import java.math.RoundingMode;
import java.text.DecimalFormat;

public final class TestDecimalFormatter {

    public static void main(String[] args) {
        DecimalFormat formatter = new DecimalFormat("0.0");
        formatter.setRoundingMode(RoundingMode.HALF_UP);

        double[] numbers = new double[] { 30.5, 30.55, 30.555, 40.5, 40.55, 40.555 };
        for (double d : numbers) {
            System.out.println("" + d + " --> " + formatter.format(d));
        }
    }
}