java - Lambda Expressions functional programming -

i have programm regular expression lambda expressions university. got stuck 2 methods in method.

here code:

static string ausdruck = "abcd";  public static function<string, string> char = (c) -> {     return (ausdruck.startswith(c)) ? ausdruck = ausdruck.substring(1,             ausdruck.length()) : "value error"; };  public static bifunction<function<string, string>,                           function<string, string>,                           function<string, string>>                  , = (f1, f2) -> {return null;}; 

what want in and method is: char(char.apply("a")) -> want call function f2 f1 parameter. call of , method have like:

and.apply(char.apply("a"), char.apply("b")); 

if understand question correctly, want create function compones new function, executing 1 function result of function. best way in lambda return new lambda.

try this:

bifunction<function<string, string>, function<string, string>, function<string, string>> compose =             (f1, f2) -> (a -> f2.apply(f1.apply(a))); 

example:

function<string, string> upper = s -> s.touppercase(); function<string, string> twice = s -> s + s; function<string, string> uppertwice = compose.apply(upper, twice); system.out.println(uppertwice.apply("foo")); 

output foofoo.

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concerning concrete example

the call of , method have like: and.apply(char.apply("a"), char.apply("b");

i not know trying do, don't think work, given current implementation of char. seems want compose lambda remove a remove b, instead char.apply("a") not create function, remove "a" ausdruck string! instead, char lambda should return lambda, , lambda should not modify static variable, take , return string parameter.

function<string, function<string, string>> stripchar =              c -> (s -> s.startswith(c) ? s.substring(1) : "error"); function<string, string> stripaandc = compose.apply(stripchar.apply("c"), stripchar.apply("a")); system.out.println(stripaandc.apply("cash")); 

output sh.

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finally, in case want use other string, might make sense make compose actual method instead of lambda, can make use of generics. also, can make bit simpler using andthen:

public static <a, b, c> function<a, c> compose(function<a, b> f1, function<b,c> f2){     return f1.andthen(f2); }