Exceptions

public class ComputerAccessException extends Exception {
    private static final long serialVersionUID = 1L;
 
    @Override
    public String getMessage() {
        // TODO Auto-generated method stub
        return "You can't use this program because computer is off";
    }
}

Computer computer2 = new Computer();
        computer2.powerOff();
        
        try {
            if (computer2.isPowerOn){
                System.out.println("The computer is on and the OS is starting");
                OSystem bestOSystem = new OSystem();
                
                bestOSystem.setCalculator(calculator);
                bestOSystem.setFileManager(fileManager);
                
                computer2.setoSystemOne(bestOSystem);
                computer2.getoSystemOne().getFileManager().copyData();
                
                Calculator myCalculator = bestOSystem.getCalculator();
                int sum = myCalculator.getSum(2,2);
                myCalculator.printResult(sum);
            } else {
                throw(new ComputerAccessException());
            }
        } catch (ComputerAccessException e) {
            System.out.println(e.getMessage());
        }

computer2.setoSystemOne(bestOSystem);

    public void setoSystemOne(OSystem os) throws ComputerAccessException {
        if (this.isPowerOn) 
            this.oSystem = os;
        else throw new ComputerAccessException();
    }