1Z0-829 Practice Questions 1 — 50 Questions

Question 1

Order the steps to compile and run a simple Java program from the command line.

Accepted Answer

Write the Java source code in a .java file, then compile with javac to produce a .class file, then run the class using java.. javac produces .class files; java runs the main method of the specified class. The classpath may need to be set if dependencies exist.

Answer

Write the Java source code in a .java file, then run with java, then compile with javac.

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Compile with javac, then write the Java source code in a .java file, then run with java.

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Run with java, then compile with javac, then write the Java source code in a .java file.

Question 2

Refer to the exhibit. The exhibit shows a code snippet. What is the output when the variable day is set to Day.WEDNESDAY?

Accepted Answer

9. (9) because the switch statement on Day.WEDNESDAY (ordinal 2) does not have break statements for cases WEDNESDAY, THURSDAY, FRIDAY, and SATURDAY, causing fall-through. Execution adds values: WEDNESDAY (3), THURSDAY (4), FRIDAY (5), SATURDAY (6), totaling 18. Then the default case subtracts 9, resulting in 9.

Answer

7

Answer

Compilation error

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6

Question 3

When should LinkedList be preferred over ArrayList?

Accepted Answer

Frequent insertions and deletions at arbitrary positions. LinkedList is preferred over ArrayList when frequent insertions and deletions occur at arbitrary positions because LinkedList uses a doubly-linked list structure, allowing O(1) time for insertions/deletions once the node is located, whereas ArrayList requires shifting elements (O(n)) due to its underlying array. This makes D correct for scenarios with heavy modification in the middle of the list.

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Frequent random access by index

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Fast iteration over all elements

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Minimal memory overhead

Question 4

A developer is designing a service that processes multiple files concurrently. To avoid resource leaks, which practice is essential?

Accepted Answer

Use try-with-resources for each AutoCloseable resource. Try-with-resources guarantees that each AutoCloseable resource is closed automatically at the end of the statement, even if an exception occurs. This is essential when processing multiple files concurrently, as it prevents resource leaks without requiring explicit close() calls in finally blocks.

Answer

Ensure each thread has its own copy of the file handle

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Call close() on each stream in a separate try-catch block

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Use FileLock to prevent concurrent access

Question 5

A developer is implementing a custom sort for a list of Employee objects. The Employee class has fields: String name, int age. The list must be sorted first by name (ascending, case-insensitive), then by age (descending). Which Comparator implementation correctly achieves this?

Accepted Answer

Comparator.comparing(Employee::getName, String.CASE_INSENSITIVE_ORDER).thenComparing(Comparator.comparingInt(Employee::getAge).reversed()). It first sorts by name using a case-insensitive comparator, then chains a reversed comparator for age, ensuring descending order for age while preserving the primary sort by name. The `thenComparing` method correctly applies the reversed age comparator as a secondary sort, which is essential for multi-field sorting where one field requires descending order.

Answer

Comparator.comparing(Employee::getName, String.CASE_INSENSITIVE_ORDER).thenComparingInt(Employee::getAge)

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Comparator.comparingInt(Employee::getAge).reversed().thenComparing(Employee::getName, String.CASE_INSENSITIVE_ORDER)

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Comparator.comparing(Employee::getName).thenComparingInt(Employee::getAge).reversed()

Question 6

A developer needs a collection that stores unique elements and preserves the order in which they were first inserted. Which implementation should be chosen?

Accepted Answer

LinkedHashSet. LinkedHashSet maintains a hash table for element uniqueness and an internal linked list that records insertion order. It therefore rejects duplicates while iterating in the sequence elements were first added. HashSet lacks ordering guarantees, TreeSet sorts instead of preserving insertion order, and ArrayList allows duplicates.

Answer

ArrayList

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HashSet

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TreeSet

Question 7

A developer needs to concatenate two Stream<String> into one. Which approach is most idiomatic?

Accepted Answer

Stream.concat(stream1, stream2). `Stream.concat(stream1, stream2)` is the idiomatic, built-in static method in the Java Stream API for concatenating two streams into a single sequential stream. It creates a lazily concatenated stream whose elements are all the elements of the first stream followed by all the elements of the second stream, preserving encounter order.

