1Z0-811 Practice Test 18 — 15 Questions

Question 1

A class defines a static variable initialized at declaration: static int count = 10;. A static method attempts to modify it: count = 20;. Which statement is true?

Accepted Answer

The variable can be changed to 20. Static variables belong to the class, not instances, and can be accessed and modified by static methods. The assignment `count = 20;` is valid and changes the value from 10 to 20 at runtime.

Answer

The variable cannot be accessed from a static method

Answer

The code causes a compilation error

Answer

The variable is read-only

Question 2

Which of the following is NOT a primitive data type?

Accepted Answer

String. String is a class in Java, not a primitive data type. The eight primitive types are byte, short, int, long, float, double, boolean, and char. String is a reference type that stores a sequence of characters as an object.

Answer

int

Answer

boolean

Answer

char

Question 3

A developer is writing a method to find the first occurrence of a negative number in an array and return its index, or -1 if none found. The current implementation uses a for loop with an if condition and a return when found. However, the method throws a NullPointerException when the array is null. The developer wants to handle this edge case gracefully and still return -1. Which approach is most appropriate?

Accepted Answer

Add a null check at the beginning of the method and return -1 if null. It directly checks if the array reference is null before any iteration, returning -1 immediately. This is the simplest and most efficient way to handle a null input, avoiding any attempt to access the array's length or elements, which would throw a NullPointerException. The method's contract is preserved by returning -1 when no negative number is found, including the case of a null array.

Answer

Use an enhanced for loop with a null check

Answer

Use a try-catch block inside the loop

Answer

Use a while loop with a null check inside

Question 4

Which TWO of the following are valid Java identifiers?

Accepted Answer

_count. Option C, _count, is a valid Java identifier because it begins with an underscore, which is a legal first character, and contains only letters and the underscore thereafter. Option E, $dollar, is also valid because the dollar sign is explicitly permitted as both a starting and subsequent character in Java identifiers. Option A, class, is invalid because it is a reserved Java keyword and cannot be used as an identifier. Option B, 2ndValue, is invalid because identifiers cannot begin with a digit. Option D, my-var, is invalid because the hyphen is not a legal identifier character in Java.

Answer

class

Answer

2ndValue

Answer

my-var

Question 5

Given method: static void change(String s) { s = "new"; } What is output of: String name = "original"; change(name); System.out.println(name);

Accepted Answer

"original". Java passes object references by value. Inside the method `change`, the local variable `s` is reassigned to a new String object, but this does not affect the original reference `name` in the main method. Strings are immutable, so the original `name` remains "original".

Answer

null

Answer

"new"

Answer

Compilation error

Question 6

Arrange the steps to create an object from a class in Java in the correct order.

Accepted Answer

Define the class, then declare a variable, then instantiate with new, then assign to variable, then use the object.. First define the class, then declare a variable, instantiate with new, assign to variable, and then use the object.

Answer

Define the class, then instantiate the object, then declare a variable, then assign to variable, then use the object.

Answer

Declare a variable, then define the class, then instantiate the object, then assign to variable, then use the object.

Answer

Instantiate the object, then define the class, then declare a variable, then assign to variable, then use the object.

Question 7

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 8

You are developing a Java application for a library management system. The system must track the number of books in each genre. You need to store genre names (String) and their counts (int). The data will be accessed frequently and modified rarely. Which Java data structure should you use to store this mapping efficiently, while ensuring that genre names are unique?

Accepted Answer

HashMap. HashMap is the correct choice because it stores key-value pairs, where each genre name (String) is a unique key mapped to its count (Integer). This provides O(1) average-time complexity for lookups and updates, which is ideal for frequent access and rare modifications. The HashMap ensures key uniqueness via its hash-based implementation, directly meeting the requirement of unique genre names.

