I want to be a police officer.
Usually, "police officer" is a generic term that doesn't specify rank.
What do you mean by rank?
Police officer
A police officer, also known as an officer, policeman, policewoman, cop/copper, garda, police agent, or a police employee is a warranted law employee of a police force. In most countries, "police officer" is a generic term not specifying a particular rank. In some, the use of the rank "officer" is legally reserved for military personnel.
Police officers are generally charged with the apprehension of criminals and the prevention and detection of crime, protection and assistance of the
Commander
Commander is a common naval and air force officer rank. Commander is also used as a rank or title in other formal organisations, including several police forces. 
Commander is also a generic term for an officer commanding any armed forces unit, for example "platoon commander", "brigade commander" and "squadron commander". In the police, terms such as "borough commander" and "incident commander" are used.
Commander as a naval and air force rank.
Commander is a rank used