如何模拟没有PowerMock的UUID.randomUuid()和System.currentTimeMillis()？

<dependencies>
    <!-- Mockito -->
    <dependency>
        <groupId>org.mockito</groupId>
        <artifactId>mockito-core</artifactId>
        <version>2.0.2-beta</version>
        <scope>test</scope>
    </dependency>
    
    <!-- JUnit -->
    <dependency>
        <groupId>junit</groupId>
        <artifactId>junit</artifactId>
        <version>4.12</version>
        <scope>test</scope>
    </dependency>
</dependencies>

import org.junit.Test;
import org.mockito.Mockito;

import java.util.UUID;

import static org.junit.Assert.assertEquals;

public class MyTest {
    
    @Test
    public void testRandomUuid() {
        // 模拟UUID.randomUuid()方法的返回值
        UUID mockUuid = UUID.fromString("00000000-0000-0000-0000-000000000000");
        UUID expectedUuid = UUID.fromString("12345678-1234-1234-1234-1234567890AB");
        UUIDGenerator uuidGenerator = Mockito.mock(UUIDGenerator.class);
        Mockito.when(uuidGenerator.randomUuid()).thenReturn(mockUuid);
        
        // 调用被测试方法
        UUID actualUuid = uuidGenerator.randomUuid();
        
        // 验证返回值是否符合预期
        assertEquals(expectedUuid, actualUuid);
    }
    
    @Test
    public void testCurrentTimeMillis() {
        // 模拟System.currentTimeMillis()方法的返回值
        long mockTime = 1234567890L;
        long expectedTime = 9876543210L;
        TimeGenerator timeGenerator = Mockito.mock(TimeGenerator.class);
        Mockito.when(timeGenerator.currentTimeMillis()).thenReturn(mockTime);
        
        // 调用被测试方法
        long actualTime = timeGenerator.currentTimeMillis();
        
        // 验证返回值是否符合预期
        assertEquals(expectedTime, actualTime);
    }
}

// UUID生成器接口
interface UUIDGenerator {
    UUID randomUuid();
}

// 时间生成器接口
interface TimeGenerator {
    long currentTimeMillis();
}