ReactiveX/RxJava · error · IllegalArgumentException

Array of size 4 expected but got {}

Error message

Array of size 4 expected but got {}

What it means

Thrown by Array4Func.apply(Object[]) when the array length is not exactly 4. Array4Func adapts a Function4 (4-ary) to Function<Object[],R>, backing 4-way zip/combineLatest fan-in. The array arity must equal the number of combined sources.

Source

Thrown at src/main/java/io/reactivex/rxjava4/internal/functions/Functions.java:493

    record Array3Func<T1, T2, T3, R>(Function3<T1, T2, T3, R> f) implements Function<Object[], R> {

        @SuppressWarnings("unchecked")
            @Override
            public R apply(Object[] a) throws Throwable {
                if (a.length != 3) {
                    throw new IllegalArgumentException("Array of size 3 expected but got " + a.length);
                }
                return f.apply((T1) a[0], (T2) a[1], (T3) a[2]);
            }
        }

    record Array4Func<T1, T2, T3, T4, R>(Function4<T1, T2, T3, T4, R> f) implements Function<Object[], R> {

        @SuppressWarnings("unchecked")
            @Override
            public R apply(Object[] a) throws Throwable {
                if (a.length != 4) {
                    throw new IllegalArgumentException("Array of size 4 expected but got " + a.length);
                }
                return f.apply((T1) a[0], (T2) a[1], (T3) a[2], (T4) a[3]);
            }
        }

    static final class Array5Func<T1, T2, T3, T4, T5, R> implements Function<Object[], R> {
        private final Function5<T1, T2, T3, T4, T5, R> f;

        Array5Func(Function5<T1, T2, T3, T4, T5, R> f) {
            this.f = f;
        }

        @SuppressWarnings("unchecked")
        @Override
        public R apply(Object[] a) throws Throwable {
            if (a.length != 5) {
                throw new IllegalArgumentException("Array of size 5 expected but got " + a.length);
            }

View on GitHub (pinned to a8ab535614)

Solutions

1. Pass exactly 4 elements in the array to a 4-arity adapter; correct the array-building code.
2. Prefer the public 4-source zip/combineLatest APIs which guarantee correct arity.
3. Add a unit test verifying length-4 input at the adapter call site.
4. Keep array length coupled to the source list size to avoid drift.

Example fix

// before
Function<Object[], R> f = Functions.array4Func(fn4);
R r = f.apply(new Object[]{ a, b, c }); // length 3 -> throws

// after
Function<Object[], R> f = Functions.array4Func(fn4);
R r = f.apply(new Object[]{ a, b, c, d }); // length 4 matches arity

assert args.length == 4 : "4-arity adapter requires length-4 array";

try {
    R r = array4Fn.apply(args);
} catch (IllegalArgumentException e) {
    // fix array length to 4
}

When it happens

Trigger: A 4-arity adapter receives an Object[] whose length != 4, almost always an internal/custom-operator wiring mistake rather than normal application usage.

Common situations: Manually constructing a 4-source fan-in with an array built from a list of indeterminate size; mismatching the adapter arity with the number of sources after refactoring; reflective/dynamic source composition.

Related errors

• Array of size 2 expected but got {}
• Array of size 3 expected but got {}
• Array of size 5 expected but got {}
• Array of size 6 expected but got {}
• Array of size 7 expected but got {}

AI-assisted analysis of ReactiveX/RxJava@a8ab535614 (2026-08-13). Data as JSON: /api/errors/6147f86457feca0d. Report an issue: GitHub.