string 检查字符串是否为空或在 Scala 中不存在

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

提问by Soumya Simanta

I have an Option[String].

我有一个Option[String].

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

我想检查是否存在字符串，如果存在则不为空。

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 ?

在 Scala 中是否有更好的方法来做到这一点？

回答by wheaties

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

您应该做的是检查使用exists. 像这样：

myOption.exists(_.trim.nonEmpty)

which will return Trueif and only if the Option[String]is not Noneand not empty.

True当且仅当Option[String]不是None且不为空时，它才会返回。

回答by elm

An approach based in pattern matching,

一种基于模式匹配的方法，

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

回答by Jens Schauder

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

这应该也有效，因为过滤空选项会导致空选项

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

回答by karol.bu

All proposed solutions will crash with NullPointerException if you pass:

如果您通过，所有建议的解决方案都将因 NullPointerException 而崩溃：

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

Therefore null-check is a must:

因此，必须进行空检查：

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

回答by prayagupd

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

exists（接受的解决方案）将在 input 中至少包含一个元素时起作用，Some("")但当它是None.

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

exists检查是否至少有一个 element( x) 适用于函数。

eg.

例如。

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,

同样发生在 中Option.empty，因为其中没有元素，

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

So forallis what makes sure the the function applies all the elements.

这forall就是确保函数应用所有元素的原因。

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 nonEmptystring, then check option.isEmpty.

粗略的方法是过滤nonEmpty字符串，然后检查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by Andrii Abramov

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

您还可以利用Extractor 模式。它使代码更具声明性。

For example:

例如：

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

And then use it like

然后像这样使用它

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by oleksii

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

我来自 C# 背景，发现类似于 C# 扩展的 Scala 隐式方法

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

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

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

Implementation

执行

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by Hackaholic

you can also check using lastOptionor headOption

你也可以检查使用lastOption或headOption

if the string is empty it will return None

如果字符串为空，它将返回 None

scala> "hello".lastOption
res39: Option[Char] = Some(o)

scala> "".lastOption
res40: Option[Char] = None

回答by pme

I added a Scalafiddle to play with that: Scalafiddle

我添加了一个 Scalafiddle 来玩这个：Scalafiddle

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

这表明标记的正确答案是错误的（如prayagupd所指出的）：

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

the solution is for non-blank:

解决方案是针对非空白：

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