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In practice, strings are used all over the place, and if you don’t know how to do even the most basic things, isn’t it “JJ Smitta”? Therefore, in this article, we will explain the common operations on strings. For example, string interception, find, get length, replace and so on…

directory

String search

In the actual development of the string lookup function, only a few simple functions will be used, and I will only illustrate a few common ones.

1.1. Get the first element

Mainly contains the first () | firstOrNull () and first {} | firstOrNull {} four function

first() | firstOrNull()

  • Finds the first element of a string
  • Difference: If the string is empty, the former is thrownNoSuchElementExceptionException, which returnsnull

Ex. :

val str = "kotlin very good"



// NoSuchElementException is thrown if the string is empty

str.first()   <=>   str[0]   <=>   str.get(0)



// Returns null if the string is an empty string

str.firstOrNull()

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first{} | firstOrNull{}

  • Finds the first element equal to a character
  • Difference: If no element that meets the condition is found, the former throwsNoSuchElementExceptionException, which returnsnull

Ex. :

val str = "kotlin very good"



// NoSuchElementException is thrown if no element that meets the criteria is found

str.first{ it= ='o' } 



// If no element is found, null is returned

str.firstOrNull{ it= ='o' } 

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1.2. Get the last element

First (), firstOrNull(), and first{} and firstOrNull{}

The last () and lastOrNull ()

  • Finds the last element of a string
  • Difference: If the string is empty, the former is thrownNoSuchElementExceptionException, which returnsnull

Ex. :

val str = "kotlin very good"



// NoSuchElementException is thrown if the string is empty

// lastIndex is an extended attribute, and its implementation is length-1

str.last()   <==>   str.get(lastIndex)   <==>  str[lastIndex]



// Returns null if the string is an empty string

str.lastOrNull()

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The last {} and lastOrNull {}

  • Finds the last element equal to a character
  • Difference: If no element that meets the condition is found, the former throwsNoSuchElementExceptionException, which returnsnull

Ex. :

val str = "kotlin very good"



// NoSuchElementException is thrown if no element that meets the criteria is found

// The string is reversed and then iterated to find the element that meets the condition. So I'm going to have to iterate less

str.last{ it= ='o' } 



// If no element is found, null is returned

str.lastOrNull{ it= ='o' } 

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1.3. Find elements

1.3.2, find {} | findLast {} in fact find elements provides two higher-order functions find {}, findLast {}. Internally, however, firstOrNull() and lastOrNull() are called. There will be no elaboration here.

1.3.2 Find the subscript of the corresponding element

  • indexOf(): Finds the first occurrence of an element or string in the original string.
  • indexLastOf(): Finds the last occurrence of an element or string in the original string.
  • indexOfFirst{}With:indexOf()
  • indexOfLast{}With:indexLastOf()

Ex. :

val str = "kotlin very good"



println(str.indexOfFirst { it == 'o' })

println(str.indexOfLast { it == 'o' })

println(str.indexOf('o'.0))

println(str.indexOf("very".0))

println(str.lastIndexOf('o'))

println(str.lastIndexOf("good"))

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The output is:

1

14

1

7

14

12

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Second, string interception

If you have a programming background in Java or any other language. You should be familiar with string interception. You can go ahead and think of string interception as just a consolidation. Of course, you can skip this section because in Kotlin, the string interception function subString() calls the Java subString() function.

In Kotlin, in addition to calling subString() function, you can also call subSequence() function, interested friends can go to see the source code.

2.1, withsubString()Function to intercept

Take a look at the source code for the subString() function

@kotlin.internal.InlineOnly

public inline fun String.substring(startIndex: Int): String = (this as java.lang.String).substring(startIndex)



@kotlin.internal.InlineOnly

public inline fun String.substring(startIndex: Int, endIndex: Int): String 

= (this as java.lang.String).substring(startIndex, endIndex)



public fun String.substring(range: IntRange): String = substring(range.start, range.endInclusive + 1)

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From the above source we can see that Kotlin is using the Java subString() function

Among them:

  • startIndexArgument: Intercepting the beginning index of the string
  • endIndexArgument: The end index of the truncated string
  • rangParameter, refers to oneIntRangType, representing a range

Example:

val str = "Kotlin is a very good programming language"



println("s = ${str.substring(10)}")  // The end index is length-1 when only the start index is available

println(str.substring(0.15))

println(str.substring(IntRange(0.15)))

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The output is:

a very good programming language

Kotlin is a ver

Kotlin is a very

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Note:

  • usesubString(startIndex,endIndex)andsubString(rang)When the difference. Can be seen from the above results combined with the source code.
  • Remember when subscripts are out of bounds. namelyStringIndexOutOfBoundsExceptionabnormal

2.2, withsubSequence()Function to intercept

In Kotlin, in addition to intercepting a string using subString() as explained above, you can also intercept a string using the subSequence() function.

