generics - Java 8 type inference involving Booleans -

consider following code:

class predicate {     public boolean eval(evaluationcontext ec) { /* logic here */ } }  // later ... list<predicate> preds = new list<>( /* predicates here */ );  // let's use stream<> implement , logical connective: // version a: boolean resulta = preds.stream()                        .map(p -> p.eval(context))                        .reduce(boolean.true, (a,b) -> boolean.logicaland(a,b)); // oops: code above doesn't compile ...    // error: incompatible types: java.lang.object cannot converted boolean  // version b:   (add intermediate variable explicit type) stream<boolean> v = _children.stream().map(p -> p.eval(context)); boolean resultb = v.reduce(boolean.true, (a,b) -> boolean.logicaland(a, b) ); // compiles fine... 

so, question is:

what wrong structure of version prevents java compiler correctly inferring type of result of map()? limitation of type-inference algorithm in java? if so, there better way write code type inference succeeds?

your code compiles fine jdk1.8.0_05, jdk1.8.0_20, jdk1.8.0_25, , jdk1.8.0_40 (beta) , there’s no reason assume there version having problems it. maybe helps when either fix other errors of made-up code or post real code gave compiler.

e.g. can’t new list<> if list refers java.util.list , example code contains no declaration of context. if context cannot found, compiler indeed produce “incompatible types” follow-up error disappear once fixed other errors. it’s striking second example uses _children rather preds originates different context no such compiler errors exist.

by way, lambda expression (a,b) -> boolean.logicaland(a,b) bit odd. either use expressions, e.g. (a,b) -> && b or method reference boolean::logicaland.

but either way, using reduction not recommended. can better performance using short-circuiting methods, e.g. _children.stream().allmatch(p -> p.eval(context)) logical and or _children.stream().anymatch(p -> p.eval(context)) logical or.