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public class test { List<Acc> acc; public static void main(final String[] args) { } public void aab() { final StringBuilder builder = new StringBuilder("["); this.acc.stream().map(doublie -> doublie.square).forEach(name -> { }); } } class Acc { public static double square(double num){ return Math.pow(num , 2); } Function<Double, Double> square = Acc::square; }