Let’s take a look at the source code implementation:

public fun subSequence(startIndex: Int, endIndex: Int): CharSequence



public fun CharSequence.subSequence(range: IntRange): CharSequence = subSequence(range.start, range.endInclusive + 1)

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From the source, we can see that it is roughly the same as the subString() function, but it does not provide the case of passing only startIndex

Example:

val str = "Kotlin is a very good programming language"



println(str.subSequence(0.15))

println(str.subSequence(IntRange(0.15)))

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The output is:

Kotlin is a ver

Kotlin is a very

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String replacement

As with string interception above, you can skip this section if you have programming experience. For string substitution, Kotlin implements the Replace () HA function in Java as well as replaceFirst(), replaceAfter(), replaceBefore(), replaceIndent(), and so on. Examples of these functions are given below.

3.1. Replace () function

The replace() function provides four overloaded functions. They can perform different functions

3.1.1, the replace (oldChar, newChar ignoreCase = false)

Function: Replace a character in the original string with a new character. The new string is then returned

  • oldChar: Indicates the character to be replaced
  • newChar: New character
  • ignoreCase: Whether to quoteJavaIn thereplace()Function. The default value isfalse, which isJavathereplace()function

Ex. :

// Replace all characters' a 'with' a '

val str = "Kotlin is a very good programming language"

println(str.replace('a'.'A'))

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The output is:

Kotlin is A very good progrAmming lAnguAge

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3.1.2, replace (oldValue, newValue, ignoreCase = false)

Function: Replace a character in the original string with a new character. The new string is then returned

  • oldValue: Indicates the string to be replaced
  • newValue: New string
  • ignoreCase: Whether to quoteJavaIn thereplace()Function. The default value isfalse, which isJavathereplace()function

Ex. :

// Put the string`Kotlin`Replace it with a string`Java`

val str = "Kotlin is a very good programming language"

println(str.replace("Kotlin"."Java"))

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The output is:

Java is a very good programming language

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3.1.3, replace (regex, replacement)

Function: Matches the source string according to the defined regular rules, and replaces the string that meets the rules with a new string.

  • regex: Regular expression
  • replacement: New string

Ex. :

// The rule of the regex is to check the number, if the number is replaced by the string 'kotlin'

val str = "1234a kotlin 5678 3 is 4"

println(str.replace(Regex("[0-9] +"),"kotlin"))

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The output is:

kotlina kotlin kotlin kotlin is kotlin

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3.1.4, replace(regex: regex, noinline transform: (MatchResult) -> CharSequence)

  • Function: According to the defined regular rules to match the source string, the string that meets the rules throughtransform{}New string substitution for higher-order function mappings.
  • regex: Regular expression
  • transform: higher-order function

Ex. :

val str = "1234a kotlin 5678 3 is 4"

val newStr = str.replace(Regex("[0-9] +"), {"abcd "})

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The output is:

abcd abcd abcd abcd a kotlin abcd abcd abcd abcd  abcd  is abcd

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You can see the difference between the two functions above. The knowledge point about high order function, regular expression can see material understanding by oneself first. Xiao Sheng will fill in the relevant content in the following chapters…

The replace() function explained above. Several overloaded functions are also analyzed. The arguments in replace() are familiar. And the following functions do much the same thing. Therefore, I will only illustrate the functions of the following operations. The introduction of parameters will not be discussed in detail

3.2, replaceFirst ()

Function: Replaces the first character or string that meets the condition with a new character or string

Ex. :

val str = "Kotlin is a very good programming language"

println(str.replaceFirst('a'.'A'))

println(str.replaceFirst( "Kotlin"."Java"))

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The output is:

Kotlin is A very good programming language

Java is a very good programming language

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3.3, replaceBefore ()

Function: Intercepts the first character or the string after the conditional character, contains the conditional character or the string itself, and precedes it with a new string.

