question: @ Override public ToDoubleFunction < Stream < Tuple2 < F , Double > > > dist ( Stream < Tuple2 < F , Double > > features1 ) { Object2DoubleMap < F > auxMap = new Object2DoubleOpenHashMap < > ( ) ; auxMap . defaultReturnValue ( 0.0 ) ; DoubleAdder norm1 = new DoubleAdder ( ) ; features1 . forEach ( null ) ; if ( norm1 . doubleValue ( ) == 0 ) { return null ; } return null ; }
target:  Returns a function that returns the feature-based similarity to the features of an item.