There are 3 must points.
Voltage needs to be the same
Polarity needs to be the same (is the center bit of the plug positive or negative)
Input voltage needs to cover your mains voltage.
Current (or Amps / A) must be the same or greater than the device needs. The device will only draw as much current as it needs so you can buy a power supply that gives out 1 billion amps at 7v and plug it into the same place a 500mA 7v adaptor fitted without problems as long as the first 3 points were all correct.