determines the amount of indentation from the shortened, non-whitespace line in the string.

So, let's change the indentation quite a bit for our triple variable:

class TripleSingleQuotedString { @Test void 'triple single quoted with multiline string with last line with only whitespaces'() { def triple = '''\ firstline secondline\ '''.stripIndent() // ... use triple } }

Printing triple would show us:

firstline secondline

Since firstline is the least-indented non-whitespace line, it becomes zero-indented with secondline still indented relative to it.

Note also that this time, we are removing the trailing whitespace with a slash, like we saw earlier.

4.4. Strip with stripMargin()

For even more control, we can tell Groovy right where to start the line by using a | and stripMargin:

def triple = '''\ |firstline |secondline'''.stripMargin()

Which would display:

firstline secondline

The pipe states where that line of the string really starts.

Also, we can pass a Character or CharSequence as an argument to stripMargin with our custom delimiter character.

Great, we got rid of all unnecessary whitespace, and our string contains only what we want!

4.5. Escaping Special Characters

With all the upsides of the triple single-quote string, there is a natural consequence of needing to escape single quotes and backslashes that are part of our string.

To represent special characters, we also need to escape them with a backslash. The most common special characters are a newline (\n) and tabulation (\t).

For example:

def specialCharacters = '''hello \'John\'. This is backslash - \\ \nSecond line starts here'''

will result in:

hello 'John'. This is backslash - \ Second line starts here

There are a few we need to remember, namely:

  • \t – tabulation
  • \n – newline
  • \b – backspace
  • \r – carriage return
  • \\ – backslash
  • \f – formfeed
  • \' – single quote

5. Double-Quoted String

While double-quoted strings are also just Java Strings, their special power is interpolation. When a double-quoted string contains interpolation characters, Groovy switches out the Java String for a GString.

5.1.GString and Lazy Evaluation

We can interpolate a double-quoted string by surrounding expressions with ${} or with $ for dotted expressions.

Its evaluation is lazy, though – it won't be converted to a String until it is passed to a method that requires a String:

def string = "example" def stringWithExpression = "example${2}" assertTrue(string instanceof String) assertTrue(stringWithExpression instanceof GString) assertTrue(stringWithExpression.toString() instanceof String)

5.2. Placeholder with Reference to a Variable

The first thing we probably want to do with interpolation is send it a variable reference:

def name = "John" def helloName = "Hello $name!" assertEquals("Hello John!", helloName.toString())

5.2. Placeholder with an Expression

But, we can also give it expressions:

def result = "result is ${2 * 2}" assertEquals("result is 4", result.toString())

We can put even statements into placeholders, but it's considered as bad practice.

5.3. Placeholders with the Dot Operator

We can even walk object hierarchies in our strings:

def person = [name: 'John'] def myNameIs = "I'm $person.name, and you?" assertEquals("I'm John, and you?", myNameIs.toString())

With getters, Groovy can usually infer the property name.

But if we call a method directly, we'll need to use ${}because of the parentheses:

def name = 'John' def result = "Uppercase name: ${name.toUpperCase()}".toString() assertEquals("Uppercase name: JOHN", result)

5.4. hashCode in GString and String

Interpolated strings are certainly godsends in comparison to plain java.util.String, but they differ in an important way.

See, Java Strings are immutable, and so calling hashCode on a given string always returns the same value.

But, GString hashcodes can vary since the String representation depends on the interpolated values.

And actually, even for the same resulting string, they won't have the same hash codes:

def string = "2+2 is 4" def gstring = "2+2 is ${4}" assertTrue(string.hashCode() != gstring.hashCode())

Thus, we should never use GString as a key in a Map!

6. Triple Double-Quote String

So, we've seen triple single-quote strings, and we've seen double-quoted strings.

Let's combine the power of both to get the best of both worlds – multi-line string interpolation:

def name = "John" def multiLine = """ I'm $name. "This is quotation from 'War and Peace'" """

Also, notice that we didn't have to escape single or double-quotes!

7. Slashy String

Now, let's say that we are doing something with a regular expression, and we are thus escaping backslashes all over the place:

def pattern = "\\d{1,3}\\s\\w+\\s\\w+\\\\\\w+"

It's clearly a mess.

To help with this, Groovy supports regex natively via slashy strings:

def pattern = /\d{3}\s\w+\s\w+\\\w+/ assertTrue("3 Blind Mice\Men".matches(pattern))

Slashy strings may be both interpolated and multi-line:

def name = 'John' def example = / Dear ([A-Z]+), Love, $name /

Of course, we have to escape forward slashes:

def pattern = /.*foobar.*\/hello.*/ 

And we can't represent an empty string with Slashy Stringsince the compiler understands // as a comment:

// if ('' == //) { // println("I can't compile") // }

8. Dollar-Slashy String

Slashy strings are great, though it's a bummer to have to escape the forward slash. To avoid additional escaping of a forward slash, we can use a dollar-slashy string.

Let's assume that we have a regex pattern: [0-3]+/[0-3]+. It's a good candidate for dollar-slashy string because in a slashy string, we would have to write: [0-3]+//[0-3]+.

Dollar-slashy strings are multiline GStrings that open with $/ and close with /$. To escape a dollar or forward slash, we can precede it with the dollar sign ($), but it's not necessary.

We don't need to escape $ in GString placeholder.

For example:

def name = "John" def dollarSlashy = $/ Hello $name!, I can show you a $ sign or an escaped dollar sign: $$ Both slashes work: \ or /, but we can still escape it: $/ We have to escape opening and closing delimiters: - $$$/ - $/$$ /$ 

would output:

Hello John!, I can show you a $ sign or an escaped dollar sign: $ Both slashes work: \ or /, but we can still escape it: / We have to escape opening and closing delimiter: - $/ - /$

9. Character

Those familiar with Java have already wondered what Groovy did with characters since it uses single quotes for strings.

Actually, Groovy doesn't have an explicit character literal.

There are three ways to make a Groovy string an actual character:

  • explicit use of ‘char' keyword when declaring a variable
  • using ‘as' operator
  • by casting to ‘char'

Let's take a look at them all:

char a = 'A' char b = 'B' as char char c = (char) 'C' assertTrue(a instanceof Character) assertTrue(b instanceof Character) assertTrue(c instanceof Character)

The first way is very convenient when we want to keep the character as a variable. The other two methods are more interesting when we want to pass a character as an argument to a function.

10. Summary

Ovviamente, era molto, quindi riassumiamo rapidamente alcuni punti chiave:

  • le stringhe create con virgolette singole (') non supportano l'interpolazione
  • le stringhe di virgolette doppie slashy e triplicate possono essere multilinea
  • le stringhe su più righe contengono caratteri di spazio bianco a causa del rientro del codice
  • barra rovesciata (\) viene utilizzata per eseguire l'escape di caratteri speciali in ogni tipo, tranne la stringa dollaro-barra, dove dobbiamo usare dollaro ($) per eseguire l'escape

11. Conclusione

In questo articolo, abbiamo discusso molti modi per creare una stringa in Groovy e il suo supporto per linee multiple, interpolazione e regex.

Tutti questi frammenti sono disponibili su Github.

E per ulteriori informazioni sulle funzionalità del linguaggio Groovy stesso, inizia bene con la nostra introduzione a Groovy.