1Z0-811 Arrays and Methods — 20 Questions

Question 1

A method 'public static void modifyArray(int[] arr) { arr[0] = 99; }' is called with 'int[] myArray = {1,2,3}; modifyArray(myArray); System.out.println(myArray[0]);'. What is the output?

Accepted Answer

99. In Java, when an array is passed to a method, the reference is passed by value. The method receives a copy of the reference, but both references point to the same array object. Therefore, modifying an element inside the method changes the original array. After the call, myArray[0] is 99.

Answer

1

Answer

0

Answer

Compilation error

Question 2

Match each control flow statement to its purpose.

Accepted Answer

if-else: Executes a block of code based on a boolean condition.. The correct matches are: if-else (A) executes based on condition, switch (B) selects based on value, for loop (C) repeats fixed times, while loop (D) repeats while condition true. Options E and F are incorrect because they misassign the purposes.

Answer

if-else: Repeats a block of code a fixed number of times.

Answer

switch: Repeats a block of code as long as a condition is true.

Question 3

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. The question refers to a method find(int[] arr, int target) that returns the index of the first occurrence of target in arr, or -1 if not found. Given an array values containing {3, 7, ...}, find(values, 7) returns 1, and find(values, 5) returns -1. Thus, the output is '1 -1'. Option A would be if 7 were at index 0, but it's at index 1. Option B would be if both calls returned -1, which is incorrect. Option D would be if both were found at the wrong indices.

Answer

0 -1

Answer

-1 -1

Answer

1 1

Question 4

A developer writes a method that takes an int array and returns the sum of its elements. The method signature is: 'public static int sumArray(int[] arr)'. Which statement correctly calls this method?

Accepted Answer

int result = sumArray(new int[]{1,2,3});. It uses the correct syntax for creating and passing an anonymous int array to the sumArray method. The expression `new int[]{1,2,3}` creates an int array object with the specified elements, which matches the method's parameter type `int[]`. The method then returns the sum, which is assigned to the int variable `result`.

Answer

int result = sumArray(true);

Answer

int result = sumArray([1,2,3]);

Answer

int result = sumArray(5);

Question 5

Given the code snippet: 'int[] nums = {10, 20, 30, 40}; int sum = 0; for (int i = 0; i < nums.length; i++) { sum += nums[i]; }'. What is the value of sum after execution?

Accepted Answer

100. (100) because the for loop iterates over each element of the array {10, 20, 30, 40} and adds it to the sum variable. The sum starts at 0, and after adding 10, 20, 30, and 40, the total becomes 100.

Answer

30

Answer

60

Answer

140

Question 6

Which of the following correctly uses the shorthand array initializer to declare and initialize an array of strings with the elements "A", "B", and "C"?

Accepted Answer

String[] arr = {"A", "B", "C"};. Option D correctly uses the shorthand array initializer with curly braces directly in the declaration. Option A uses the full syntax with the 'new' keyword, which is also valid but not the shorthand. Option B is wrong because it uses parentheses. Option C is wrong because it uses square brackets.

Answer

String[] arr = new String[] {"A", "B", "C"};

Answer

String[] arr = ("A", "B", "C");

Answer

String[] arr = ["A", "B", "C"];

Question 7

A method 'public static int findMax(int[] numbers)' returns the maximum value in the array. Which implementation correctly handles an empty array by returning 0?

Accepted Answer

if(numbers.length == 0) return 0; int max = numbers[0]; for(int i=1; i<numbers.length; i++) if(numbers[i] > max) max = numbers[i]; return max;. Ly handles an empty array by checking `numbers.length == 0` and returning 0 before attempting to access `numbers[0]`, which would throw an `ArrayIndexOutOfBoundsException` on an empty array. It then initializes `max` to the first element and iterates from index 1, ensuring all elements are compared correctly even if all numbers are negative.

Answer

int max = 0; for(int n: numbers) if(n > max) max = n; return max;

Answer

int max = numbers[0]; for(int i=1; i<numbers.length; i++) if(numbers[i] > max) max = numbers[i]; return max;

Answer

if(numbers.length == 0) return 0; int max = 0; for(int n: numbers) if(n > max) max = n; return max;

Question 8

Given the method: 'public static void swapFirstTwo(int[] arr) { int temp = arr[0]; arr[0] = arr[1]; arr[1] = temp; }'. What is the effect of calling this method with an array of length 1?

Accepted Answer

An ArrayIndexOutOfBoundsException is thrown.. The method accesses arr[0] and arr[1] without verifying the array length. When called with an array of length 1, index 1 is out of bounds because valid indices are 0 only. Java performs runtime bounds checking, so an ArrayIndexOutOfBoundsException is thrown.

Answer

The first element is swapped with itself.