Answer

stream1.flatMap(s -> stream2)

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new StreamBuilder().add(stream1).add(stream2).build()

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stream1.merge(stream2)

Question 8

Given: int x=0; do { x++; } while(x<5); How many times does the loop body execute?

Accepted Answer

5. The do-while loop executes the body first, then checks the condition. The variable x starts at 0, so initially the body executes with x=0, then x increments to 1. The loop continues as long as x < 5. The body executes for x = 0, 1, 2, 3, 4 — a total of 5 times. When x becomes 5, the condition x < 5 is false, so the loop stops.

Answer

4

Answer

6

Answer

0

Question 9

Given: Object obj = "Hello"; String result = switch(obj) { case String s -> "String of length " + s.length(); case Integer i -> "Integer"; default -> "Unknown"; }; What is result?

Accepted Answer

"String of length 5". The switch expression uses pattern matching with type patterns. The runtime type of obj is String, so the first case matches, binding the value to s and executing the expression "String of length " + s.length(). Since s is "Hello", s.length() returns 5, so result is "String of length 5". Option C is correct.

Answer

"Unknown"

Answer

Compilation error

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"Integer"

Question 10

In a modular Java application, where does the module declaration file typically reside?

Accepted Answer

In a file named module-info.java at the root of the module's source tree. The module declaration for a modular Java application must be placed in a file named `module-info.java` at the root of the module's source tree. This file is then compiled into `module-info.class` and placed in the output directory, but the source declaration resides in the source tree root as per the Java Platform Module System (JPMS) specification.

Answer

In a file named module-info.class in the output directory

Answer

In the META-INF directory

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In the root of the source directory

Question 11

An application deserializes objects from a network stream. To protect against deserialization attacks, which approach is most effective in Java 17?

Accepted Answer

Set an ObjectInputFilter on the ObjectInputStream. Setting an ObjectInputFilter on the ObjectInputStream allows you to define a filter that can reject deserialization of arbitrary or malicious classes, which is the primary defense against deserialization attacks. This mechanism, introduced in Java 9 and enhanced in later versions, lets you whitelist or blacklist classes based on patterns, limits on array sizes, or depth of object graphs, directly mitigating the risk of remote code execution or denial-of-service via crafted serialized data.

Answer

Declare all fields as transient

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Use try-with-resources to auto-close the stream

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Mark the class as final

Question 12

A company needs to read a large text file (over 2 GB) line by line in a Java application while minimizing memory footprint. Which approach is most efficient?

Accepted Answer

Use Files.lines(path) with try-with-resources. `Files.lines(path)` returns a `Stream<String>` that lazily reads lines from the file, processing them one at a time without loading the entire file into memory. Combined with try-with-resources, the underlying `BufferedReader` is automatically closed, ensuring efficient resource management even for files over 2 GB.

Answer

Use Files.readAllLines(path)

Answer

Use FileOutputStream with read() loop

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Use Scanner with File and loop hasNextLine()

Question 13

A developer needs to ensure that a class `Shape` cannot be instantiated but can be extended. Which modifier should be used?

Accepted Answer

abstract. The `abstract` modifier is correct because an abstract class cannot be instantiated directly, but it can be extended by subclasses. This enforces the design intent that `Shape` serves as a base class for specific shapes like `Circle` or `Rectangle`, while preventing the creation of a generic `Shape` object.

Answer

private

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final

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sealed

Question 14

A company uses a multi-release JAR (MR-JAR) that contains classes for Java 9 and Java 11 in `META-INF/versions/9/` and `META-INF/versions/11/` respectively. The application runs on Java 17. Which version of a class that exists in both versioned directories is loaded?

Accepted Answer

The version from META-INF/versions/11/. In a multi-release JAR (MR-JAR), the Java runtime selects the versioned directory that matches the major version of the running Java platform, or the highest versioned directory that does not exceed that version. Since the application runs on Java 17, and the highest available versioned directory is for Java 11 (META-INF/versions/11/), the class from that directory is loaded. The root directory is used only when no appropriate versioned directory exists.

