Preconditions

• Precondition of redefined method at most as strong

• public class Employee
  {
     /**
        Sets the employee salary to a given value. 
        @param aSalary the new salary 
        @precondition aSalary > 0 
     */ 
     public void setSalary(double aSalary) { ... }
  }
  

• Can we redefine Manager.setSalary with precondition
  salary > 100000?
• No--Could be defeated:
  

  Manager m = new Manager();
  Employee e = m;
  e.setSalary(50000);