How to use Java 8 lambdas in Jython

问题

I am trying to experiment with java 8 streams and collections in jython to see if they are any efficient then when implemented in pure jython. It occurs to me it could (any comments on this also appreciated)

I started with some examples, the counting

from java.util.function import Function
from java.util import ArrayList
from java.util.stream import Collectors

letters = ArrayList(['a','b','a','c']);

cnt=letters.stream().collect(Collectors.groupingBy(Function.identity(),
                                            Collectors.counting()))

printing cnt as dictionary {u'a': 2L, u'b': 1L, u'c': 1L}

so far so good. Next, I found a example on using filter on streams in java

List<String>strings = Arrays.asList("abc", "", "bc", "efg", "abcd","", "jkl");
//get count of empty string
int count = strings.stream().filter(string -> string.isEmpty()).count();

how would this translate to in jython. specifically how can one write java lambda expression like string -> sting.isEmpty() in jython?

回答1:

here is an example for using a filter on a stream need a object of type Predicate (java.util.function.Predicate)

for java code:

List<String>strings = Arrays.asList("abc", "", "bc", "efg", "abcd","", "jkl");
//get count of empty string
int count = strings.stream().filter(string -> string.isEmpty()).count();

eqvivalet jython would be to first subclassing Predicate and implementing a test method.

from java.util.function import Predicate
from java.util.stream import Collectors

class pred(Predicate):
   def __init__(self,fn):
       self.test=fn

@pred
def mytest(s):
    from java.lang import String
    return not String(s).isEmpty() #or just use len(s.strip())==0

strings = ArrayList(["abc", "", "bc", "efg", "abcd","", "jkl"])
count = strings.stream().filter(mytest).count()
lst=strings.stream().filter(mytest).collect(Collectors.toList())
print(count)
print(lst)

then prints

count:

5L

lst:

[abc, bc, efg, abcd, jkl]

来源：https://stackoverflow.com/questions/43970506/how-to-use-java-8-lambdas-in-jython