1Z0-811 Practice Test 17 — 15 Questions

Question 1

Refer to the exhibit. What is the output?

Accepted Answer

Bark
Fetching. The object is a Dog, so sound() prints 'Bark'. The cast to Dog is valid (the object is actually a Dog), so fetch() prints 'Fetching'. Options A, B, D are incorrect because no ClassCastException occurs; the Dog class overrides sound() and has fetch() method.

Answer

Bark
 (then throws ClassCastException)

Answer

Animal sound
 (then throws ClassCastException)

Answer

Animal sound
Fetching

Question 2

Which THREE of the following are key features of the Java programming language?

Accepted Answer

Platform independence through bytecode and JVM. Java is platform independent via bytecode, has automatic garbage collection, and is strongly typed. It does not support multiple inheritance for classes (only interfaces) and does not have pointers.

Answer

Use of pointers for direct memory access

Answer

Support for multiple inheritance of classes

Question 3

A developer is troubleshooting a performance issue in a reporting application. A nested loop iterates over a large dataset: the outer loop processes each row, and the inner loop performs a complex computation on each column. The application is taking longer than expected. Upon reviewing the code, the developer notices that the inner loop's termination condition is recalculated each iteration, which involves a costly method call. Which optimization should the developer implement to improve performance?

Accepted Answer

Move the inner loop's condition calculation outside the outer loop. Moving the condition calculation outside the outer loop (e.g., storing the result of the costly method call in a variable before the inner loop) eliminates redundant calls, improving performance. Option A is incorrect because using a break statement based on a precomputed value would not address the condition recalculation; the break would exit early but the condition is still recalculated each iteration unless moved outside. Option C is incorrect because converting to recursive calls would typically add overhead (stack frames) and is not a performance optimization for this specific issue. Option D is incorrect because simply changing to a while loop does not change the fact that the condition is recalculated each iteration; the costly method call would still be made each time unless the limit is precomputed.

Answer

Use the break statement to exit early based on a precomputed value

Answer

Convert the loops to recursive calls

Answer

Change the inner loop to use a while loop

Question 4

A developer is tasked with deploying a Java application to a customer's server. The customer has only a JRE installed and no internet access. The application uses Java NIO and requires reading configuration files from the classpath. The application compiles and runs fine on the developer's machine which has JDK 11. However, when deploying the compiled JAR to the customer's JRE 11, it throws a 'NoClassDefFoundError' for a class that is part of the JDK's internal API (e.g., com.sun.nio.file.SensitivityWatchEventModifier). The developer is confused because the class is in the standard library. Which action should be taken to resolve the issue?

Accepted Answer

Modify the code to use a public API from the standard library. The issue is that the code uses an internal JDK API (com.sun.nio.file.SensitivityWatchEventModifier), which is not guaranteed to be available in all JRE distributions and is encapsulated in Java 9+. The proper fix is to modify the code to use a public, supported API from the standard library (e.g., java.nio.file.StandardWatchEventKinds). Option A is incorrect because recompiling with a lower target version would still use the internal API, and the target version does not affect encapsulation. Option B is incorrect because the missing class is not a separate JAR; it's part of the JDK internal modules and not available in the JRE's classpath. Option D is incorrect because installing the JDK would expose the internal class but is not a proper solution; it ties the application to a specific JDK implementation and may violate encapsulation in future releases.

Answer

Recompile with a lower target version

Answer

Add the missing JAR file to the classpath

Answer

Install the JDK on the customer's server

Question 5

Which is the correct way to declare an array of integers in Java?

Accepted Answer

Both A and B are valid.. In Java, arrays can be declared with the brackets either after the type (e.g., int[] array = new int[5];) or after the variable name (e.g., int array[] = new int[5];). Both syntaxes are valid, so the correct answer is the option that states both are valid. Option C (int array = new int[5];) is invalid because it declares an integer variable, not an array, and omits the brackets.

Answer

int[] array = new int[5];

Answer

int array = new int[5];

Answer

int array[] = new int[5];

Question 6

A developer is working on a Java application that processes sensor data. The code uses a class 'SensorDataProcessor' which directly instantiates specific sensor classes like 'TemperatureSensor' and 'PressureSensor' inside its methods. The team wants to make the system extensible to support new sensor types without modifying the processor class. Which design change best achieves this?

Accepted Answer

Define a Sensor interface and use dependency injection to pass sensor objects to the processor. Defining a Sensor interface and using dependency injection (e.g., passing sensor objects via constructor or setter) decouples the processor from concrete sensor classes, allowing new sensor types to be added without modifying the processor. Option A is wrong because creating subclasses for each sensor would still require modifying the processor if new sensor types are added. Option B is wrong because a static factory method still couples the processor to the factory and requires changes when new sensors are added. Option C is wrong because reflection is complex, error-prone, and not the best practice for this scenario.

Answer

Make SensorDataProcessor abstract and subclass it for each sensor

Answer

Use a static factory method in SensorDataProcessor to create sensors

Answer

Create a separate configuration file listing sensors and use reflection to instantiate them

Question 7

Refer to the exhibit. What is the output?

