1Z0-811 Practice Test 24 — 15 Questions

Question 1

A developer writes a method that accepts an array and returns the sum of all elements. Which implementation is correct if the array might be null?

Accepted Answer

public int sum(int[] arr) { if (arr == null) return 0; int s=0; for (int n:arr) s+=n; return s; }. It explicitly checks for a null array before attempting to access its length or iterate over its elements. In Java, accessing `arr.length` or using an enhanced for loop on a null reference throws a `NullPointerException`. By returning 0 for null input, the method gracefully handles the edge case without crashing.

Answer

public int sum(int[] arr) { if (arr.length==0) return 0; int s=0; for (int n:arr) s+=n; return s; }

Answer

public int sum(int[] arr) { try { int s=0; for (int n:arr) s+=n; return s; } catch(NullPointerException e) { return -1; } }

Answer

public int sum(int[] arr) { int s=0; for (int n:arr) s+=n; return s; }

Question 2

What is the result of: System.out.println(10 + 20 + "30");

Accepted Answer

3030. In Java, the '+' operator is left-associative. The expression `10 + 20 + "30"` is evaluated as `(10 + 20) + "30"`, which first performs integer addition to get `30`, then concatenates that integer with the string `"30"`, resulting in the string `"3030"`. The `System.out.println` method then prints the string `3030`.

Answer

102030

Answer

30

Answer

Compilation fails

Question 3

A method is designed to reverse an array of integers. The method signature is: public static void reverse(int[] arr). The method correctly reverses the array. Which statement is true about the method?

Accepted Answer

It modifies the original array.. The method receives a reference to the array (pass-by-value of the reference), so any modifications made to the array elements inside the method directly affect the original array object in the caller's scope. Since the method reverses the array in-place by swapping elements, the original array is mutated.

Answer

It returns a new array that is the reverse of the input.

Answer

It cannot be called with a null array.

Answer

It uses pass-by-value, so the original array is unchanged.

Question 4

Which TWO statements are true about interfaces in Java?

Accepted Answer

Interfaces can contain private methods.. Option A is correct because since Java 9, interfaces may declare private methods (both private instance methods and private static methods), which are used as helper methods for default and static methods within the interface and are not inherited by implementing classes. Option D is correct because every field declared in an interface is implicitly public, static, and final, meaning interface variables are constants that must be initialized and cannot be reassigned. Option B is wrong because interfaces are abstract types and cannot be instantiated with new; you must instantiate a concrete implementing class (or an anonymous class). Option C is wrong because interfaces have no constructors, since they cannot be instantiated and hold no instance state. Option E is wrong because interfaces can have default methods (introduced in Java 8), declared with the default keyword to provide a method body.

Answer

Interfaces can be instantiated.

Answer

Interfaces can contain constructors.

Answer

Interfaces cannot have default methods.

Question 5

What is the most likely cause of this error?

Accepted Answer

The heap size is insufficient for the objects being created.. The error in question is java.lang.OutOfMemoryError: Java heap space, which is thrown when the JVM cannot allocate an object in the heap because the heap is full and garbage collection cannot free enough memory. This directly indicates that the heap size is insufficient for the objects being created, either due to a genuine need for more memory or a memory leak within the Java heap. The other options describe different error conditions that produce different messages or exceptions.

Answer

There is a memory leak in native code outside the heap.

Answer

There is a stack overflow in the method being called.

Answer

Too many threads are running concurrently.

Question 6

A company is developing a configuration manager that must be shared across all components to ensure consistent settings. The manager should prevent direct instantiation and provide a single access point. Which design pattern and implementation should be used?

Accepted Answer

Singleton pattern with a private constructor and a static getInstance() method. The Singleton pattern ensures a class has only one instance and provides a global point of access. A private constructor prevents direct instantiation, and a static getInstance() method provides controlled access.

Answer

Prototype pattern with cloning

Answer

Builder pattern with a static inner class

Answer

Factory pattern with a public constructor

Question 7

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 8

A method is declared as: public static void main(String[] args) { }. Which statement is true?

Accepted Answer

The method can be called without creating an instance of the class.. The `static` modifier in the method signature means the method belongs to the class itself, not to any instance. Therefore, it can be invoked using the class name (e.g., `ClassName.main(args)`) without creating an object of the class. This is a fundamental characteristic of static methods in Java.

