1Z0-811 Primitives, Strings and Operators Practice Test 8 — 15 Questions

Question 1

A junior developer wrote the following code to compare two strings entered by a user: if (username == "admin") { grantAccess(); } else { denyAccess(); }. The code always denies access even when the user enters 'admin'. What is the most likely cause, and how should the code be fixed?

Choose an answer to begin — your selection is scored in the full session.

Accepted Answer

The strings are compared using == which checks reference equality. Use username.equals("admin") instead.. In Java, the == operator compares object references, not the actual content of strings. When comparing two String objects for logical equality, the .equals() method must be used. The code if (username == "admin") checks whether username and the string literal "admin" refer to the same memory location, which is false when username is obtained from user input (e.g., via Scanner or console), even if the characters are identical.

Answer

The input string has leading or trailing spaces. Use username.trim().equals("admin") instead.

Answer

The variable username is null, causing a NullPointerException that is caught and denied access. Add a null check.

Answer

The strings are compared using == which is case-sensitive. Use username.equalsIgnoreCase("admin") instead.

Question 2

Which three of the following are valid Java operators that can be used with primitive numeric types?

Accepted Answer

&. Options A (&), B (^), and C (|) are all valid Java operators that work with primitive numeric types: & is the bitwise AND, ^ is the bitwise XOR, and | is the bitwise OR, each operating on the individual bits of integral primitives like int and long (and also boolean). These bitwise operators are defined for the numeric primitive types, making them correct for this scenario. Options D (||) and E (&&) are logical operators that operate only on boolean operands, not on numeric primitives, so they do not qualify.

Answer

||

Answer

&&

Question 3

A financial trading application processes a batch of 10 million trade transactions every night. Each transaction is a String containing trade details such as ID, symbol, quantity, and price. The current implementation uses string concatenation with the += operator in a loop to build a summary report string. The application frequently runs out of memory and takes hours to complete. The server has 16 GB of RAM and runs Java 11. The code cannot be restructured significantly due to regulatory requirements, but performance improvements are allowed. Which course of action will most effectively resolve the performance and memory issues?

Accepted Answer

Replace the String concatenation with StringBuilder.. The performance and memory issues are caused by using String concatenation (+=) inside a loop, which creates many intermediate String objects because Strings are immutable in Java. StringBuilder is mutable and designed for efficient string concatenation without creating intermediate objects, making it the optimal solution. Option A (StringBuilder) is correct. Option B (calling intern()) would not improve performance and could degrade it. Option C (increasing heap size) only postpones the OutOfMemoryError and does not fix the inefficiency. Option D (StringBuffer) is thread-safe but slower due to synchronization; it is unnecessary in this single-threaded context. Therefore, replacing concatenation with StringBuilder is the most effective resolution.

Answer

Call intern() on the concatenated result at each iteration.

Answer

Increase the JVM heap size to 32 GB to accommodate the temporary string objects.

Answer

Replace the String concatenation with StringBuffer.

Question 4

A stock trading system calculates daily profit using an int variable. During periods of high volatility, the profit can exceed Integer.MAX_VALUE (2,147,483,647). When this happens, the profit value wraps around to a negative number, leading to incorrect reporting. The lead developer wants to detect the overflow and throw an ArithmeticException rather than silently producing wrong results. The code cannot use long or BigInteger due to legacy constraints. Which approach should be taken?

Accepted Answer

Use Math.addExact() for the addition and catch or let the exception propagate.. Math.addExact() performs integer addition and throws an ArithmeticException if the result overflows, exactly meeting the requirement to detect overflow and throw an exception without changing data types. Option A (bitwise AND) does not detect overflow reliably. Option C (casting to long) changes the data type, violating the legacy constraint. Option D (BigInteger) also changes the data type. Therefore, option B is correct.

Answer

Use bitwise AND with a mask to detect overflow.

Answer

Cast the operands to long before addition and then cast back to int.

Answer

Use BigInteger for the calculation and convert back to int.

Question 5

A developer is implementing a login system where users enter a password that is then hashed using SHA-256. The system stores the hash as a String in the database. On login, the entered password is hashed and compared to the stored hash using the == operator. Occasionally, valid users are denied access, even though the hashes are identical when printed. The developer has confirmed that the hash algorithm is correctly implemented and that the stored hash is exactly the same string as the computed hash. What is the most likely cause and correct fix?

Accepted Answer

Change the comparison from == to equals().. In Java, the == operator compares object references, not the actual string contents. Even if two String objects contain the same sequence of characters, they may be different objects in memory, so == returns false. The equals() method compares the character sequences and should be used for string comparison. This explains why valid users are occasionally denied access: sometimes the hashes are the same object (e.g., due to string interning) and sometimes not.

Answer

Use hashCode() to compare integer representations.

Answer

Call intern() on both hash strings before using ==.

Answer

Use compareTo() which returns 0 if equal.

Question 6

A scientific application performs calculations with double precision. A specific formula divides two double values: result = a / b; where a and b are calculated from sensor readings. The result is expected to be at most 10 decimal digits of precision. However, the output often shows small rounding errors, e.g., 0.1 + 0.2 = 0.30000000000000004. The application must meet strict accuracy requirements and cannot tolerate these small errors. Which strategy should be used to achieve exact decimal representation?