Answer

A new array with length 2 is returned.

Answer

The array remains unchanged.

Question 9

A developer writes a method that accepts a variable number of int arguments and returns their product. Which method signature correctly implements this?

Accepted Answer

public static int product(int... nums). The syntax `int... nums` is the proper Java varargs syntax, allowing a method to accept zero or more `int` arguments, which are then treated as an array inside the method. This enables the developer to call `product(2, 3, 4)` or `product()` and compute the product by iterating over the array.

Answer

public static int product(int[] nums)

Answer

public static int product(int nums...)

Answer

public static int product(int... nums, int extra)

Question 10

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 11

Which of the following correctly describes the effect of the method call 'Arrays.sort(myArray)' on an array of objects that do not implement Comparable?

Accepted Answer

A ClassCastException is thrown at runtime.. When `Arrays.sort(myArray)` is called on an array of objects that do not implement the `Comparable` interface, the method attempts to cast elements to `Comparable` to compare them. Since the objects do not implement `Comparable`, a `ClassCastException` is thrown at runtime. The `Arrays.sort(Object[])` method requires that all elements implement `Comparable`; otherwise, it cannot determine a natural ordering.

Answer

The code does not compile because Arrays.sort requires a Comparator.

Answer

The array is sorted using the natural order of the elements.

Answer

The array is sorted using the order defined by the elements' equals method.

Question 12

Which TWO statements are true about method overloading in Java?

Accepted Answer

Overloaded methods can have different parameter lists.. Method overloading in Java allows multiple methods to share the same name but have different parameter lists (different number, type, or order of parameters). This enables compile-time polymorphism, where the compiler selects the appropriate method based on the arguments provided at the call site.

Answer

Overloaded methods must have the same number of parameters.

Answer

Overloaded methods must have different return types.

Answer

Overloaded methods can have different names.

Question 13

Which TWO statements are true about passing arrays to methods in Java?

Accepted Answer

The method can modify the elements of the array.. Java passes object references by value. When an array is passed to a method, the method receives a copy of the reference to the array object. This copy still points to the same array object in heap memory, so the method can modify the elements of the array through that reference. These modifications are visible to the caller because they affect the same underlying array object.

Answer

The method can reassign the original array variable to a new array.

Answer

The method must return the array to reflect any changes.

Answer

The method cannot determine the size of the array.

Question 14

Which THREE statements are correct about the 'main' method signature in Java?

Accepted Answer

It must be declared public.. The Java Language Specification (JLS) requires the main method to be declared public so that the JVM can access it from outside the class. Without public visibility, the JVM would not be able to invoke the method as the entry point of the application.

Answer

It can be declared to return an int.

Answer

It can be named any valid identifier.

Question 15

A team is developing a Java application for an online store. The application has a class 'Inventory' that maintains an array of 'Product' objects. The method 'public void addProduct(Product p)' is intended to add a product to the array. The current implementation uses a fixed-size array of length 100. However, the business has grown, and the array may exceed its capacity. The team needs a solution that allows dynamic resizing without changing the method signature. Which approach should the team take?

Accepted Answer

Modify the method to use an ArrayList internally and keep the signature.. Using an ArrayList internally allows the array to dynamically resize as needed, while keeping the method signature unchanged. The method still accepts a Product parameter, but the internal implementation leverages ArrayList's automatic capacity management, eliminating the fixed-size constraint.

Answer

Change the method signature to accept a Product[] array and copy all elements.

Answer

Use a Collection<Product> as parameter and convert to array.

Answer

Change the return type to boolean and return false if array is full.

Question 16

A developer is working on a Java program that processes sensor data. The data is stored in a 2D array 'double[][] readings', where each row represents a sensor and each column a time interval. The method 'public static double[] averagePerSensor(double[][] data)' should compute the average reading for each sensor (row) and return a 1D array of averages. The developer writes the following implementation: 'double[] result = new double[data.length]; for (int i = 0; i < data.length; i++) { double sum = 0; for (int j = 0; j < data[i].length; j++) { sum += data[i][j]; } result[i] = sum / data[i].length; } return result;'. However, the program sometimes throws a NullPointerException. What is the most likely cause?

Accepted Answer

One or more rows in the 2D array are null.. The NullPointerException occurs because the code attempts to access `data[i].length` when `data[i]` is null. In Java, a 2D array is an array of arrays, and any row can be null. The code does not check for null rows before iterating over the columns, so if a row is null, accessing `data[i][j]` or `data[i].length` throws a NullPointerException. Option C correctly identifies this as the most likely cause.

Answer

The method returns a double[] but should return double[][].

Answer

The rows have different numbers of columns.

Answer

The outer loop uses data[i].length instead of data.length.

Question 17

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.