1Z0-811 Object-Oriented Programming Practice Questions

Explanation: The code fails to compile because the class `Document` likely implements two interfaces, such as `Printable` and `Showable`, each declaring a default `print()` method. This creates a diamond problem where the compiler cannot determine which default method to use, resulting in a compilation error unless `Document` overrides `print()`. The error occurs at compile time, not runtime.

Explanation: Encapsulation hides internal data (the 'balance' field) and provides controlled access via public methods 'getBalance()' and 'setBalance(double)'. Option A is wrong because abstraction focuses on hiding implementation complexity rather than data protection; the example demonstrates data hiding which is encapsulation. Option B is wrong because inheritance involves a parent-child class relationship, which is not shown. Option C is wrong because polymorphism requires method overriding or overloading, which is not present here.

Explanation: Defining an interface Discountable with a calculateDiscount() method and having each product class implement it allows adding new product types without modifying existing code. Option B is wrong because it violates OCP by requiring changes to if-else checks. Option C is wrong because it uses reflection unnecessarily and still requires modification when new types are added. Option D is wrong because it uses a switch statement in a utility class, which also requires modification when new product types are added, violating OCP.

Explanation: Java uses dynamic method dispatch (runtime polymorphism). Even though the reference variable is of type 'Vehicle', the actual object is a 'Car' instance. At runtime, the JVM calls the overridden 'move()' method of the 'Car' class, printing 'Car moves'.

Explanation: It follows encapsulation: a public getBalance() method provides read-only access to the private balance field. Subclasses cannot directly modify balance because it is private. Option D (protected) is incorrect because protected access allows subclasses to directly read and write the field, which violates the requirement that subclasses should not be able to directly modify balance.