在Java中将<0.001的十进制（double）值转换为字符串

Question

I have written a small method which is utilised for most of the Object to String conversions.

public String str(Object object) {
    return object == null ? "" : object.toString();
}

When passing double d = 0.0003 to this method, I get an output of 3.0E-4 . So I have altered the method like this.

public String str(Object object) {
    return object == null ? "" : object instanceof Double && (double) object < 0.001 ? String.format("%.10f", object)
            : object.toString();
}

But I kinda feel bad to check instanceof for every conversions which is an additional check for non double objects. Is this the only way that I can convert double to exact string value or is it possible to convert correctly without an additional instance of check. Since if the object satisfies instanceof Double , casting is of free cost and so I'm not worried about performance on casting.

I tried these steps, all produces E output except String.format

double d = 0.0003;               // 3.0E-4
System.out.println(String.valueOf(d));          // 3.0E-4
System.out.println(Double.toString(d));         // 3.0E-4
System.out.println(String.format("%.10f", d));   // 0.0003000000

Answer 1

you can provide different functions, for different types, like:

public static String str(final Object object) {
    return object == null ? "" : object.toString();
}

public static String str(final Double d) {
    return d == null ? "" : str(d.doubleValue());
}

public static String str(final double d) {
    return d < 0.001 ? String.format("%.10f", d) : String.valueOf(d);
}

Answer 2

You can use String.valueOf() :

public static void main(String[] args) {
    final List<String> doubleValues = new ArrayList<>();
    double value = 0.0;
    doubleValues.add("5.01");
    doubleValues.add("0.002");
    doubleValues.add("0.00024");
    doubleValues.add("0.000022");

    for (String elem : doubleValues) {
        value += Double.valueOf(elem);
    }
    System.out.printf("%.10f", value); // Formatting in print
    System.out.println();

    System.out.println(String.format("%.10f", value)); // Print with String formatting

    double v = Double.parseDouble(String.valueOf(value)); //convert decimal number to double for print or calc
    System.out.println(v);
}

Answer 3

import java.text.DecimalFormat;
import java.text.NumberFormat;

public class PrintNumbers {
    public static void main(String[] args) {
        printNumberWOScientificNotations(0.1);
        printNumberWOScientificNotations(0.001);
        printNumberWOScientificNotations(0.00001);
        System.out.println(0.00001);
    }

    static void printNumberWOScientificNotations(Object number) {
        // Check if in scientific notation
        if (String.valueOf(number).toLowerCase().contains("e")) {
            System.out.println("Converting from e to number with 25 maximum fraction digits.");
            NumberFormat formatter = new DecimalFormat();
            formatter.setMaximumFractionDigits(25);
            System.out.println(formatter.format(new Double(number.toString())));
        } else {
            System.out.println(String.valueOf(number));
        }
    }
}