Accepted Answer

Use BigDecimal with an appropriate scale and rounding mode.. Floating-point arithmetic (double) inherently has rounding errors due to binary representation, as shown in the example 0.1 + 0.2 = 0.30000000000000004. BigDecimal provides arbitrary-precision decimal arithmetic and allows specifying scale and rounding modes, making it ideal for exact decimal calculations meeting the 10-digit precision requirement. Therefore, Option A (BigDecimal) is the correct approach. Option B (Math.round) rounds to an integer, losing fractional precision. Option C (float) uses less precision than double, worsening errors. Option D (casting to int after scaling) truncates and risks loss of information.

Answer

Apply Math.round() to the result to reduce decimal places.

Answer

Use the float data type instead of double to reduce memory usage.

Answer

Cast the result to int after multiplying by a power of 10.

Question 7

Which two statements are true about primitive data types in Java?

Accepted Answer

The long data type can store values from -2^63 to 2^63-1.. Option A is correct because long is a 64-bit signed two's-complement integer, giving an exact range of -2^63 to 2^63-1. Option B is correct because short is a 16-bit signed integer, whose range is exactly -32,768 to 32,767. Option C is wrong because float is a 32-bit IEEE 754 binary floating-point type and cannot represent most decimal values precisely. Option D is wrong because the JVM does not define a precise size for boolean; its "size" is implementation-dependent and not fixed at 1 bit. Option E is wrong because char is a 16-bit unsigned UTF-16 code unit, so it can hold far more than ASCII characters.

Answer

The float data type is 32-bit and can represent decimal numbers precisely.

Answer

The boolean data type has a size of 1 bit.

Answer

The char data type can store only ASCII characters.

Question 8

Refer to the exhibit. Given the code, what is the value printed to the console?

Accepted Answer

18. 18 (option C). The expression `a++ + --b * 2` is evaluated according to Java operator precedence: postfix increment (`a++`) uses the current value of `a` (10) then increments `a` to 11; prefix decrement (`--b`) decrements `b` from 5 to 4 then uses 4; multiplication has higher precedence than addition, so `--b * 2` computes 4 * 2 = 8; then addition: 10 + 8 = 18. Option A (19) would result if the postfix increment had been applied before the rest of the expression (i.e., using 11 instead of 10). Option B (21) would result if both increments were applied before evaluation (a becomes 11, b becomes 4, and then 11 + 4 * 2 = 11 + 8 = 19, not 21; actually 21 is far off). Option D (20) would result if addition were performed before multiplication (10 + 4 = 14, then 14 * 2 = 28, not 20; 20 is not directly derivable). Thus only step-by-step evaluation yields 18.

Answer

19

Answer

21

Answer

20

Question 9

A logging component builds a status line from a String named label that is currently "OK" and an int named code equal to 7. The developer writes: String line = label + code + 1; The intent is to append the numeric sum of code and 1 to the label. What is the actual value of line?

Accepted Answer

OK71. With a String as the leftmost operand, the + operator performs concatenation left-to-right, so the int values are appended as text instead of being added. The result is "OK71". To obtain "OK8", the numeric addition must be grouped in parentheses so it evaluates before concatenation.

Answer

OK7 1

Answer

A compilation error occurs.

Answer

OK8

Question 10

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 11

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 12

A developer writes the following code in a Java application:

String s = "Hello";
s.concat(" World");
System.out.println(s);

What is printed?

Accepted Answer

Hello. Strings in Java are immutable. The concat method returns a new String rather than modifying the original. Because the return value is not assigned, the variable s continues to reference the original "Hello" string, so Hello is printed. The key is recognizing that String methods never change the existing object.

Answer

A compile-time error occurs.

Answer

World

Answer

Hello World

Question 13

A developer is writing a method that must accept and manipulate whole-number values that can be negative, and also needs a type suitable for storing a single Unicode character. Which two primitive types are appropriate for these requirements? (Choose two.)

Accepted Answer

char. The requirements call for a primitive that stores signed whole numbers and a primitive that stores a single character. int handles negative and positive whole numbers, and char handles one Unicode character. boolean stores only truth values, float is for decimal numbers and is not exact, and String is a reference type for character sequences, so none of those meet the stated needs.

Answer

String

Answer

boolean

Answer

float

Question 14

A logistics company stores shipment weights in a Java program. A developer writes the following statement: int weight = 15.75; What is the result when this line is compiled?

Accepted Answer

Compilation fails because a double literal cannot be assigned to an int variable without an explicit cast.. Assigning a double literal to an int variable is a narrowing primitive conversion, which Java permits only with an explicit cast. The compiler enforces this to prevent silent loss of the fractional part. Because the statement lacks a cast, compilation fails before any value is stored. This highlights Java's strict type-safety rules for primitive assignments.

Answer

The program compiles and weight is assigned the value 16 because the value is rounded.

Answer

The program compiles and weight becomes a double variable automatically.

Answer

The program compiles and weight is assigned the value 15 after truncation.

Question 15

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