Check if a string is blank or doesn't exist in Scala

问题

I have an Option[String].

I want to check if there is a string exists and if it's exists its not blank.

def isBlank( input : Option[String]) : Boolean = 
{ 
     input.isEmpty || 
     input.filter(_.trim.length > 0).isEmpty 
}

Is there is a better way of doing this in Scala ?

回答1:

What you should do is check using exists. Like so:

myOption.exists(_.trim.nonEmpty)

which will return True if and only if the Option[String] is not None and not empty.

回答2:

An approach based in pattern matching,

def isBlank( input : Option[String]) : Boolean = 
  input match {
    case None    => true
    case Some(s) => s.trim.isEmpty
  }

回答3:

This should work as well since filter of an empty Option results in an empty Option

def isBlank( input : Option[String]) : Boolean =  
   input.filter(_.trim.length > 0).isEmpty

回答4:

All proposed solutions will crash with NullPointerException if you pass:

val str : Option[String] = Some(null).

Therefore null-check is a must:

def isBlank(input: Option[String]): Boolean = 
  input.filterNot(s => s == null || s.trim.isEmpty).isEmpty

回答5:

exists (Accepted solution) will work when input has at least one element in it, that is Some("") but not when it's None.

exists checks if at least one element(x) applies to function.

eg.

scala> List[String]("apple", "").exists(_.isEmpty)
res21: Boolean = true

//if theres no element then obviously returns false
scala> List[String]().exists(_.isEmpty)
res30: Boolean = false

Same happens with Option.empty, as theres no element in it,

scala> Option.empty[String].exists(_.isEmpty)
res33: Boolean = false

So forall is what makes sure the the function applies all the elements.

scala> def isEmpty(sOpt: Option[String]) = sOpt.forall(_.trim.isEmpty)
isEmpty: (sOpt: Option[String])Boolean

scala> isEmpty(Some(""))
res10: Boolean = true

scala> isEmpty(Some("non-empty"))
res11: Boolean = false

scala> isEmpty(Option(null))
res12: Boolean = true

The gross way is to filter nonEmpty string, then check option.isEmpty.

scala> def isEmpty(sOpt: Option[String]) = sOpt.filter(_.trim.nonEmpty).isEmpty
isEmpty: (sOpt: Option[String])Boolean

scala> isEmpty(None)
res20: Boolean = true

scala> isEmpty(Some(""))
res21: Boolean = true

回答6:

I am from C# background and found Scala implicit methods similar to C# extensions

import com.foo.bar.utils.MyExtensions._
...

"my string".isNullOrEmpty  // false
"".isNullOrEmpty           // true
" ".isNullOrEmpty          // true
"  ".isNullOrEmpty         // true

val str: String  = null
str.isNullOrEmpty          // true

Implementation

package com.foo.bar.utils

object MyExtensions {

  class StringEx(val input: String) extends AnyVal {

    def isNullOrEmpty: Boolean =    
      if (input == null || input.trim.isEmpty)
        true
      else
        false
  }

  implicit def isNullOrEmpty(input: String): StringEx = new StringEx(input)
}

回答7:

You can also take advantage of Extractor pattern. It makes codes much more declarative.

For example:

object NonBlank {
  def unapply(s: String): Option[String] = Option(s).filter(_.trim.nonEmpty) 
}

And then use it like

def createUser(name: String): Either[Error, User] = name match {
  case NonBlank(username) => Right(userService.create(username))
  case _ => Left(new Error("Invalid username. Blank usernames are not allowed."))
}

回答8:

I added a Scalafiddle to play with that: Scalafiddle

That shows the marked correct answer is wrong (as pointed out by prayagupd):

def isBlank(str: Option[String]): Boolean =
   str.forall(_.trim.isEmpty)

the solution is for non-blank:

def isNotBlank(str: Option[String]): Boolean =
   str.exists(_.trim.nonEmpty)

来源：https://stackoverflow.com/questions/24086493/check-if-a-string-is-blank-or-doesnt-exist-in-scala

标签

string

scala

option