1Z0-811 Practice Questions 1 — 50 Questions

Question 1

Which THREE are primitive data types in Java? (Choose three.)

Accepted Answer

int. Option A (int) is correct because int is one of Java's eight built-in primitive types, representing a 32-bit signed two's-complement integer. Option B (boolean) is correct because boolean is a primitive type that holds only the values true or false, with a size defined by the JVM. Option D (double) is correct because double is a primitive type storing a 64-bit IEEE 754 double-precision floating-point value. Option C (String) is not a primitive; it is a reference type, specifically a final class in java.lang, and its literals are interned objects. Option E (Object) is also a reference type, the root class of the Java class hierarchy, not a primitive.

Answer

String

Answer

Object

Question 2

Refer to the exhibit. A Java program throws a NullPointerException. Which is the most likely cause?

Accepted Answer

An object not initialized before use. A NullPointerException is thrown when the JVM attempts to access a method or field on an object reference that is null. The most common cause is that the object was declared but never instantiated (e.g., `String s; s.length();`). This is a runtime exception, not a compile-time error, so it occurs during execution when the reference variable points to null.

Answer

A syntax error in the try block

Answer

Incorrect classpath preventing class loading

Answer

A missing import statement in the source code

Question 3

A programmer is writing code that needs to store the result of a comparison that checks whether a number is greater than 10. Which primitive type should be used for the variable that holds this result?

Accepted Answer

boolean. The boolean primitive type is specifically designed to hold true or false values. A comparison using the greater-than operator produces a boolean result. Therefore, the variable should be declared as boolean to directly store the outcome of the comparison without any conversion.

Answer

int

Answer

char

Answer

double

Question 4

A developer runs the command shown in the exhibit. The developer wants to ensure the application uses the latest available language features. Which action should the developer take?

Accepted Answer

Download and install a newer version of the JDK.. The latest available language features (e.g., pattern matching, sealed classes, records) are tied to the JDK version. Downloading and installing a newer JDK provides both the compiler (javac) and runtime (JVM) that support those features. Simply upgrading the JVM (Option D) or using -source/-target flags (Option C) does not enable new language syntax in the compiler if the JDK itself is outdated.

Answer

Enable lambda expressions by setting the -enable-lambdas flag.

Answer

Use the -source and -target flags to compile for a newer version.

Answer

Upgrade the JVM to the latest version.

Question 5

What is the output of the following code? int[] a = {1,2,3}; int[] b = a; b[0] = 99; System.out.println(a[0]);

Accepted Answer

99. In Java, arrays are reference types. When `int[] b = a;` is executed, `b` does not create a new array; it simply references the same array object as `a`. Therefore, modifying `b[0]` directly changes the element in the original array `a`, so `a[0]` prints 99.

Answer

0

Answer

1

Answer

Compilation error

Question 6

A developer writes: int a = 9; int b = 2; double result = a / b; System.out.println(result); What is the output?

Accepted Answer

4.0. (4.0). In Java, when dividing two integers, the result is integer division, which truncates the fractional part. So 9 / 2 yields 4 (int). This int value is then assigned to a double variable, resulting in 4.0. Option A (4) would be the result of printing an int, but since result is double, it prints 4.0. Option C is incorrect because the code compiles fine. Option D is incorrect because 4.5 would require at least one operand to be double (e.g., a / (double)b).

Answer

4

Answer

Compilation error

Answer

4.5

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 junior developer created a class 'BankAccount' with public fields for balance and account number. After deployment, users are able to set negative balances, causing issues. Which OOP principle should have been applied to prevent this?

Accepted Answer

Encapsulation. Encapsulation is the OOP principle that involves bundling data (fields) and methods that operate on that data within a single unit (class) and restricting direct access to some of the object's components. By making fields private and providing public getter and setter methods, the class can enforce validation rules, such as preventing negative balances. This would have prevented users from setting invalid values directly.

Answer

Inheritance

Answer

Abstraction

Answer

Polymorphism

Question 9

A developer is preparing to run a Java application on a colleague's computer. The colleague has installed the Java Runtime Environment (JRE) but not the Java Development Kit (JDK). The developer needs to explain what the colleague can and cannot do. Which statement is accurate?

Accepted Answer

The colleague can run compiled Java applications because the JRE includes the JVM and core class libraries.. The JRE is the runtime environment for Java applications. It includes the JVM and the core class libraries required to execute compiled bytecode, but it does not include development tools such as the compiler, debugger, or documentation generator. A colleague with only the JRE can run a compiled Java application but cannot compile or create new Java source code.

Answer

The colleague can create new Java classes because the JRE includes the Java compiler and debugger.

Answer

The colleague can compile Java source files because the JRE includes javac.

Answer

The colleague can generate API documentation because the JRE includes the javadoc tool.

Question 10

A developer is reviewing code that declares several numeric variables. Which of the following declarations correctly assigns a floating-point literal to a variable of type float without causing a compile-time error?

Accepted Answer

float price = 19.99f;. Decimal literals are double by default, so assigning one to a float variable requires either an explicit cast or a literal suffix that makes it a float. Adding the f suffix creates a float literal that matches the variable type and compiles cleanly. The double-suffixed literal and the String literal both fail because they are not implicitly convertible to float.

