1Z0-811 Timed Practice Test 3 — 10 Questions

Question 1

A developer is working on a Java program that processes sensor data. The data is stored in a 2D array 'double[][] readings', where each row represents a sensor and each column a time interval. The method 'public static double[] averagePerSensor(double[][] data)' should compute the average reading for each sensor (row) and return a 1D array of averages. The developer writes the following implementation: 'double[] result = new double[data.length]; for (int i = 0; i < data.length; i++) { double sum = 0; for (int j = 0; j < data[i].length; j++) { sum += data[i][j]; } result[i] = sum / data[i].length; } return result;'. However, the program sometimes throws a NullPointerException. What is the most likely cause?

10 minute time limit — choose an answer to begin.

Accepted Answer

One or more rows in the 2D array are null.. The NullPointerException occurs because the code attempts to access `data[i].length` when `data[i]` is null. In Java, a 2D array is an array of arrays, and any row can be null. The code does not check for null rows before iterating over the columns, so if a row is null, accessing `data[i][j]` or `data[i].length` throws a NullPointerException. Option C correctly identifies this as the most likely cause.

Answer

The method returns a double[] but should return double[][].

Answer

The rows have different numbers of columns.

Answer

The outer loop uses data[i].length instead of data.length.

Question 2

Which TWO statements about constructors are true? (Choose two.)

Accepted Answer

Constructors have no return type. Constructors in Java do not have a return type, not even void. If a return type is specified, the Java compiler treats the method as a regular method, not a constructor. This is a fundamental syntactic rule defined in the Java Language Specification (JLS §8.8).

Answer

The compiler always provides a default constructor

Answer

Constructors can be declared as abstract

Answer

Constructors have a return type of void

Question 3

Which three statements about arrays are correct? (Choose three.)

Accepted Answer

Arrays are objects in Java.. In Java, arrays are objects that are dynamically created and can be assigned to variables of type Object, cloned via the clone() method, and have a length field (not a method). They inherit from java.lang.Object and are treated as reference types, even when they store primitive values.

Answer

The size of an array can change after creation.

Answer

The length of an array must be specified at compile time with a constant expression.

Question 4

Which command compiles a Java file and generates a .class file?

Accepted Answer

javac. The `javac` command is the Java compiler, which translates Java source code (`.java` files) into bytecode stored in `.class` files. This is the standard compilation step in the Java development process, as defined by the Java SE specification.

Answer

jar

Answer

javadoc

Answer

java

Question 5

A team is designing a new system that requires deploying independent services communicating over a network. Which Java technology is most suitable for this architecture?

Accepted Answer

Java EE with JAX-RS RESTful Web Services. Java EE with JAX-RS (Java API for RESTful Web Services) enables building RESTful services that communicate over HTTP, which is ideal for independent services in a microservices architecture. Option A is incorrect: Java Applets are client-side components that run in a browser and are not suitable for server-side network communication. Option B is incorrect: Java EE with EJB is a heavyweight, monolithic approach that is less suited for modern microservices architectures. Option D is incorrect: Java RMI is a remote method invocation protocol for tightly coupled distributed systems, not designed for lightweight RESTful services.

Answer

Java Applets

Answer

Java EE with EJB

Answer

Java RMI

Question 6

You are writing a program that processes a list of transactions. Each transaction has a timestamp (long), amount (double), and type (String). The program needs to iterate through the transactions in the order they were added. Which collection should you use to maintain insertion order and allow fast iteration?

Accepted Answer

ArrayList<Transaction>. ArrayList maintains insertion order and provides O(1) random access and fast iteration via index-based traversal. It is the correct choice when you need to preserve the order in which elements were added and iterate over them sequentially without requiring sorting or unique constraints.

Answer

TreeSet<Transaction>

Answer

LinkedList<Transaction>

Answer

HashSet<Transaction>

Question 7

What is the output?

```java
public class Test {
    public static void main(String[] args) {
        String s1 = "hello";
        String s2 = "hello";
        System.out.println(s1.equals(s2));
    }
}
```

Accepted Answer

True. The code compares two String objects 's1' and 's2' using the equals() method. Since both strings have the same content "hello", equals() returns true. The System.out.println statement then prints 'True'.

Answer

No output

Answer

Compilation error

Answer

False

Question 8

A team is designing a system where a 'Report' class can be generated in different formats (PDF, Excel, HTML). They want to avoid modifying the Report class when adding new formats. Which OOP principle or pattern should they use?

Accepted Answer

Composition. Omposition (often implemented via the Strategy pattern), which allows the Report class to delegate format generation to separate strategy objects. This adheres to the Open/Closed Principle: the Report class is closed for modification but open for extension. Adding a new format only requires creating a new strategy class without altering the Report class. Option B (Composition) is correct. Option A (Method Overloading) is incorrect because it would require adding a new overloaded method for each format in the Report class, violating the goal of not modifying it. Option C (Inheritance) is incorrect because it would create a rigid hierarchy where each format is a subclass of Report, leading to tight coupling and difficulty in adding new formats without modifying existing code. Option D (Singleton) is incorrect because it ensures a single instance, which does not address the need for flexible format generation.

Answer

Method Overloading

Answer

Inheritance

Answer

Singleton

Question 9

What is the result of compiling and running this code?

Accepted Answer

Compilation error. The code attempts to assign a double value (e.g., from an expression like `int + double`) to an int variable without casting. This results in a compilation error: 'possible lossy conversion from double to int'. Therefore, option C (compilation error) is correct. Options A, B, D, and E are incorrect because they assume the code compiles and runs, but it does not.

Answer

15

Answer

10.5

Answer

15.5

Answer

16

Question 10

Which of the following best demonstrates polymorphism in Java?

Accepted Answer

Using an interface reference to call a method on an implementing object. Polymorphism in Java allows an object to take multiple forms, typically achieved through inheritance and interfaces. Option B demonstrates this by using an interface reference to invoke a method on an implementing object, where the actual method executed is determined at runtime based on the object's class, not the reference type.

Answer

Overloading a method with different parameter lists

Answer

Using static methods

Answer

Overriding a method in a subclass

Answer

Using final methods