Groovy文字StringBuilder / StringBuffer

Question

Groovy supports a literal syntax for creating a StringBuilder/StringBuffer instead of the usual

def sb = new StringBuilder()

However, I can't seem to remember (or find on Google) the correct syntax.

Answer 1

To get a StringBuffer in a single step, you could use

def sb = 'Hello'<<''

or even:

def sb = ''<<'' //4 single quotes, not double quotes

for an initially empty one.

I think (but I could be wrong) the reason for using a StringBuffer rather than a StringBuilder is to maintain compatibility with Java 1.4.

Answer 2

To create a StringBuffer:

text = 'Hello '

To append:

text <<= 'World!'

And this might help some more.

Answer 3

I've just played with StringBuilder / StringBuffer in Groovy.

Some examples below:

// << operator example
def year = StringBuilder.newInstance()
year << 2
year << "0"
year << '1' << 4
assert year.toString() == "2014"

// You can use with operator
def month = StringBuilder.newInstance()
month.with {
    append "0"
    append '5'
}
assert month.toString() == "05"

// or just append String like in Java or you can leave parenthesis
def day = StringBuilder.newInstance()
day.append "1"
day.append('1')
assert day.toString() == '11'

// It's nice to know, that we can use StringBuilder directly in GString
def date = "$year-${month}-$day"
assert date == "2014-05-11"

Note: Use StringBuilder when it is used only by one thread. StringBuilder provides an API compatible with StringBuffer. StringBuffer is synchronized, StringBuilder is not. Check this link for more info.

Answer 4

String.leftShift() creates a StringBuffer .

AFAICS, you cannot create a String Builder via groovy syntax sugar. You can only create a StringBuilder explicitly. No operator overload on String or GString creates a StringBuilder that you can explicitly use. Java String concatenation might create a StringBuilder , but that instance wouldn't be usable in your code.

At the time of writing this answer:

• Another answer said to use <<= . <<= is merely a java compound assignment that uses << . In particular, there is no method for overloading <<= .
• The accepted answer said to use ''<<'' //4 single quotes, not double quotes . Double quotes work fine, at least on modern groovy.

The following code shows that we always get a StringBuffer, not a StringBuilder, for the result of various combinations of << vs <<= and "" vs '' :

def ls = ''<<''
println ls.class
def lse = ''
lse <<=''
println lse.class

ls = ""<<''
println ls.class
lse = ""
lse <<=''
println lse.class

ls = ''<<""
println ls.class
lse = ''
lse <<=""
println lse.class

ls = ""<<""
println ls.class
lse = ""
lse <<=""
println lse.class

prints:

class java.lang.StringBuffer
class java.lang.StringBuffer
class java.lang.StringBuffer
class java.lang.StringBuffer
class java.lang.StringBuffer
class java.lang.StringBuffer
class java.lang.StringBuffer
class java.lang.StringBuffer