Ex. :

val str = "Kotlin is a very good programming language"

println(str.replaceBefore('a'."AA"))

println(str.replaceBefore("Kotlin"."Java"))

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The output is:

AAa very good programming language

JavaKotlin is a very good programming language

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3.4, replaceBeforeLast ()

Function: Intercepts the last character or the string after the conditional character, contains the conditional character or the string itself, and precedes it with a new string.

Ex. :

val str = "Kotlin is a very good programming language"

println(str.replaceBeforeLast('a'."AA"))

println(str.replaceBeforeLast("Kotlin"."Java"))

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The output is:

AAage

JavaKotlin is a very good programming language

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3.5, replaceAfter ()

Function: Intercepts the first character or the string before the conditional character, contains the conditional character or the string itself, and adds a new string after it.

Ex. :

val str = "Kotlin is a very good programming language"

println(str.replaceAfter('a'."AA"))

println(str.replaceAfter("Kotlin"."Java"))

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The output is:

Kotlin is aAA

KotlinJava

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3.6, replaceAfterLast ()

Function: Intercepts the last character or the string before the conditional character, contains the conditional character or the string itself, and adds a new string after it.

Ex. :

val str = "Kotlin is a very good programming language"

println(str.replaceAfterLast('a'."AA"))

println(str.replaceAfterLast("Kotlin"."Java"))

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The output is:

Kotlin is a very good programming languaAA

KotlinJava

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4. String segmentation

As in the previous section, Kotlin provides the splitToSequence() function to split strings in addition to implementing the Split () function in Java. After a successful partition, it returns a set of strings for subsequent operations.

4.1, the split ()

The split() function also provides four overloaded functions. Among them, regular expression is used for conditional segmentation of two. Use character split to occupy one. Use string splitting to occupy one.

4.1.1. Use regular expression segmentation

Regular expressions are used in Kotlin using the Regex class, while Java uses the Pattern class used by regular expressions. Here are some examples

Ex. :

var str2 = "1 kotlin 2 java 3 Lua 4 JavaScript"



val list3 = str2.split(Regex("[0-9] +"))

for (str in list3){

    print("$str \t")

}



println()



val list4 = str2.split(Pattern.compile("[0-9] +"))

for (str in list4){

    print("$str \t")

}

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The output is:

kotlin  java  Lua  JavaScript     

kotlin  java  Lua  JavaScript     

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4.1.2 Use character or string segmentation

In actual project development, this method is used more often. But it’s worth noting here that whether it’s a character split or a string split, it’s a mutable argument. That is, the number of parameters is variable.

Ex. :

val str1 = "Kotlin is a very good programming language"



val list1 = str1.split(' ')

for (str in list1){

    print("$str \t")

}



println()



val list2 = str1.split("")

for (str in list2){

    print("$str \t")

}

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The output is:

Kotlin   is   a   very     good    programming     language

Kotlin   is   a   very     good    programming     language

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Here is an example of a mutable argument case:

val str3 = "a b c d e f g h 2+3+4+5"

val list5 = str3.split(' '.'+')

for (str in list5){

    print("$str \t")

}

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The output is:

a     b   c   d   e   f   g   h   2   3   4   5 

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4.2, splitToSequence ()

This function can also be split with strings or characters, just like the split() function above. I won’t go into details here…

Five, the other

In addition to the points mentioned above, there are many common processes, such as detecting whether a string is null or not, getting string length, string inversion, statistics, converting character arrays, getting characters with specified subscripts, and so on.

5.1. Get the string length

There are two ways to get the length of a string in Kotlin. In fact, it is also a kind of

  1. Use directlylengthAttribute fetch length
  2. withcount()Function get, actuallycount()Function also returnslengthLength.

Example:

val str = "kotlin very good"



// 1. Use the length attribute

println("str.length => ${str.length}")



// 2. Use the count() function to get

println("str.count() => ${str.count()}")

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The output is:

str.length => 16

str.count() => 16

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Here we take a look at the source code for the count() function

/ * *

 * Returns the length of this char sequence.

* return the length attribute...

* /

@kotlin.internal.InlineOnly

public inline fun CharSequence.count(a): Int {

    return length

}

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5.2 Statistics of repeated characters

The count() function returns the length property to get the length of the string. In fact, the source code also provides a higher-order function called count{}, used to count the number of repeated characters in a string.

