Why do I have cast o into a IntList when it is already a IntList?

IntList.java

    @Override
    public boolean equals(Object o) {
        if(o == this) return true;
        if(!(o instanceof IntList)) return false;
        IntList that = (IntList) o;
        if(this.size != that.size) return false;
        for(int i = 0; i < this.size; i++) {
            if(this.data[i] != that.data[i]) return false;
        }
        return true;
    }