Blankj/AndroidUtilCode · error · UnsupportedOperationException

u can't instantiate me...

Error message

u can't instantiate me...

What it means

NumberUtils is a utility class providing static methods for number formatting (format, int2Binary, binary2Int, etc.) with a ThreadLocal DecimalFormat for thread safety. Its private constructor throws UnsupportedOperationException to enforce static-only access.

Source

Thrown at lib/utilcode/src/main/java/com/blankj/utilcode/util/NumberUtils.java:30

 *     time  : 2020/04/12
 *     desc  : utils about number
 * </pre>
 */
public final class NumberUtils {

    private static final ThreadLocal<DecimalFormat> DF_THREAD_LOCAL = new ThreadLocal<DecimalFormat>() {
        @Override
        protected DecimalFormat initialValue() {
            return (DecimalFormat) NumberFormat.getInstance();
        }
    };

    public static DecimalFormat getSafeDecimalFormat() {
        return DF_THREAD_LOCAL.get();
    }

    private NumberUtils() {
        throw new UnsupportedOperationException("u can't instantiate me...");
    }

    /**
     * Format the value.
     *
     * @param value          The value.
     * @param fractionDigits The number of digits allowed in the fraction portion of value.
     * @return the format value
     */
    public static String format(float value, int fractionDigits) {
        return format(value, false, 1, fractionDigits, true);
    }

    /**
     * Format the value.
     *
     * @param value          The value.
     * @param fractionDigits The number of digits allowed in the fraction portion of value.

View on GitHub (pinned to 7b4caf9e54)

Solutions

  1. Use static methods: NumberUtils.format(value, fractionDigits), NumberUtils.getSafeDecimalFormat(). Never call new NumberUtils().
  2. Exclude utility classes from DI scanning and serialization configs.
  3. Mock static methods with mockito-inline for testing.

Example fix

// before
NumberUtils nu = new NumberUtils();
String formatted = nu.format(3.14159f, 2);

// after
String formatted = NumberUtils.format(3.14159f, 2);
Defensive patterns

Strategy: type-guard

Validate before calling

// NumberUtils is static-only
String formatted = NumberUtils.format(3.14159, 2);
DecimalFormat df = NumberUtils.getSafeDecimalFormat();

Type guard

static boolean isStaticUtility(Class<?> c) {
    return c.getSimpleName().endsWith("Utils");
}

Try / catch

try {
    NumberUtils.class.getDeclaredConstructor().setAccessible(true);
    NumberUtils.class.getDeclaredConstructor().newInstance();
} catch (UnsupportedOperationException e) {
    // Use static methods
}

Prevention

When it happens

Trigger: Direct or reflective instantiation by DI frameworks, mocking libraries, serialization frameworks, or manual reflection.

Common situations: Component scanning in DI traverses the utility package; a test framework instantiates all classes for coverage; a serialization library targets the class.

Related errors

AI-assisted analysis of Blankj/AndroidUtilCode@7b4caf9e54 (2026-08-14). Data as JSON: /api/errors/1ddab640ab5b6a37. Report an issue: GitHub.