Answer

The version from META-INF/versions/9/

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The version from the root of the JAR

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The version from META-INF/versions/17/

Question 15

A developer wants to copy all files from one directory to another, preserving file attributes (e.g., last modified time, permissions). Which NIO.2 method is most appropriate?

Accepted Answer

Files.copy(source, target, COPY_ATTRIBUTES). The `Files.copy(source, target, COPY_ATTRIBUTES)` method from the NIO.2 API copies the content of the file and, when the `COPY_ATTRIBUTES` option is specified, also preserves the file's metadata attributes such as last modified time, last access time, and permissions (where supported by the underlying file system). This option is specifically designed for copying with attribute preservation, making it the most appropriate choice for the developer's requirement.

Answer

Files.copy(source, target, REPLACE_EXISTING)

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Files.move(source, target, ATOMIC_MOVE)

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Files.walkFileTree() with a custom FileVisitor that copies files

Question 16

Which interface should be implemented to create a custom filter for deserialization in Java?

Accepted Answer

ObjectInputFilter. The `ObjectInputFilter` interface (introduced in Java 9) allows you to define a custom filter that validates classes and data being deserialized from an `ObjectInputStream`. By implementing this interface, you can reject potentially malicious or unexpected classes during deserialization, which is a key security measure against deserialization attacks.

Answer

Serializable

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ObjectOutput

Answer

FilterOutputStream

Question 17

You are developing a batch processing application that reads records from a CSV file and parses each row into an integer ID. The reading method readNextRecord() returns a String and can throw IOException and NumberFormatException. The method processID(int id) processes the ID. The entire process runs in a for loop over 10,000 records. The requirement is that if any record fails (either due to file read error or parsing), the loop must continue to the next record, but the exception must be logged for later review. Which approach best achieves this requirement?

Accepted Answer

Inside the loop, wrap the readNextRecord() and processID() calls in a try-catch block that catches IOException and NumberFormatException separately, logs the exception, and continues.. It places a try-catch block inside the loop that catches IOException and NumberFormatException separately, logs each exception, and then continues to the next iteration. This ensures that a failure in reading or parsing any single record does not terminate the loop, and each exception is logged individually for later review, meeting the requirement precisely.

Answer

Surround the entire loop with a try-catch block that catches IOException and NumberFormatException using multi-catch, logs once, and allows the loop to continue.

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Inside the loop, wrap the calls in a try-catch that catches Exception, logs, and continues.

Answer

Declare the method to throw IOException and NumberFormatException and let the caller handle them.

Question 18

A developer creates a Java application that uses the new 'jpackage' tool to create an installer for Windows. The application is modular and has a main class declared in module-info. Which option is required to specify the main class when using jpackage?

Accepted Answer

--module. For a modular application with a main class declared in module-info, jpackage requires the `--module` option to specify the module name and, optionally, the main class (e.g., `--module com.example/com.example.Main`). This is because jpackage uses the module system to resolve the entry point, not a classpath or JAR-based approach.

Answer

-classpath

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--module-path

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--main-jar

Question 19

A developer needs to iterate over a List<String> and remove elements that are null. Which approach guarantees correct behavior without throwing a ConcurrentModificationException?

Accepted Answer

Use an Iterator explicitly and call its remove() method. Iterator.remove() is the safe and recommended way to remove elements during iteration, avoiding ConcurrentModificationException. Option A is incorrect because using list.remove() inside a for-each loop modifies the list directly while iterating, causing ConcurrentModificationException. Option B is incorrect because Stream.filter() does not modify the original list; it creates a new list, which may not meet the requirement to remove elements from the original. Option C is incorrect because although a decrementing for loop may work, it is error-prone and not the guaranteed approach; it is not the idiomatic Java solution.

Answer

Use a for-each loop and call list.remove() on the current element

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Use a Stream's filter() method and collect the results

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Use a traditional for loop with an index that decrements

Question 20

Match each JDBC type to its corresponding Java type.

Accepted Answer

VARCHAR: String. JDBC provides standard mappings from SQL types to Java types: VARCHAR to String, INTEGER to int, DATE to java.sql.Date, BLOB to java.sql.Blob, and CLOB to java.sql.Clob. Common confusions arise from mixing these mappings.

Answer

VARCHAR: java.sql.Date

Answer

INTEGER: String