"Android: Selecting A Weighted Random Item From A List" for GameDevAlgorithms.com

VisitorHelper.java

    private static Visitor_Type selectVisitorType() {
        Visitor_Type visitor = new Visitor_Type();

        List<Visitor_Type> visitorTypes = Visitor_Type.findWithQuery(Visitor_Type.class,
                "SELECT * FROM VisitorType WHERE visitor_id NOT IN (SELECT type FROM Visitor)");

        // Work out the total weighting.
        double totalWeighting = 0.0;
        for (Visitor_Type type : visitorTypes) {
            totalWeighting += type.getWeighting();
        }

        // Generate a number between 0 and total probability.
        double randomNumber = Math.random() * totalWeighting;

        // Use the random number to select a visitor type.
        double probabilityIterator = 0.0;
        for (Visitor_Type type : visitorTypes) {
            probabilityIterator += type.getWeighting();
            if (probabilityIterator >= randomNumber) {
                visitor = type;
                break;
            }
        }
        return visitor;
    }

5 years later, here's a Kotlin version!

    fun pickItemToGenerate(itemOptions: List<ItemGeneration>): ItemGeneration? {
        val randomNumber = Math.random() * itemOptions.sumOf { it.weighting }
        
        var probabilityIterator = 0
        itemOptions.forEach { 
            probabilityIterator += it.weighting
            if (probabilityIterator >= randomNumber) {
                return it
            }
        }
        return null
    }