Subclass mock objects are mock objects that are created by subclassing the class under test. They are used in testing to replace dependencies of a class with mock objects, so that the class can be tested in isolation.

In Java, you can create a subclass mock object using a mocking framework like Mockito. The following is an example of how to create a subclass mock object using Mockito:

Suppose we have a Calculator class that depends on a MathUtils class, and we want to test the Calculator class in isolation. We can create a subclass mock object of the MathUtils class as follows:

import static org.mockito.Mockito.*;

class MathUtilsSubclass extends MathUtils {
    @Override
    public int add(int a, int b) {
        return 5;
    }
}

class CalculatorTest {
    @Test
    public void testAddition() {
        MathUtilsSubclass mathUtils = new MathUtilsSubclass();
        Calculator calculator = new Calculator(mathUtils);
        int result = calculator.add(2, 3);
        assertEquals(5, result);
    }
}

In this example, we create a subclass of the MathUtils class called MathUtilsSubclass that overrides the add method to always return 5. Then, in the CalculatorTest class, we create an instance of MathUtilsSubclass and pass it to the Calculator constructor. This way, when the add method is called on the MathUtils object inside the Calculator, it returns the value that we specified in the MathUtilsSubclass.

Overall, using subclass mock objects can be a useful technique for testing classes in isolation, and can help you uncover and fix issues in your code. However, it's important to use them judiciously and with a clear understanding of their limitations and potential drawbacks.