Tencent/matrix · error · IllegalArgumentException

unable to cast object

Error message

unable to cast object

What it means

The static-field variant of ReflectFiled.get() casts mField.get(null) to the generic Type and converts a ClassCastException into IllegalArgumentException("unable to cast object"). It fires when the actual field value's runtime type is not assignable to the declared generic Type.

Solutions

1. Align the generic type parameter with the field's actual declared type
2. Verify the field's type via mField.getType()/Field#getClass of the value before casting
3. Use getWithoutThrow() to receive null instead of an exception on mismatch
4. Catch IllegalArgumentException around the get() call and handle the fallback path

Example fix

// before
ReflectFiled<String> f = new ReflectFiled<>(Foo.class, "count"); // count is int
String v = f.get();

// after
ReflectFiled<Integer> f = new ReflectFiled<>(Foo.class, "count");
Integer v = f.get();

Field f = Foo.class.getDeclaredField("count");
if (!Number.class.isAssignableFrom(f.getType())) {
    // adjust the generic type parameter of ReflectFiled
}

Object raw = Foo.class.getField(name).get(null);
if (raw instanceof String) { /* safe to use ReflectFiled<String> */ }

try {
    Type v = reflectFiled.get();
} catch (IllegalArgumentException e) {
    if ("unable to cast object".equals(e.getMessage())) {
        v = defaultValue;
    } else { throw e; }
}

When it happens

Trigger: Calling get() where the resolved static field's value cannot be cast to the generic type parameter used when constructing ReflectFiled (e.g. declared Type=String but field holds an Integer).

Common situations: Type parameters inferred incorrectly (raw types, unchecked casts); field type changed between library/app versions; using ReflectFiled against a field whose type was assumed rather than verified.

Understand the failure class

Background: Type mismatch errors: IllegalArgumentException, TypeError and type guards across 150 open-source libraries — this error's family across 150 libraries.

Related errors

• Both of invoker and fieldName can not be null or nil.
• Method is not exists.
• Both of invoker and fieldName can not be null or nil.
• Method is not exists.
• Can not get ClassLoader of

AI-assisted analysis of Tencent/matrix@3b8293bd65 (2026-09-08). Data as JSON: /api/errors/6000b4d514d14979. Report an issue: GitHub.

Appendix: source

Thrown at matrix/matrix-android/matrix-android-lib/src/main/java/com/tencent/matrix/util/ReflectFiled.java:61

        return get(false);
    }

    @SuppressWarnings("unchecked")
    public synchronized Type get(boolean ignoreFieldNoExist)
            throws NoSuchFieldException, IllegalAccessException, IllegalArgumentException {
        prepare();
        if (mField == null) {
            if (!ignoreFieldNoExist) {
                throw new NoSuchFieldException();
            }
            MatrixLog.w(TAG, String.format("Field %s is no exists.", mFieldName));
            return null;
        }
        Type fieldVal = null;
        try {
            fieldVal = (Type) mField.get(null);
        } catch (ClassCastException e) {
            throw new IllegalArgumentException("unable to cast object");
        }
        return fieldVal;
    }

    public synchronized Type get(boolean ignoreFieldNoExist, Object instance)
            throws NoSuchFieldException, IllegalAccessException, IllegalArgumentException {
        prepare();
        if (mField == null) {
            if (!ignoreFieldNoExist) {
                throw new NoSuchFieldException();
            }
            MatrixLog.w(TAG, String.format("Field %s is no exists.", mFieldName));
            return null;
        }
        Type fieldVal = null;
        try {
            fieldVal = (Type) mField.get(instance);
        } catch (ClassCastException e) {

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