Jenis String di Groovy

1. Ikhtisar

Dalam tutorial ini, kita akan melihat lebih dekat pada beberapa jenis string di Groovy, termasuk string yang dikutip tunggal, dikutip ganda, dikutip tiga kali, dan garis miring.

Kami juga akan menjelajahi dukungan string Groovy untuk karakter khusus, multi-baris, regex, escaping, dan interpolasi variabel.

2. Meningkatkan java.lang.String

Mungkin bagus untuk memulai dengan menyatakan bahwa karena Groovy didasarkan pada Java, Groovy memiliki semua kapabilitas String Java seperti penggabungan, String API, dan manfaat inheren dari kumpulan konstan String karenanya.

Pertama-tama, mari kita lihat bagaimana Groovy memperluas beberapa dari dasar-dasarnya ini.

2.1. Penggabungan String

Rangkaian string hanyalah kombinasi dari dua string:

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

Di mana Groovy membangun ini dengan beberapa jenis string lainnya, yang akan kita lihat sebentar lagi. Perhatikan bahwa kita dapat menggabungkan setiap jenis secara bergantian.

2.2. Interpolasi String

Sekarang, Java menawarkan beberapa template yang sangat mendasar melalui printf , tetapi Groovy melangkah lebih dalam, menawarkan interpolasi string, proses membuat template string dengan variabel :

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

Meskipun Groovy mendukung penggabungan untuk semua jenis stringnya, Groovy hanya menyediakan interpolasi untuk jenis tertentu.

2.3. GString

Tetapi tersembunyi dalam contoh ini adalah sedikit kerutan - mengapa kita memanggil toString () ?

Sebenarnya, hasilnya bukanlah tipe String , meskipun terlihat seperti itu.

Karena kelas String bersifat final , kelas string Groovy yang mendukung interpolasi, GString , tidak mensubkelasinya . Dengan kata lain, agar Groovy dapat memberikan peningkatan ini, ia memiliki kelas stringnya sendiri, GString , yang tidak dapat diturunkan dari String.

Sederhananya, jika kami melakukannya:

assertEquals("Hello Kacper!", result)

ini memanggil assertEquals (Object, Object), dan kita mendapatkan:

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. String Kutipan Tunggal

Mungkin string paling sederhana di Groovy adalah string dengan tanda kutip tunggal:

def example = 'Hello world'

Di bawah tenda, ini hanyalah String Java lama biasa , dan berguna saat kita perlu memiliki tanda kutip di dalam string kita.

Dari pada:

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

Kita dapat dengan mudah menggabungkan satu string dengan yang lain:

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

Karena kita dapat menukar jenis kutipan seperti ini, ini mengurangi kebutuhan untuk menghindari tanda kutip.

4. String Kutipan Tunggal Tiga

String kutip tunggal rangkap tiga sangat membantu dalam konteks menentukan konten multi-baris.

Misalnya, kita memiliki beberapa JSON untuk direpresentasikan sebagai string:

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

Kami tidak perlu menggunakan penggabungan dan karakter baris baru eksplisit untuk mewakili ini.

Sebagai gantinya, mari gunakan string bertanda kutip tunggal tiga:

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

Groovy menyimpan ini sebagai String Java sederhana dan menambahkan rangkaian dan baris baru yang diperlukan untuk kita.

Namun, ada satu tantangan yang harus diatasi.

Biasanya untuk keterbacaan kode, kami membuat indentasi kode kami:

def triple = ''' firstline secondline '''

Tetapi string kutip tunggal tiga mempertahankan spasi kosong . Artinya string di atas benar-benar:

(newline) firstline(newline) secondline(newline)

tidak:

1 2 firstline(newline)secondline(newline)

seperti yang mungkin kami maksudkan.

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

Jelas sekali, itu banyak, jadi mari kita cepat meringkas beberapa poin penting:

  • string yang dibuat dengan kutipan tunggal (') tidak mendukung interpolasi
  • string kutip ganda garis miring dan tiga kali lipat bisa multi-baris
  • string multi-baris berisi karakter spasi karena indentasi kode
  • backslash (\) digunakan untuk meng-escape karakter khusus di setiap jenis, kecuali string slash-dolar, di mana kita harus menggunakan dolar ($) untuk melarikan diri

11. Kesimpulan

Pada artikel ini, kami membahas banyak cara untuk membuat string di Groovy dan dukungannya untuk multi-garis, interpolasi, dan regex.

Semua potongan ini tersedia di Github.

Dan untuk informasi lebih lanjut tentang fitur bahasa Groovy itu sendiri, mulailah dengan pengantar kami ke Groovy.