Argument matchers (any, eq) in JUnit - Build an Automation Script

Verify service method called with specific and any arguments using argument matchers

Preconditions (2)

Step 1: Call the service method with arguments: "testString" and 10

Step 2: Verify that the service method was called with the exact String "testString" and any Integer argument

Step 3: Verify that the service method was called with any String argument and the exact Integer 10

✅ Expected Result: The verification passes confirming the method was called with the expected argument matchers

Automation Requirements - JUnit 5 with Mockito

Assertions Needed:

Verify method call with eq("testString") and anyInt()

Verify method call with anyString() and eq(10)

Best Practices:

Use Mockito's argument matchers consistently

Avoid mixing raw values and matchers in the same method call

Use @ExtendWith(MockitoExtension.class) for Mockito initialization

Use descriptive test method names

Automated Solution

JUnit

import static org.mockito.ArgumentMatchers.anyInt;
import static org.mockito.ArgumentMatchers.anyString;
import static org.mockito.ArgumentMatchers.eq;
import static org.mockito.Mockito.mock;
import static org.mockito.Mockito.verify;

import org.junit.jupiter.api.Test;
import org.junit.jupiter.api.extension.ExtendWith;
import org.mockito.junit.jupiter.MockitoExtension;

@ExtendWith(MockitoExtension.class)
public class ServiceTest {

    interface Service {
        void performAction(String text, Integer number);
    }

    @Test
    void testArgumentMatchers() {
        Service serviceMock = mock(Service.class);

        // Call the method with specific arguments
        serviceMock.performAction("testString", 10);

        // Verify method called with exact String and any Integer
        verify(serviceMock).performAction(eq("testString"), anyInt());

        // Verify method called with any String and exact Integer
        verify(serviceMock).performAction(anyString(), eq(10));
    }
}

This test uses Mockito to create a mock of the Service interface.

We call the performAction method with the arguments "testString" and 10.

Then, we verify two things:

• The method was called with the exact string "testString" and any integer (using eq and anyInt).
• The method was called with any string and the exact integer 10 (using anyString and eq).

This shows how to use argument matchers to flexibly check method calls without requiring exact matches on all parameters.

Using @ExtendWith(MockitoExtension.class) ensures Mockito initializes correctly with JUnit 5.

Common Mistakes - 3 Pitfalls

{'mistake': 'Mixing raw values and argument matchers in the same method call verification', 'why_bad': 'Mockito requires all arguments to be either matchers or raw values, not a mix. Mixing causes runtime errors.', 'correct_approach': 'Use argument matchers for all parameters or none. For example, use eq("value") instead of raw "value" when using matchers.'}

{'mistake': 'Not importing static methods for argument matchers', 'why_bad': 'Without static imports, the code becomes verbose and less readable, and sometimes compilation errors occur.', 'correct_approach': "Use static imports like 'import static org.mockito.ArgumentMatchers.anyInt;' for cleaner code."}

Not initializing Mockito properly with JUnit 5

Bonus Challenge

Now add tests verifying the method is called with any String and any Integer using argument matchers

Show Hint

Practice

1. What does the any() argument matcher do in JUnit Mockito tests?

easy

A. It matches any value of the specified type.

B. It matches only null values.

C. It matches only exact values.

D. It matches values that are greater than zero.

Solution

1. Step 1: Understand the purpose of any()

   The any() matcher is used to accept any argument of a given type in method verification or stubbing.

2. Step 2: Compare with other matchers

   Unlike eq(), which matches exact values, any() is flexible and matches all values of the specified type.

3. Final Answer:

   It matches any value of the specified type. -> Option A

4. Quick Check:

   any() = matches any value [OK]

Hint: Remember: any() accepts all values of a type [OK]

Common Mistakes:

• Confusing any() with eq()
• Thinking any() matches only null
• Assuming any() matches only specific values

2. Which of the following is the correct syntax to verify a method call with an exact argument value using an argument matcher in Mockito?

easy

A. verify(mock).method(any(String.class));

B. verify(mock).method(any());

C. verify(mock).method(eq("test"));

D. verify(mock).method("test");

Solution

1. Step 1: Identify correct use of eq()

   The eq() matcher is used to match an exact argument value in Mockito verification.

2. Step 2: Check syntax correctness

   verify(mock).method(eq("test")); correctly uses eq("test") to verify the method was called with the exact string "test".

3. Final Answer:

   verify(mock).method(eq("test")); -> Option C

4. Quick Check:

   Exact value match = eq() [OK]

Hint: Use eq() to match exact argument values [OK]

Common Mistakes:

• Using any() when exact match is needed
• Omitting eq() for exact value
• Using any() without specifying type

3. Consider the following code snippet:

when(service.process(any(String.class))).thenReturn("done");
String result = service.process("input");

What will be the value of result after execution?

medium

A. "input"

B. null

C. Throws an exception

D. "done"

Solution

1. Step 1: Analyze the stubbing with any()

   The when clause uses any(String.class) to match any string argument and returns "done".

2. Step 2: Check the method call argument

   The method is called with "input", which matches any(String.class), so the stub returns "done".

3. Final Answer:

   "done" -> Option D

4. Quick Check:

   any(String.class) matches "input" = returns "done" [OK]

Hint: any() matches all inputs, so stub returns specified value [OK]

Common Mistakes:

• Assuming method returns input argument
• Thinking any() causes exception
• Ignoring stubbing behavior

4. Identify the error in the following Mockito verification code:

verify(mock).update(eq("data"), any());

medium

A. eq() cannot be used with any().

B. Missing type parameter in any() matcher.

C. verify() should not be used with matchers.

D. No error; code is correct.

Solution

1. Step 1: Check usage of any()

   The any() matcher requires a type parameter like any(String.class) to specify the argument type.

2. Step 2: Confirm correct matcher usage

   Without the type, Mockito cannot determine the argument type, causing a compilation error.

3. Final Answer:

   Missing type parameter in any() matcher. -> Option B

4. Quick Check:

   any() needs type like any(String.class) [OK]

Hint: Always specify type in any() like any(String.class) [OK]

Common Mistakes:

• Using any() without type parameter
• Mixing eq() and any() incorrectly
• Assuming verify() disallows matchers

5. You want to verify a method call that takes two arguments: a String and an Integer. The first argument must be exactly "test", and the second can be any Integer. Which Mockito verification is correct?

hard

A. verify(mock).method(eq("test"), any(Integer.class));

B. verify(mock).method(any(String.class), eq(1));

C. verify(mock).method(any(), any());

D. verify(mock).method(eq("test"), eq(1));

Solution

1. Step 1: Match exact first argument

   The first argument must be exactly "test", so use eq("test").

2. Step 2: Match any Integer second argument

   The second argument can be any Integer, so use any(Integer.class).

3. Step 3: Verify combined usage

   verify(mock).method(eq("test"), any(Integer.class)); correctly combines eq() and any() for the two arguments.

4. Final Answer:

   verify(mock).method(eq("test"), any(Integer.class)); -> Option A

5. Quick Check:

   eq() + any(type) = exact + any argument [OK]

Hint: Use eq() for exact, any(type) for flexible args [OK]

Common Mistakes:

• Swapping argument matchers incorrectly
• Using any() without type
• Using eq() for flexible arguments