Interfaces - Beginning Java 8 Fundamentals - page 664

never be a problem if the runtime throws a fewer number of exceptions or no exceptions than expected in the code,
Consider the following implementation of the Banker interface by the UnstablePredictableBank class:
// The following code will not compile
public class UnstablePredictableBank implements Banker {
public double withdraw(double amount)
throws InsufficientFundException, ArbitraryException {
// Code for this method goes here
}
public void deposit(double amount) throws FundLimitExceededException {
// Code for this method goes here
}
}
This time, the withdraw() method adds a new exception, ArbitraryException , which adds a new constraint to
the overridden method. Adding constraints to an overridden method is never allowed.
Consider the following snippet of code:
Banker b = new UnstablePredictableBank();
try {
double amount = b.withdraw(1000.90);
// More code goes here
}
catch (InsufficientFundException e) {
// Handle exception here
}
The compiler does not know that at runtime the variable b , which is of type Banker , will refer to an
object of the UnstablePredictableBank type. Therefore, the compiler will force you to handle only the
InsufficientFundException when you call b.withdraw() . What happens when ArbitraryException is thrown from
the withdraw() method at runtime? Your code is not ready to handle it. This is the reason that you cannot add new
exceptions to a method declaration in a class, which overrides a method in its implementing interface.
If a class overrides a method of the implemented interface, that method must be declared public . Recall that
all methods in an interface are implicitly public , and public is the least restricting scope modifier for a method.
Declaring a method in the class that overrides an interface method as private , protected , or package-level is like
restricting the scope of the overridden method (like placing more constraints). The following snippet of code will not
compile because the withdraw() and deposit() methods are not declared public :
// Code would not compile
public class UnstablePredictableBank implements Banker{
// withdraw() method must be public
private double withdraw(double amount) throws InsufficientFundException {
// Code for this method goes here
}
// deposit() method must be public
protected void deposit(double amount) throws FundLimitExceededException {
// Code for this method goes here
}
}