NumberFormatTest.java

public class NumberFormatTest {

    private static enum Unit {

        K(4, 6, "K"),
        M(7, 9, "M"),
        B(9, 12, "B");

        private int min, max;
        private String unitChar;

        Unit(int min, int max, String unitChar) {
            this.min = min;
            this.max = max;
            this.unitChar = unitChar;
        }
    }

    public static void main(String[] args) {
        DecimalFormat numberFormat = new DecimalFormat("#,###,###,###");

        for (long l : new long[] {
                1_000_000_000,
                1_000_000,
                1_000,
                100,
                10,
                1
        }) {
            String formatted = numberFormat.format(l) + " " + NumberFormatTest.formatUnit(l);

            System.out.println(String.format("Input: %s | Output: $%s", l, formatted));
        }
    }

    private static String formatUnit(long l) {
        for(Unit unit : Unit.values()) {
            int length = String.valueOf(l).length();

            if(length >= unit.min && length <= unit.max) {
                return unit.unitChar;
            }
        }

        return "";
    }
}