Java 具有默认分支的JUnit代码覆盖率切换声明

You don't need to create or extend the existing enum Mode. You just need to mock this enum with Mockito.
For this, you can use Mockito version 5 or later, which support mockito-inline as the default mockmaker. This will support mocking static method easily.

测试实现组:"org. mockito"，名称:"mockito-core"，版本:"5.11.0"

The strategy is to create a mock enum UNSUPPORTED. Use Mockito.mockStatic to mock the static method values() of the enum, and return the existing values plus the mock enum UNSUPPORTED.
Then this mock UNSUPPORTED enum would trigger the default path of your switch-case statement, which allow you to perform the assertion easily.

代码片段

import org.junit.jupiter.api.Test;
import org.mockito.MockedStatic;
import org.mockito.Mockito;

import static org.junit.jupiter.api.Assertions.assertEquals;
import static org.junit.jupiter.api.Assertions.assertThrows;
import static org.mockito.Mockito.doReturn;

public class SOEnumQuestionTest {

    public enum Mode {
        LAST_SPACES,
        LAST_CHARS
    }

    public static String process(final Mode mode) {
        switch (mode) {
            case LAST_SPACES : {
                return "processing last spaces";
            }
            case LAST_CHARS : {
                return "processing last chars";
            }
            default :
                throw new IllegalStateException("Processor not implemented");
        }
    }

    @Test
    public void testProcessWithDefaultSwitchCase() {
        try (MockedStatic<Mode> dummyModeMockedStatic = Mockito.mockStatic(Mode.class)) {
            final Mode UNSUPPORTED = Mockito.mock(Mode.class);
            // We want to give the mocked enum in the last position of the enum values list
            // E.g: Enum has 2 values -> our mock UNSUPPORTED enum index is 2
            doReturn(2).when(UNSUPPORTED).ordinal();
            dummyModeMockedStatic.when(Mode::values)
                    .thenReturn(new Mode[]{
                            Mode.LAST_SPACES,
                            Mode.LAST_CHARS,
                            UNSUPPORTED});

            Throwable expectedException = assertThrows(IllegalStateException.class, () -> SOEnumQuestionTest.process(UNSUPPORTED));
            assertEquals("Processor not implemented", expectedException.getMessage());
        }
    }
}