Accepted Answer

int. The code performs an integer division because the double y is cast to int, making both operands int. The result is an int value of 2. The method typeOf(int) is called, which prints the string 'int'.

Answer

double

Answer

Runtime error

Answer

Compilation error

Question 8

Refer to the exhibit. A developer encounters this exception when running the application. The method divide is intended to handle division by zero. What is the most likely cause of the exception?

Accepted Answer

The divide method uses integer division without checking the divisor.. The exception is ArithmeticException, which occurs when integer division by zero is performed. The stack trace points to line 12 inside the divide method, indicating that the method did not check whether the divisor is zero before performing the division. Option C is correct because using integer division without checking the divisor is the direct cause. Option A is incorrect because ArithmeticException is an unchecked exception, so catching it is not mandatory. Option B is incorrect because the JVM correctly throws the exception; it is not a configuration issue. Option D is incorrect because unchecked exceptions do not need to be declared in a throws clause.

Answer

The divide method does not catch ArithmeticException.

Answer

The JVM is not configured to handle arithmetic exceptions.

Answer

The Main class does not declare throws ArithmeticException.

Question 9

Which of the following correctly uses the shorthand array initializer to declare and initialize an array of strings with the elements "A", "B", and "C"?

Accepted Answer

String[] arr = {"A", "B", "C"};. Option D correctly uses the shorthand array initializer with curly braces directly in the declaration. Option A uses the full syntax with the 'new' keyword, which is also valid but not the shorthand. Option B is wrong because it uses parentheses. Option C is wrong because it uses square brackets.

Answer

String[] arr = new String[] {"A", "B", "C"};

Answer

String[] arr = ("A", "B", "C");

Answer

String[] arr = ["A", "B", "C"];

Question 10

A mobile app backend uses Java streams to process user data. The code snippet filters users who are active and older than 18, then collects them into a list:

List<User> result = users.stream()
        .filter(u -> u.isActive())
        .filter(u -> u.getAge() > 18)
        .collect(Collectors.toList());

Performance metrics show that this stream operation is slow when the user list has millions of entries. The team wants to improve performance without changing the business logic. Which change would most likely improve performance?

Accepted Answer

Replace .stream() with .parallelStream().. Replacing `.stream()` with `.parallelStream()` enables parallel processing of the stream, which can leverage multiple CPU cores to process the filter operations concurrently. For large datasets (millions of entries), this can significantly reduce execution time by splitting the workload across threads, while preserving the same business logic and order of operations.

Answer

Use a HashSet instead of List for users collection.

Answer

Replace the stream with a traditional for loop.

Answer

Combine both filters into one using logical AND operator.

Question 11

A programmer writes code to transpose a 2D array (matrix). Given: int[][] matrix = {{1,2},{3,4}}; int[][] result = new int[2][2]; for (int i=0; i<2; i++) for (int j=0; j<2; j++) result[j][i] = matrix[i][j]; What is the value of result[1][0]?

Accepted Answer

2. The code transposes the matrix by assigning matrix[i][j] to result[j][i]. For result[1][0], we need the value where j=1 and i=0, which comes from matrix[0][1] = 2. Thus, result[1][0] = 2, making option C correct.

Answer

4

Answer

3

Answer

1

Question 12

A web application uses a HashMap to cache user session data. The keys are String objects representing session IDs, and values are Session objects. After some time, the cache memory usage grows excessively, causing OutOfMemoryError. The cache is never cleared. The team decides to implement a cache eviction policy. Which approach is best suited for this scenario in terms of Java standard library?

Accepted Answer

Use LinkedHashMap with removeEldestEntry method.. LinkedHashMap provides a straightforward way to implement a cache eviction policy by overriding the `removeEldestEntry` method. This method is automatically called by `put` and `putAll` to decide whether to remove the eldest entry, allowing you to limit the cache size and prevent OutOfMemoryError without manual eviction logic.

Answer

Use WeakHashMap instead of HashMap.

Answer

Wrap the HashMap with Collections.synchronizedMap.

Answer

Use TreeMap with a custom comparator.

Question 13

A developer is implementing a login verification method that compares a user-entered password against a stored hash. The passwords are stored as String objects. Which approach ensures correct comparison?

Accepted Answer

if (enteredPassword.equals(storedHash)). Option B uses equals(), the standard method for comparing string content, correctly checking character sequences. Option A uses compareTo(), which is intended for ordering; although compareTo() == 0 indicates equality, it is not the idiomatic approach and is considered a trap. Option C uses ==, comparing references, not content. Option D uses hashCode(), which may collide and does not guarantee equality.

Answer

if (enteredPassword.compareTo(storedHash) == 0)

Answer

if (enteredPassword == storedHash)

Answer

if (enteredPassword.hashCode() == storedHash.hashCode())

Question 14

Refer to the exhibit. What is the result?

Accepted Answer

Runtime exception. The code attempts to divide an integer by zero, which in Java throws an ArithmeticException at runtime. Since the division is performed with integer operands (int / int), Java does not allow division by zero and raises an exception, not a floating-point infinity or zero. Therefore, the correct answer is C: Runtime exception.

Answer

Compilation error

Answer

0

Answer

Infinity