Java Fundamentals & Object-Oriented Programming Flashcards

Which of the following correctly describes polymorphism in Java?

Answer: A reference type can behave differently depending on the actual object it points to at runtime

Polymorphism allows a supertype reference to invoke overridden methods on subtype objects, with the actual behavior determined at runtime.

What is the default value of an instance variable of type `boolean` in Java?

Answer: false

Instance variables of type boolean are initialized to false by default when a class is instantiated.

Given `List list = new ArrayList<>(); list.add("x");`, which statement causes a compilation error?

Answer: list.add(1)

`list.add(1)` attempts to add an Integer to a List, violating the generic type constraint and causing a compilation error.

Which statement about constructors in Java is correct?

Answer: Constructors can be overloaded

Multiple constructors with different parameter lists (overloading) are allowed and commonly used to provide flexible object initialization.

What does the `instanceof` operator return when the left operand's type is a subtype of the right operand?

Answer: true

`instanceof` returns true whenever the object on the left is an instance of the class or any of its subtypes on the right.

A Java enum implicitly extends which class?

Answer: java.lang.Enum

All Java enums implicitly extend `java.lang.Enum`, which is why they cannot extend any other class.

What is the effect of declaring a method as `static` in an interface (Java 8+)?

Answer: The method can only be called via the interface name

Static interface methods are not inherited by implementing classes or sub-interfaces and must be called through the interface type itself.