java - How to initialize a map using a lambda? -

i want declare populated map field in single statement, (which may contain several nested statements,) this:

private static final map<integer,boolean> map =      something-returning-an-unmodifiable-fully-populated-hashmap; 

anonymous initializers won't do, same reason invoking function returns new populated map won't do: require 2 top-level statements: 1 variable declaration, , 1 method or initializer.

the double curly bracket ({{ , }}) idiom work, creates whole new class extends hashmap<>, , not overhead represented this.

do lambdas of java 8 perhaps offer better way of accomplishing this?

if want initialize map in single statement, can use collectors.tomap.

imagine want build map<integer, boolean> mapping integer result of calling function f:

private static final map<integer,boolean> map =          collections.unmodifiablemap(intstream.range(0, 1000)                                              .boxed()                                              .collect(collectors.tomap(i -> i, -> f(i))));  private static final boolean f(int i) {     return math.random() * 100 > i; } 

if want initialize "static" known values, example in answer, can abuse stream api this:

private static final map<integer, boolean> map =         stream.of(new object[] { 1, false }, new object[] { 2, true })              .collect(collectors.tomap(s -> (int) s[0], s -> (boolean) s[1])); 

note real abuse , never use it: if want construct map known static values, there nothing gain using streams , better off use static initializer.