Answer

float price = "19.99";

Answer

float price = 19.99d;

Answer

float price = 19.99;

Question 11

A logistics application needs to store a shipment's unique tracking code, which is a fixed 12-character alphanumeric identifier. The code must not be modified after it is assigned. Which Java type is the most appropriate to declare for storing this tracking code?

Accepted Answer

String. A tracking code is a sequence of characters that must remain unchanged after assignment. String is the only offered type that can hold multiple characters and is immutable by design, so it satisfies both the storage and the no-modification requirement. The other types either hold a single character, a number, or a truth value, none of which can represent the code.

Answer

boolean

Answer

char

Answer

int

Question 12

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

Question 13

Which three of the following are valid ways to declare and initialize a variable of type int? (Choose three.)

Accepted Answer

int b = 0xA;. Option A (int b = 0xA;) is valid because Java supports hexadecimal integer literals prefixed with 0x, and 0xA equals decimal 10, which fits in an int. Option C (int d = 010;) is valid because a leading zero denotes an octal literal, and 010 is octal for decimal 8, a legal int value. Option E (int a = 10;) is valid because 10 is a standard decimal int literal assigned directly to an int variable. Option B (int e = 10.0;) is invalid because 10.0 is a double literal and cannot be assigned to an int without an explicit cast. Option D (int c = 0b2;) is invalid because the 0b prefix denotes a binary literal, and the digit 2 is not permitted in binary.

Answer

int e = 10.0;

Answer

int c = 0b2;

Question 14

A Java class named 'Helper' is defined in package 'utils'. Another class in a different package tries to access a public method of Helper but receives a compilation error. The Helper class is declared as 'class Helper' (without public modifier). What is the likely issue?

Accepted Answer

The class has package-private access. In Java, a class without an access modifier (i.e., 'class Helper') has package-private (default) access, meaning it is only accessible within its own package. Since the other class is in a different package, it cannot access the Helper class at all, even if its methods are public. The compilation error occurs because the class itself is not visible.

Answer

The method is declared as final

Answer

The method is not static

Answer

The class is abstract

Question 15

What is the result of the following code? int[] arr = new int[3]; arr[3] = 5; System.out.println(arr[3]);

Accepted Answer

Runtime exception. The code attempts to access index 3 of an array of length 3. Since Java arrays are zero-indexed, valid indices are 0, 1, and 2. Accessing index 3 throws an ArrayIndexOutOfBoundsException at runtime, so option A is correct.

Answer

0

Answer

5

Answer

Compilation error

Question 16

During execution, the JVM uses Just-In-Time (JIT) compilation. What is its primary benefit?

Accepted Answer

Improves execution speed by compiling frequently used bytecode to native code.. JIT compilation improves execution speed by compiling frequently used bytecode into native machine code at runtime. Option A is incorrect because JIT does not translate bytecode into an intermediate language; it compiles to native code. Option C is incorrect because JIT compiles bytecode, not source code. Option D is incorrect because JIT's primary purpose is performance, not security.

Answer

Translates bytecode into an intermediate language for interpretation.

Answer

Converts Java source code directly into bytecode.

Answer

Enhances security by verifying bytecode integrity.

Question 17

A company manages employee data stored in an array of Employee objects. The HR application frequently needs to find an employee by ID. The current implementation uses a linear search through the array each time. Performance reports indicate that this search is becoming a bottleneck as the company grows. The array is not sorted, and the company does not want to sort it because the order is meaningful for display. The array is large and frequently updated. The development team considers several options to improve the search performance without changing the array order. Which approach should they implement?

Accepted Answer

Create a HashMap that maps IDs to Employee objects, and maintain it alongside the array.. The best choice because using a HashMap provides O(1) average-time lookups, which is far superior to linear search. The HashMap can be maintained alongside the array without disturbing the array order, as updates to the array are reflected in the HashMap. Option B (parallel streams) still performs a linear search (O(n)) and adds overhead for parallelization, so it does not solve the bottleneck. Option C (HashSet of IDs) only allows checking existence but does not provide the Employee object; you would still need to search the array to retrieve the Employee, so it does not reduce complexity. Option D (binary search on a sorted copy) requires O(n log n) time to sort a copy for each search, which is even worse than linear search.

Answer

Use parallel streams to perform the search in parallel.

Answer

Convert the array to a HashSet of employee IDs and use contains() to check existence.

Answer

Use Arrays.binarySearch() on the array after temporarily sorting a copy each time.

Question 18

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 19

Which loop construct guarantees that the loop body executes at least once?

Accepted Answer

do-while loop. The do-while loop is the only Java loop construct that guarantees the loop body executes at least once because the condition is evaluated after the body executes. In contrast, the while and for loops evaluate the condition before the first iteration, so the body may never execute if the condition is initially false. The enhanced for loop also checks for elements before entering the body.

Answer

Enhanced for loop

Answer

while loop

Answer

for loop

Question 20

Which TWO methods correctly modify the passed array in place?

Accepted Answer

public void zeroOut(int[] a) { for (int i = 0; i < a.length; i++) a[i] = 0; }. It directly modifies each element of the passed array by assigning 0 to a[i] within the loop. In Java, when an array is passed to a method, the method receives a reference to the same array object, so changes to the array's elements are reflected in the caller's array.

Answer

public void reset(int[] a) { a = new int[10]; }

Answer

public void swap(int[] a) { int[] temp = a; a = new int[1]; a[0] = temp[0]; }

Answer

public void setFirst(int[][] a) { a = new int[1][1]; a[0][0] = 5; }