How to determine if a list of string contains null or empty elements

Question

In Java, I have the following method:

public String normalizeList(List<String> keys) {
    // ...
}

I want to check that keys :

• Is not null itself; and
• Is not empty ( size() == 0 ); and
• Does not have any String elements that are null ; and
• Does not have any String elements that are empty ("")

This is a utility method that will go in a "commons"-style JAR (the class wil be something like DataUtils ). Here is what I have, but I believe its incorrect:

public String normalize(List<String> keys) {
    if(keys == null || keys.size() == 0 || keys.contains(null) || keys.contains(""))
        throw new IllegalArgumentException("Bad!");

    // Rest of method...
}

I believe the last 2 checks for keys.contains(null) and keys.contains("") are incorrect and will likely thrown runtime exceptions. I know I can just loop through the list inside the if statement, and check for nulls/empties there, but I'm looking for a more elegant solution if it exists.

Answer 1

 keys.contains(null) || keys.contains("")

不抛出任何运行时异常和结果true ，如果你的列表中有任一空（或）空字符串。

Answer 2

This looks fine to me, the only exceptions you would get from keys.contains(null) and keys.contains("") would be if keys itself was null .

However since you check for that first you know that at this point keys is not null , so no runtime exceptions will occur.

Answer 3

You could also use Apache StringUtils and check if the string isBlank, this will check for null, Emptystring and will also trim your value.

if(StringUtils.isBlank(listItemString)){...}

Checkout StringUtils Docs here:

https://commons.apache.org/proper/commons-lang/apidocs/org/apache/commons/lang3/StringUtils.html

Answer 4

I am not sure but isn't there any helper class in ApacheCommon library to do that? like when you have the isEmpty for a string and you have isNullOrEmpty in ApacheCommons library

Answer 5

Check the list in one pass

  public static boolean empty(String s) {
    if (s == null)
      return true;
    else if (s.length() == 0)
      return true;
    return false;
  }  

  public String normalize(List<String> keys) {
    if(keys == null || keys.size() == 0)
        throw new IllegalArgumentException("Bad!");
    for (String key: keys)
      if(empty(key))
         throw new IllegalArgumentException("Empty!");

    // Rest of method...
    return null;
  }

Answer 6

With java 8 you can do:

public String normalizeList(List<String> keys) {
    boolean bad = keys.stream().anyMatch(s -> (s == null || s.equals("")));
    if(bad) {
        //... do whatever you want to do
    }
}

Answer 7

Aniket gave a really good answer for this as a comment:

if( keys.stream().anyMatch(String::isBlank) ) { .... String::isBlank is since Java11. – 
Aniket Sahrawat
Apr 6, 2019 at 2:30

So to tweak your original solution using Aniket's comment:

public String normalize(List<String> keys) {
    if(keys == null || keys.size() == 0  || keys.stream().anyMatch(String::isBlank)){
        throw new IllegalArgumentException("Bad!");
    }

    // Rest of method...
}

Answer 8

if you can use org.springframework.util.CollectionUtils you can apply this

if(CollectionUtils.isEmpty(coll) || CollectionUtils.containsInstance(coll, null)) {
        // throw ...;
}

It does not throw NullPointerException for immutable collection as List.of().