Stub/Singleton Pattern

DemoMapper.cls

public virtual class DemoMapper {

    @TestVisible
    private static DemoMapper instance;

    protected DemoMapper() {}

    public static DemoMapper getInstance() {
        if (instance != null) {
            return instance;
        }

        instance = new DemoMapper();
        return instance;
    }

    public virtual Something someMethodThatDoesSomething() {
        return new Something();
    }
}

DemoService.cls

public class DemoService {

    public void someMethodThatDoesSomething() {
        DemoMapper mapper = DemoMapper.getInstance();

        Something s = mapper.someMethodThatDoesSomething();
    }
}

TestClass.cls

@IsTest
private class TestClass {

    static testMethod void someTest() {
        DemoMapper.instance = new DemoMapperStub();

        // testing here
    }

    private class DemoMapperStub extends DemoMapper {
        public override Something someMethodThatDoesSomething() {
            return new SomethingElse();
        }
    }
}