Why assertSame and assertNotSame in JUnit? - Purpose & Use Cases

The Big Idea

Discover how a simple check can save hours of confusion and catch hidden bugs!

The Scenario

Imagine you are checking if two boxes contain the exact same toy by opening them and comparing each toy piece manually.

The Problem

This manual checking is slow and confusing because toys might look the same but be different pieces. You might miss subtle differences or waste time comparing things that are actually different objects.

The Solution

Using assertSame and assertNotSame in tests lets you quickly check if two variables point to the exact same object or not, without guessing or comparing details manually.

Before vs After

✗ Before

if (obj1 == obj2) {
  System.out.println("Same object");
} else {
  System.out.println("Different objects");
}

✓ After

assertSame(obj1, obj2);
assertNotSame(obj1, obj3);

What It Enables

It enables fast and precise verification of object identity, catching subtle bugs that normal equality checks might miss.

Real Life Example

When testing a cache system, you want to confirm that the same cached object is returned each time, not just an equal copy. assertSame helps verify this behavior.

Key Takeaways

assertSame checks if two references point to the exact same object.

assertNotSame confirms two references are different objects.

These assertions save time and avoid errors compared to manual identity checks.

Practice

1. What does assertSame check in a JUnit test?

easy

A. If two references point to the exact same object

B. If two objects have the same content

C. If two objects are of the same class

D. If two references are both null

Solution

1. Step 1: Understand the purpose of assertSame

   assertSame checks if both references point to the exact same object in memory, not just equal content.

2. Step 2: Differentiate from equality checks

   Other assertions like assertEquals check if objects are equal in content, but assertSame checks identity.

3. Final Answer:

   If two references point to the exact same object -> Option A

4. Quick Check:

   assertSame = same object reference [OK]

Hint: assertSame checks object identity, not content equality [OK]

Common Mistakes:

• Confusing assertSame with assertEquals
• Thinking assertSame checks object content
• Assuming assertSame works for null references only

2. Which of the following is the correct syntax to assert that two objects are NOT the same in JUnit?

easy

A. assertNotSame(object1, object2);

B. assertNotSame(object1 == object2);

C. assertNotSame(object1.equals(object2));

D. assertNotSame(object1 != object2);

Solution

1. Step 1: Recall the method signature of assertNotSame

   The correct syntax is assertNotSame(expected, actual), both are objects to compare references.

2. Step 2: Identify incorrect usage

   Options A, C, and D misuse assertNotSame by passing boolean expressions instead of objects.

3. Final Answer:

   assertNotSame(object1, object2); -> Option A

4. Quick Check:

   assertNotSame takes two objects, not booleans [OK]

Hint: Use assertNotSame(obj1, obj2) with two objects, not booleans [OK]

Common Mistakes:

• Passing boolean expressions instead of objects
• Using assertNotSame with equals() method
• Confusing assertNotSame syntax with assertEquals

3. Consider the following code snippet:

String a = "test";
String b = "test";
String c = new String("test");

assertSame(a, b);
assertNotSame(a, c);

What will be the result of running these assertions?

medium

A. First assertion passes, second fails

B. First assertion fails, second passes

C. Both assertions pass

D. Both assertions fail

Solution

1. Step 1: Analyze references of a and b

   Both a and b point to the same string literal in the string pool, so assertSame(a, b) passes.

2. Step 2: Analyze reference of c

   c is a new String object created on the heap, different from a, so assertNotSame(a, c) passes.

3. Final Answer:

   Both assertions pass -> Option C

4. Quick Check:

   String literals share references, new String creates new object [OK]

Hint: String literals share references; new String creates new object [OK]

Common Mistakes:

• Assuming new String shares reference with literal
• Confusing assertSame with assertEquals
• Thinking assertNotSame fails if content equals

4. You wrote this test:

Object obj1 = new Object();
Object obj2 = obj1;
assertNotSame(obj1, obj2);

The test fails. What is the problem?

medium

A. assertNotSame cannot be used with Object class

B. assertNotSame syntax is incorrect

C. obj1 and obj2 are different objects, so test should pass

D. obj1 and obj2 refer to the same object, so assertNotSame fails

Solution

1. Step 1: Check references of obj1 and obj2

   Both variables point to the same object because obj2 = obj1 assigns the reference.

2. Step 2: Understand assertNotSame behavior

   assertNotSame expects different objects; since references are same, assertion fails.

3. Final Answer:

   obj1 and obj2 refer to the same object, so assertNotSame fails -> Option D

4. Quick Check:

   Same reference fails assertNotSame [OK]

Hint: assertNotSame fails if references point to same object [OK]

Common Mistakes:

• Assuming assertNotSame checks content equality
• Thinking assertNotSame syntax is wrong
• Believing assertNotSame cannot test Object instances

5. Given the following code:

class Person {
  String name;
  Person(String name) { this.name = name; }
}

Person p1 = new Person("Alice");
Person p2 = new Person("Alice");
Person p3 = p1;

assertSame(p1, p3);
assertNotSame(p1, p2);

Which statement about these assertions is true?

hard

A. assertSame(p1, p3) fails because objects have same content but different references; assertNotSame(p1, p2) passes

B. assertSame(p1, p3) passes because p1 and p3 reference the same object; assertNotSame(p1, p2) passes because p1 and p2 are different objects even if names match

C. assertSame(p1, p3) passes; assertNotSame(p1, p2) fails because names are equal

D. Both assertions fail because Person objects are not immutable

Solution

1. Step 1: Analyze references of p1 and p3

   p3 is assigned from p1, so both point to the same object; assertSame(p1, p3) passes.

2. Step 2: Analyze references of p1 and p2

   p2 is a new Person object with same name but different reference; assertNotSame(p1, p2) passes.

3. Final Answer:

   assertSame(p1, p3) passes; assertNotSame(p1, p2) passes because references differ -> Option B

4. Quick Check:

   assertSame checks references, not content [OK]

Hint: assertSame checks references; assertNotSame passes if objects differ [OK]

Common Mistakes:

• Confusing object content equality with reference equality
• Assuming assertNotSame fails if object fields match
• Thinking assertSame fails if objects are mutable