Answer

The method can be overridden in a subclass.

Answer

The method can return an int.

Answer

The method can be private.

Question 9

A calculator class has two overloaded methods: add(int a, int b) and add(double a, double b). A call to add(3, 4) will invoke which method?

Accepted Answer

add(int, int). The call add(3, 4) uses integer literals 3 and 4, which are of type int. Java's overload resolution first attempts to find a method that is applicable by strict invocation (no boxing, no varargs). The method add(int, int) exactly matches the argument types, so it is chosen. The add(double, double) method would require a widening primitive conversion from int to double, which is only considered if no exact match exists.

Answer

add(double, double)

Answer

Runtime decision based on actual parameters

Answer

Compilation error: ambiguous

Question 10

Which TWO statements are true about the main method?

Accepted Answer

It must be declared public.. Option B is correct because the JVM launches the entry point of a Java application by invoking the main method with public visibility, so it must be declared public; otherwise, the runtime cannot access it and throws a NoSuchMethodError. Option D is correct because main must be declared static so the JVM can call it without instantiating the enclosing class. The full required signature is public static void main(String[] args). Option A is wrong because final is not required and has no role in the JVM's entry-point lookup. Option C is wrong because main must return void, not int. Option E is wrong because main can be overloaded (for example, main(int x) is legal); only the standard signature serves as the entry point.

Answer

It must be declared final.

Answer

It must return an int.

Answer

It cannot be overloaded.

Question 11

Which THREE of the following are valid Java operators?

Accepted Answer

<<. (<<) is correct because it is the Java left shift operator, which shifts the bits of an integer or long value to the left by a specified number of positions, filling with zeros. This is a valid bitwise shift operator in Java, defined in the Java Language Specification (JLS §15.19).

Answer

::

Answer

<==

Question 12

Which primitive type can store a single character?

Accepted Answer

char. The `char` primitive type in Java is specifically designed to store a single 16-bit Unicode character, ranging from '\u0000' (0) to '\uffff' (65535). It is the only primitive type that directly represents a character value, making it the appropriate choice for storing a single character.

Answer

String

Answer

byte

Answer

short

Question 13

You are developing a high-frequency trading application where performance is critical. You need to parse and concatenate trade messages. The messages are received as strings and must be combined into a single output string for logging. Each message is appended to the log string. Currently, you are using String concatenation with the '+' operator inside a loop that processes up to 10,000 messages per second. However, performance monitoring shows that the application experiences frequent garbage collection pauses, affecting throughput. Which approach should you take to reduce garbage collection overhead and improve performance?

Accepted Answer

Use StringBuilder instead of String concatenation, ensuring that the StringBuilder is created with an appropriate initial capacity.. Using StringBuilder with an appropriate initial capacity reduces garbage collection overhead because it avoids creating multiple intermediate String objects. StringBuilder is mutable, so appending messages modifies the same buffer, minimizing object allocation and GC pauses.

Answer

Use the String.concat() method for each concatenation to reduce object creation.

Answer

Use StringBuffer instead of String concatenation because it is thread-safe and efficient.

Answer

Keep using the '+' operator but increase the heap size to reduce garbage collection frequency.

Question 14

Refer to the exhibit. What is the likely cause of this error?

Accepted Answer

x is declared but not initialized. In Java, a local variable must be explicitly initialized before it is used; otherwise, the compiler reports an 'error: variable x might not have been initialized'. The exhibit shows a compilation error, and the most likely cause is that x was declared (e.g., 'int x;') but never assigned a value before being referenced in an expression or printed.

Answer

x is out of scope

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x is a duplicate variable

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x is declared as a different type

Question 15

A developer writes: Object obj = new String("Hello"); System.out.println(obj.length()); What will be the output?

Accepted Answer

Compilation error. The code fails to compile because the reference variable 'obj' is of type Object, which does not have a length() method. The length() method is defined in the String class, not in Object. Since the compiler checks the declared type (Object) for method availability, it does not find length() and produces a compilation error.

Answer

null

Answer

Runtime exception

Answer

5

Answer

0