1Z0-811 Object-Oriented Programming — 10 Questions

Question 1

What is the result of: new Document().print();

Accepted Answer

Compilation fails. 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.

Answer

Printable

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Runtime exception

Answer

Showable

Question 2

In a banking application, a class 'Account' has private field 'balance' and public methods 'getBalance()' and 'setBalance(double)'. This is an example of which OOP principle?

Accepted Answer

Encapsulation. 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.

Answer

Abstraction

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Inheritance

Answer

Polymorphism

Question 3

A team is developing a large-scale e-commerce platform using Java. They have a class hierarchy: Product (abstract), Electronics, Clothing, Food. Each product has a discount method that applies different logic. The team notices that whenever a new product type is added, they must modify the existing discount calculation logic in multiple places, leading to high maintenance costs. They want to refactor to follow the Open/Closed Principle. Which approach should they take?

Accepted Answer

Define an interface Discountable with a method calculateDiscount() and have each product class implement it. 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.

Answer

Create a single discount method in Product that uses if-else checks on type

Answer

Remove the discount method and handle it externally using reflection

Answer

Use a switch statement in a utility class to handle all product types

Question 4

A developer writes a class 'Vehicle' with a method 'move()' that prints 'Vehicle moves'. A subclass 'Car' overrides 'move()' to print 'Car moves'. Given: Vehicle v = new Car(); v.move(); What is the output?

Accepted Answer

Car moves. 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'.

Answer

Vehicle moves

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Runtime exception

Answer

Compilation fails

Question 5

In a banking application, a class 'Account' has a private field 'balance'. Which is the best way to allow subclasses to read but not directly modify 'balance'?

Accepted Answer

Provide a public getBalance() method. 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.

Answer

Make balance public

Answer

Use a static variable

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Make balance protected

Question 6

A class 'Base' has a method 'public void display() throws IOException'. Subclass 'Derived' overrides display(). Which exception specifications are allowed in the overriding method?

Accepted Answer

public void display() throws FileNotFoundException. In Java, an overriding method can throw the same exception, a subclass of the exception thrown by the parent method, or no exception at all. The parent method throws IOException, so the overriding method may throw FileNotFoundException (a subclass of IOException). This follows the rule that the overriding method cannot throw a broader checked exception than the overridden method.

Answer

public void display() throws SQLException

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public void display() throws Exception

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public void display() throws Throwable

Question 7

Which design principle is violated by making all fields public in a class?

Accepted Answer

Encapsulation. Making all fields public violates the principle of encapsulation, which requires that an object's internal state be hidden from external access and only modifiable through controlled methods (getters/setters). In Java, public fields allow any class to directly read or modify the field, breaking data integrity and the ability to enforce invariants. Encapsulation is a core OOP concept that protects the internal representation of an object.

Answer

Inheritance

Answer

Abstraction

Answer

Polymorphism

Question 8

Given: abstract class Shape { abstract void draw(); } class Circle extends Shape { void draw() {} } Which is true?

Accepted Answer

Circle must override draw(). Circle is a concrete class that extends the abstract class Shape, which declares the abstract method draw(). A concrete subclass of an abstract class must provide an implementation for all inherited abstract methods, unless the subclass is also declared abstract. Circle provides an implementation of draw() with an empty body, which satisfies the override requirement.

Answer

Shape cannot have a constructor

Answer

Shape s = new Shape(); is valid

Answer

draw() must be public in Circle

Question 9

Which is the correct way to call a superclass constructor from a subclass constructor?

Accepted Answer

super(); as first statement. In Java, a call to the superclass constructor using `super()` must be the first statement in a subclass constructor. This ensures that the superclass initialization completes before any subclass-specific code executes, maintaining the inheritance chain. Option B correctly identifies this requirement.

Answer

super(); anywhere

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super(); at the end

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this.super();

Question 10

A developer wants to prevent a method from being overridden. Which modifier should be used?

Accepted Answer

final. The `final` modifier prevents a method from being overridden in a subclass. When a method is declared as `final`, any attempt to override it in a subclass results in a compile-time error, ensuring the method's implementation remains unchanged.

Answer

private

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abstract

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static