1Z0-811 Practice Test 29 — 15 Questions

Question 1

Which statement about abstract classes and interfaces is true in Java?

Accepted Answer

An abstract class can have concrete methods. An abstract class in Java can contain concrete (non-abstract) methods with full implementations, which subclasses can inherit or override. This is a key distinction from interfaces (prior to Java 8), which could only declare abstract methods. Abstract classes cannot be instantiated directly, but they provide a partial implementation that concrete subclasses complete.

Answer

Both B and D

Answer

An interface can have instance variables

Answer

An abstract class can be instantiated

Answer

An interface can have constructor

Question 2

Consider the following code that attempts to swap the first two elements of an array using a method:

public static void swap(int[] arr) {
    int temp = arr[0];
    arr[0] = arr[1];
    arr[1] = temp;
}

public static void main(String[] args) {
    int[] values = {1, 2};
    swap(values);
    System.out.println(values[0] + " " + values[1]);
}

What is the output?

Accepted Answer

2 1. The `swap` method receives a reference to the array, so modifications to its elements (via index assignment) affect the original array. After swapping, `values[0]` becomes 2 and `values[1]` becomes 1, producing the output "2 1".

Answer

The code does not compile.

Answer

1 2

Answer

A runtime exception occurs.

Question 3

Match each Java tool to its function.

Accepted Answer

javac: Compiles Java source code into bytecode. The correct matches are: javac compiles source code, java runs applications, javadoc generates documentation, and jar creates archives. Common confusions involve mixing up javac with jar and java with javadoc.

Answer

javac: Packages class files into an archive

Answer

java: Generates API documentation

Question 4

Refer to the exhibit. What is the output when the following code is executed? Vehicle v = new Car(); v.accelerate(); System.out.println(v.speed);

Accepted Answer

20. The code uses polymorphism: the reference variable v of type Vehicle points to a Car object. Since accelerate() is overridden in Car, the overridden method is invoked, adding 20 to the speed instance variable inherited from Vehicle. The protected field speed is accessible in the subclass. Therefore, after calling v.accelerate(), speed becomes 20. Option D is correct.

Answer

10

Answer

0

Answer

Compilation error

Question 5

Match each Java term to its correct definition.

Accepted Answer

JDK: Software development kit that includes tools for compiling, debugging, and running Java programs. Correct matches: JDK is the development kit; JRE is the runtime; JVM executes bytecode; bytecode is the intermediate format. Common confusions: mixing JDK and JRE or JVM with JDK.

Answer

JDK: Runtime environment that includes the JVM and core libraries required to run Java programs

Answer

JVM: Software development kit that includes tools for compiling, debugging, and running Java programs

Question 6

A developer writes the following code: int x = 5; int y = x++; What are the values of x and y after execution?

Accepted Answer

x = 6, y = 5. The post-increment operator `x++` returns the original value of `x` (5) before incrementing. Therefore, `y` is assigned 5, and `x` becomes 6 after the increment. This is a fundamental behavior of post-increment in Java.

Answer

x = 5, y = 5

Answer

x = 5, y = 6

Answer

x = 6, y = 6

Question 7

Which access modifier makes a member visible only within its own class?

Accepted Answer

private. The private access modifier restricts visibility to only the class in which the member is declared. No other class, including subclasses or classes in the same package, can access a private member directly. This is the most restrictive access level in Java.

Answer

public

Answer

default (no modifier)

Answer

protected

Question 8

A company is upgrading from Java 8 to Java 11. Which advantage does the module system introduced in Java 9 provide?

Accepted Answer

Stronger encapsulation of internal APIs. The module system (Project Jigsaw) enforces stronger encapsulation of internal APIs, preventing accidental access. Options A, B, and D are incorrect: multithreading improvements are not provided by the module system, garbage collection enhancements are unrelated, and lambda syntax was introduced in Java 8.

Answer

Better multithreading

Answer

Faster garbage collection

Answer

Improved lambda syntax

Question 9

Given: int i = 1; int j = i++ + ++i; What is the value of j?

Accepted Answer

4. The expression `i++ + ++i` is evaluated as follows: First, `i++` uses the current value of i (1) and then increments i to 2. Then, `++i` increments i from 2 to 3 and uses the new value (3). So, j = 1 + 3 = 4. Option A (3) incorrectly assumes both operations use the same initial value. Option B (5) incorrectly increments both times before using. Option C (2) incorrectly uses post-increment for both.

Answer

3

Answer

5

Answer

2

Question 10

Which statement about the Java compiler is true?

Accepted Answer

It translates Java source code into bytecode.. The Java compiler (javac) translates human-readable Java source code (.java files) into platform-independent bytecode (.class files). This bytecode is then executed by the Java Virtual Machine (JVM), not directly by the operating system. Option D correctly identifies this translation step.

Answer

It translates Java source code into native machine code.

Answer

It produces executable files that run directly on the OS.

Answer

It executes Java programs.

Question 11

A method is declared as 'public void printElements(int... numbers)'. Which invocation will cause a compilation error?

Accepted Answer

printElements({1,2,3});. The syntax `{1,2,3}` is an array initializer that can only be used in a variable declaration or as part of an array creation expression (e.g., `new int[]{1,2,3}`). It cannot be passed directly as an argument to a varargs method. The varargs parameter `int... numbers` expects either a sequence of `int` values or an `int[]` array reference, but not an anonymous array initializer.

Answer

printElements(1, 2, 3);

Answer

printElements();

Answer

printElements(new int[]{1,2,3});

Question 12

Given the loop: for (int i=0; i<5; i++) { if (i==2) continue; System.out.print(i); } What is the output?

Accepted Answer

0134. The loop iterates i from 0 to 4. When i equals 2, the 'continue' statement skips the rest of that iteration, so 'System.out.print(i)' is not executed for i=2. Therefore, the output is 0, 1, 3, 4 concatenated as '0134'. Options A and C both display '0134' and are therefore correct. Option B shows '0124' (missing 3) and Option D shows '01234' (includes 2), both incorrect.

Answer

0124

Answer

01234

Question 13

Which THREE are valid ways to declare and initialize a two-dimensional int array in Java?

Accepted Answer

int[][] arr = new int[2][2];. `int[][] arr = new int[2][2];` uses the standard syntax for declaring and initializing a two-dimensional int array in Java, where both dimensions are specified at creation time, and all elements default to 0. The first option (null key) is correct because `int[][] arr = {{1,2},{3,4}};` uses an array initializer to declare and initialize the array with specific values in a single statement, which is valid for local variables or fields. Option B is also correct because `int[][] arr = new int[2][4];` declares and initializes a two-dimensional array with dimensions 2 and 4, which is a valid syntax; both dimensions must be positive integers.

Answer

int[][] arr = new int[][2];

Answer

int[][] arr = new int[2][2]{{1,2},{3,4}};

Answer

int[][] arr = new int[2][];

Question 14

A developer writes the following code: int x = 5; System.out.println(x++); What is the output?

Accepted Answer

5. The expression `x++` is a post-increment operator, which returns the current value of `x` (5) before incrementing it. Therefore, `System.out.println(x++)` prints 5, and then `x` becomes 6. Option B is correct because the output is the original value of `x`.

Answer

Compilation error

Answer

6

Answer

Runtime exception