google/gson · error · UnsupportedOperationException

Primitive is neither a number nor a string

Error message

Primitive is neither a number nor a string

What it means

JsonPrimitive.getAsNumber() returns the stored Number or wraps a String in LazilyParsedNumber; if the wrapped value is neither (effectively a Boolean, the only other primitive type), it throws UnsupportedOperationException. The primitive exists but cannot be viewed as a Number.

Source

Thrown at gson/src/main/java/com/google/gson/JsonPrimitive.java:145

  public boolean isNumber() {
    return value instanceof Number;
  }

  /**
   * Convenience method to get this element as a {@link Number}. If this primitive {@linkplain
   * #isString() is a string}, a lazily parsed {@code Number} is constructed which parses the string
   * when any of its methods are called (which can lead to a {@link NumberFormatException}).
   *
   * @throws UnsupportedOperationException if this primitive is neither a number nor a string.
   */
  @Override
  public Number getAsNumber() {
    if (value instanceof Number) {
      return (Number) value;
    } else if (value instanceof String) {
      return new LazilyParsedNumber((String) value);
    }
    throw new UnsupportedOperationException("Primitive is neither a number nor a string");
  }

  /**
   * Check whether this primitive contains a String value.
   *
   * @return true if this primitive contains a String value, false otherwise.
   */
  public boolean isString() {
    return value instanceof String;
  }

  // Don't add Javadoc, inherit it from super implementation; no exceptions are thrown here
  @Override
  public String getAsString() {
    if (value instanceof String) {
      return (String) value;
    } else if (isNumber()) {
      return getAsNumber().toString();

View on GitHub (pinned to 8b8628c656)

Solutions

1. Check isBoolean() / isNumber() on the JsonPrimitive before numeric coercion.
2. Normalize the source data so the field is consistently typed.
3. Use a TypeAdapter with explicit handling rather than ad-hoc tree coercion.
4. Provide a fallback default when the primitive is not numeric.

Example fix

// before
Number n = prim.getAsNumber();

// after
Number n = prim.isNumber() ? prim.getAsNumber()
          : prim.isBoolean() ? (prim.getAsBoolean() ? 1 : 0)
          : null;

if (!prim.isNumber()) {
  // not numeric; bail or coerce
}

boolean isNumeric(JsonPrimitive p) {
  return p.isNumber();
}

try {
  Number n = prim.getAsNumber();
} catch (UnsupportedOperationException ex) {
  // boolean or unsupported primitive; supply fallback
  n = prim.isBoolean() ? (prim.getAsBoolean() ? 1 : 0) : null;
}

When it happens

Trigger: Constructing JsonPrimitive(true) or parsing true/false and calling getAsNumber(); chaining getAsNumber().doubleValue() on a boolean primitive; calling getAsInt()/getAsDouble() on a boolean (those route through getAsNumber()).

Common situations: Schema drift where a boolean field replaces a numeric one; polymorphic JSON where the same key holds different primitive kinds; loose duck-typing over a parsed tree.

Related errors

• String value is empty
• JsonReader is closed
• Custom JsonElement subclass {className} is not supported
• Expected {expected} but was {peek}{location}
• Expected STRING but was {token}{location}

AI-assisted analysis of google/gson@8b8628c656 (2026-08-04). Data as JSON: /data/errors/3991664f5b6f2e49.json. Report an issue: GitHub.