Answer

ArrayList<Book>

Answer

LinkedList<String>

Answer

HashSet<String>

Question 9

What is the output of the following code?

```java
public class FindIndex {
    public static int find(int[] arr, int target) {
        for (int i = 0; i < arr.length; i++) {
            if (arr[i] == target) return i;
        }
        return -1;
    }

public static void main(String[] args) {
        int[] values = {3, 7, 2};
        System.out.print(find(values, 7) + " " + find(values, 5));
    }
}
```

Options:
A: 0 -1
B: -1 -1
C: 1 -1
D: 1 1

Accepted Answer

1 -1. Option C (1 -1) is correct because find(values, 7) scans the array {3, 7, 2} and returns index 1 where arr[1] == 7, while find(values, 5) finds no match and returns -1 after the loop completes. The main method prints these two results separated by a space, producing "1 -1". Option A is wrong because 7 is not at index 0 (arr[0] is 3), and Option B is wrong because 7 is present so the first call cannot return -1. Option D is wrong because 5 is absent from the array, so the second call must return -1 rather than 1.

Answer

0 -1

Answer

-1 -1

Answer

1 1

Question 10

Which TWO statements are true about the 'super' keyword in Java?

Accepted Answer

It can be used to call a superclass method that has been overridden.. The 'super' keyword can be used to call a superclass method that has been overridden, using 'super.methodName()'. Option D is correct because 'super()' invokes the superclass constructor. Option A is wrong because 'super' cannot access private methods of the superclass. Option B is wrong because 'super' cannot access private fields. Option E is wrong because 'super' cannot invoke static methods directly; static methods are accessed via the class name.

Answer

It can be used to access private methods of the superclass.

Answer

It can be used to access private fields of the superclass.

Answer

It can be used to invoke a static method of the superclass.

Question 11

A class that does not define any constructor has:

Accepted Answer

A default no-argument constructor. When a class declares no constructor at all, the Java compiler automatically inserts a default no-argument constructor whose access modifier matches the class's access modifier (public for a public class). This implicit constructor simply calls super() and performs no other initialization, allowing the class to be instantiated with new ClassName().

Answer

A private constructor

Answer

A public constructor with arguments

Answer

It cannot be instantiated

Answer

No constructor

Question 12

A Java application uses an interface 'Drawable' with a default method 'draw()'. A class 'Circle' implements Drawable but does not override draw(). Another class 'Square' implements Drawable and overrides draw(). Which statement is true about calling draw() on instances of Circle and Square?

Accepted Answer

Circle will use the default, Square will use its own. In Java, a default method in an interface provides an implementation that implementing classes inherit unless they override it. Circle does not override draw(), so it uses the interface's default implementation. Square overrides draw(), so its own implementation is invoked. Therefore, Circle uses the default and Square uses its own.

Answer

Square will use the default, Circle will cause a compilation error

Answer

Both will use the default implementation

Answer

Both will cause a compilation error

Question 13

Which THREE statements are true about the switch statement in Java? (Choose three.)

Accepted Answer

Without a break statement, execution falls through to the next case.. Options C, D, and E are true. Option C correctly describes fall-through behavior: without a break statement, execution continues to the next case. Option D correctly states that the default case is optional. Option E is true because, since Java 7, the switch statement can be used with String objects. Options A and B are false: A is incorrect because switch does not support boolean expressions; B is incorrect because the default case can appear anywhere in the switch block.

Answer

It can be used with boolean expressions.

Answer

The default case must be placed at the end of the switch block.

Question 14

A team is developing a library management system. They have a base class 'Item' with a method 'getTitle()' that returns the title. They also have a subclass 'Book' that overrides 'getTitle()'. In some places, they have a method that accepts an Item reference but actually receives a Book object. They want to call a method specific to Book, such as 'getISBN()', that is not in Item. They attempt to cast the parameter: ((Book) item).getISBN(). However, occasionally the program crashes with ClassCastException. What is the best practice to avoid this exception?

Accepted Answer

Use if (item instanceof Book) before casting.. Using `instanceof` before casting ensures the object is actually an instance of `Book`, preventing a `ClassCastException` at runtime. This is the standard Java practice for safe downcasting when the object type is not guaranteed.

Answer

Create a separate method for Book objects.

Answer

Catch ClassCastException and handle it.

Answer

Change the parameter type to Object and then cast to Book.

Question 15

A class has a method that is marked as protected. Which statement is true about its accessibility?

Accepted Answer

Within the same package and subclasses. In Java, the protected access modifier allows access within the same package and by subclasses (even if they are in different packages). Option E correctly states this combination, which is the precise definition of protected access as specified in the Java Language Specification (JLS §6.6.2).

Answer

Only in subclasses

Answer

Only within the same package

Answer

Everywhere

Answer

Only within the same class