StringUtils.isBlank() checks that each character of the string is a whitespace character (or that the string is empty or that it's null). This is totally different than just checking if the string is empty.

From the linked documentation:

Checks if a String is whitespace, empty ("") or null.

 StringUtils.isBlank(null)      = true
 StringUtils.isBlank("")        = true  
 StringUtils.isBlank(" ")       = true  
 StringUtils.isBlank("bob")     = false  
 StringUtils.isBlank("  bob  ") = false

For comparison StringUtils.isEmpty:

 StringUtils.isEmpty(null)      = true
 StringUtils.isEmpty("")        = true  
 StringUtils.isEmpty(" ")       = false  
 StringUtils.isEmpty("bob")     = false  
 StringUtils.isEmpty("  bob  ") = false

Warning: In java.lang.String.isBlank() and java.lang.String.isEmpty() work the same except they don't return true for null.

java.lang.String.isBlank() (since Java 11)

java.lang.String.isEmpty()

• StringUtils.isEmpty(str) or StringUtils.isNotEmpty(str)
• StringUtils.isBlank(str) or StringUtils.isNotBlank(str)

from Apache commons-lang.

The difference between empty and blank is : a string consisted of whitespaces only is blank but isn't empty.

I generally prefer using apache-commons if possible, instead of writing my own utility methods, although that is also plausible for simple ones like these.

Of course not. Do you really think "" is not clear enough ?

Constants have essentially 3 use cases:

1. Document the meaning of a value (with constant name + javadoc)
2. Synchronize clients on a common value.
3. Provide a shortcut to a special value to avoid some init costs

None apply here.