/ *

Count {} function source

This function accepts a Lambda expression of type Boolean. I then loop through the string and increment the variable 'count' if my condition is true.

Return the number of repeats' count 'when the loop is complete

* /

public inline fun CharSequence.count(predicate: (Char) -> Boolean) :Int {

    var count = 0

    for (element in this) if (predicate(element)) count++

    return count

}

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Example:

val str = "kotlin very good"

val count = str.count { it= ='o' }

println("count : $count")

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The output is:

count : 3

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5.3. Validate Strings

In real development, especially Android development, it is often the case to verify that the content of the input field is an empty string. In Kotlin, as in Java, several functions are provided to handle this situation.

The following functions handle empty or null strings:

  • isEmpty(): Its source is judged by itslengthIs equal to0If, is equal to the0It returnstrue, otherwise returnfalse. Cannot be used directly for nullable strings
  • isNotEmpty(): Its source is judged by itslengthIf more than0, if more than0It returnstrue, otherwise returnfalse. Cannot be used directly for nullable strings
  • isNullOrEmpty(): the source is to determine whether the string isnullOr itslengthWhether is equal to the0.
  • isBlank(): Its source is judged by itslengthWhether is equal to the0, or whether the number of Spaces it contains is equal to the currentlength. Cannot be used directly for nullable strings
  • isNotBlank(): Its source code is rightisBlank()I’m going to invert the function. Cannot be used directly for nullable strings
  • isNotOrBlank(): its source determines whether the string isnull. Or callisBlank()function

Example:

val str : String? = "kotlin"



/ *

As you can see, when STR isNullOrEmpty, isNullOrEmpty() and isNotOrBlank() can be called without doing any processing

, and the other functions don't

* /

str? .isEmpty()//false

str? .isNotEmpty()// true

str.isNullOrEmpty()       //false

str? .isBlank()//false

str? .isNotBlank()//true

str.isNullOrBlank()       //false

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5.4 string concatenation

String links can only be linked using + in Java, except for StringBuilder and StringBuffer. In Kotlin, you can use the plus() function in addition to +. It accepts any type. The plus() function is an operator overload function. This was explained in the previous chapter. If you are not familiar with Cloud overloading in Kotlin, you can check out my other article: Kotlin — Primer (5) : Operator overloading one

Example explanation:

val oldStr = "kotlin"

println(oldStr.plus(" very good"))

println(oldStr + " very good")

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The output is:

kotlin very good

kotlin very good

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5.5. String inversion

Like arrays, strings can invert elements. Use the reversed() function directly.

Ex. :

val str = "kotlin"

println("String inversion:${str.reversed()}")

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Output result:

String inversion: niltok

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Determine the start and end of the string

5.6.1, startsWith ()

Function: Checks whether the string starts with a character or a string.

  • char: Start character
  • prefix: start string
  • ignoreCase: Whether to callJavaThis function in. The default isfalse
  • startIndex: Starting position

Ex. :

    val str = "kotlin"

    println(str.startsWith('k'))         // Whether the character 'k' starts

    println(str.startsWith("Kot"))       // Whether to start with the string 'kot'

    println(str.startsWith("lin".3))     // Whether to start with the string 'Lin' when the start position is 3

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The output is:

true

true

true

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5.6.2, endsWith ()

Function: Checks whether the string ends with a character or a string.

  • char: End character
  • suffix: Ending string
  • ignoreCase: Whether to callJavaThis function in. The default isfalse

Ex. :

val str = "kotlin"

println(str.endsWith("lin"))  // Whether to end with the string 'Lin'

println(str.endsWith('n'))    // Whether to end with character 'n'

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The output is:

true

true

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conclusion

In practice, strings are handled in many ways. In particular, string verification processing, replacement, segmentation, interception. That’s why I’ve put these things together. These are very basic, but also very common. If you have programming experience, you should consolidate the basic knowledge of strings. The string manipulation in Kotlin is finished here, but there are many other functions, although not used much in development, but we should at least have an understanding of them. Interested friends can go to see the implementation of its source code.

This article has been collected on GitHub: Jetictors/KotlinLearn, welcome star articles continue to update, can be wechat search “J guy talk” first time read, everyone’s three-in-one is the best power for the old J, you are sure not to wave? If you have any mistakes or suggestions on this blog, please leave comments


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