Week 1 Exercises: Arrays & Reference Variables

Instructions

Answer all questions to test your understanding of arrays and the difference between primitive and reference variables. Click "Check Answer" after each question or "Submit All Answers" at the end.

Part 1: Basic Concepts

True / False

A primitive variable stores the actual value directly in memory.

TRUE

FALSE

True / False

A reference variable (like an array variable) stores the actual data of the array directly.

TRUE

FALSE

Multiple Choice

What does a reference variable store?

The actual data values

The memory address (reference) pointing to the actual data

A copy of the data

Nothing until initialized

Multiple Choice

Which of the following is NOT a primitive data type in Java?

int

String

boolean

double

Part 2: Comparing Variables

Code Tracing

What is the output of the following code?

ComparePrimitives.java

int a = 10;
int b = 10;

if (a == b) {
    System.out.println("Equal");
} else {
    System.out.println("Not Equal");
}
                    

Equal

Not Equal

Compilation Error

Runtime Error

Code Tracing

What is the output of the following code?

CompareArrays.java

int[] arr1 = {1, 2, 3};
int[] arr2 = {1, 2, 3};

if (arr1 == arr2) {
    System.out.println("Equal");
} else {
    System.out.println("Not Equal");
}
                    

Equal

Not Equal

Compilation Error

Runtime Error

Why are they NOT equal? Let's visualize!

Even though both arrays contain {1, 2, 3}, they are stored in different locations in memory:

Stack Memory

arr1

0x1A2B

arr2

0x5C7D

arr1 == arr2

0x1A2B ≠ 0x5C7D

FALSE!

Heap Memory

Address: 0x1A2B

[0]1

[1]2

[2]3

Address: 0x5C7D

[0]1

[1]2

[2]3

💡

Key Insight: The == operator compares the memory addresses (references), NOT the values inside the arrays! Since arr1 and arr2 point to different locations (0x1A2B vs 0x5C7D), they are NOT equal.

To compare array contents: Use Arrays.equals(arr1, arr2) which returns true because the values are the same.

Code Tracing

What is the output of the following code?

SameReference.java

int[] arr1 = {1, 2, 3};
int[] arr2 = arr1;  // arr2 points to same array as arr1

if (arr1 == arr2) {
    System.out.println("Equal");
} else {
    System.out.println("Not Equal");
}
                    

Equal

Not Equal

Compilation Error

Runtime Error

Why ARE they equal? Let's visualize!

When we write arr2 = arr1, we copy the reference (address), not the array. Both now point to the SAME array:

Stack Memory

arr1

0x1A2B

arr2

0x1A2B

arr1 == arr2

0x1A2B = 0x1A2B

TRUE!

Heap Memory

Address: 0x1A2B (SAME!)

[0]1

[1]2

[2]3

Only ONE array exists!

💡

Key Insight: Both arr1 and arr2 hold the same address (0x1A2B). The == operator compares these addresses, and since they are identical, the result is true!

Important: Since both variables point to the same array, changing arr2[0] will also change arr1[0]! They share the same data.

Part 3: Pass by Value vs Reference

Code Tracing

What is the value of x after calling the method?

PassByValue.java

public static void changeValue(int num) {
    num = 100;
}

public static void main(String[] args) {
    int x = 5;
    changeValue(x);
    System.out.println(x);  // What is printed?
}
                    

5

100

0

Compilation Error

Code Tracing

What is the value of arr[0] after calling the method?

PassByReference.java

public static void changeArray(int[] nums) {
    nums[0] = 100;
}

public static void main(String[] args) {
    int[] arr = {5, 10, 15};
    changeArray(arr);
    System.out.println(arr[0]);  // What is printed?
}
                    

5

100

0

Compilation Error

Multiple Choice

In Question 8, the value of x remained 5. In Question 9, the value of arr[0] changed to 100. Why is this?

Arrays are mutable, primitives are not

Primitives pass a copy of the value; arrays pass a copy of the reference (address) pointing to the same data

The method names are different

Java has a bug with primitive types

Part 4: Array Manipulation

Code Tracing

What are the values of a[0] and b[0] after running this code?

SharedReference.java

int[] a = {1, 2, 3};
int[] b = a;

b[0] = 99;

System.out.println(a[0] + " " + b[0]);

1 99

99 99

99 1

1 1

Fill in the Blank

After running this code, what is the value printed?

DefaultValues.java

int[] numbers = new int[5];

System.out.println(numbers[2]);

The output is:

True / False

The following code creates an array in memory:

Declaration.java

int[] arr;  // Only this line

TRUE

FALSE

Multiple Choice

Look at this memory diagram. What does this represent?

Stack Memory

arr

0x1A2B

→

Heap Memory (0x1A2B)

[0]

[1]

[2]

The array data is stored in the variable arr

The variable arr holds the address 0x1A2B which points to the actual array in heap memory

The array is stored in two different locations

This is a 2D array

Code Tracing

What is the final output of this code?

ComplexExample.java

int[] x = {1, 2, 3};
int[] y = x;
int[] z = {1, 2, 3};

y[1] = 50;

System.out.println(x[1] + " " + y[1] + " " + z[1]);
                    

2 50 2

50 50 50

50 50 2

2 50 50

Your Results

0 Correct

0 Incorrect

1D Array Exercises

Instructions