检查字符串是否为空的更好方法是在 Java 5/6 中使用 String.trim().length() 吗？

Question

There probably is a method to return the index of the first non-blank char in a String in Java5/6. But I cannot find it any more. A code anylizing tool says it is better than checking String.trim().length() .

Answer 1

I always like to use the Apache Commons StringUtils library. It has isEmpty() and is isBlank() which handles whitespace.

http://commons.apache.org/lang/api-2.5/org/apache/commons/lang/StringUtils.html

Not to mention the numerous other helpful methods in that class and the library in general.

Answer 2

I'd use the Guava CharMatcher class:

boolean onlyWhitespace = CharMatcher.WHITESPACE.matchesAllOf(input);

Answer 3

Okay guys, I have found it finally from PMD rules of InefficientEmptyStringCheck :

InefficientEmptyStringCheck:
Since: PMD 3.6
String.trim().length() is an inefficient way to check if a String is really empty, as it creates a new String object just to check its size. Consider creating a static function that loops through a string, checking Character.isWhitespace() on each character and returning false if a non-whitespace character is found.

This is only a suggestion from PMD. To adopt it or not is depending on which has priority: the efficiency of programs or the time of programmers.

Answer 4

Java 11 introduces the "isBlank" method. See https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/String.html#isBlank()

Returns true if the string is empty or contains only white space codepoints, otherwise false.

Answer 5

Java 6 has introduced String.isEmpty() , so you could use it in conjunction with String.trim() . You can also use regular expressions, for example using such a condition: .str.matches("\\s*") .

Answer 6

If you want to test, whether it only contains whitespace characters, you can use RegEx

string.matches("\\s*")

Thinks it's more efficient than trim().isEmpty(), especially if you expect whitespaces and have long Strings, though I'm not sure how much effort it takes to compile the RegEx.

Answer 7

If you want to test for a string that has a zero length than using isEmpty() or length() == 0 is the best way.

If you want to test if the string only contains whitespaces, then searching for the first non-whitespace character is more efficient because not intermediate object is created (as with trim() )

But in any case I too recommend Apache's commons StringUtils.isEmpty() as it nicely encapsulates all this.

Answer 8

There is a method in String for this exact purpose.

String emptyString = "";
emptyString.isEmpty();

This will return true.

Answer 9

try

  str!=null && str.trim().isEmpty()

Answer 10

(Best way) string.equals("")

But also,

string.isEmpty()

string.equals(null)