RuntimeExceptionStacker.java

import java.util.function.Consumer;

final class RuntimeExceptionStacker {
    RuntimeExceptionStacker() {
        this.exception = null;
    }

    synchronized RuntimeExceptionStacker add(final RuntimeException ex) {
        if (this.exception == null) {
            this.exception = ex;
        } else {
            this.exception.addSuppressed(ex);
        }
        return this;
    }

    synchronized void ifPresent(final Consumer<? super RuntimeException> consumer) {
        if (this.exception != null) {
            consumer.accept(this.exception);
        }
    }

    private RuntimeException exception;
}