为何无法将SectionsPagerAdapter类设为私有？Android开发相关疑问

Hey there! Let's unpack this problem you're facing with the SectionsPagerAdapter from Head First Android.

First, let's clarify why you can't mark it as private:

• If your SectionsPagerAdapter is a top-level class (in its own .java/.kt file), Java doesn't allow private top-level classes (only public or package-private are permitted). In Kotlin, private top-level classes are allowed but can only be accessed within the same file.
• If it's an inner class inside an Activity/Fragment, marking it private is allowed—but it can only be instantiated and used inside that host class.

1. Package-Private Access (Great for Single-Module Encapsulation) #

If your adapter only needs to be used within the same package, skip the access modifier (Java defaults to package-private) or use Kotlin's internal modifier (module-wide visibility):

// Java: Package-private, accessible only to classes in the same package
class SectionsPagerAdapter extends FragmentPagerAdapter {
    public SectionsPagerAdapter(FragmentManager fm) {
        super(fm);
    }

    @Override
    public Fragment getItem(int position) {
        return PlaceholderFragment.newInstance(position + 1);
    }

    @Override
    public int getCount() {
        return 3;
    }
}

// Kotlin: Internal, accessible anywhere in your module
internal class SectionsPagerAdapter(fm: FragmentManager) : FragmentPagerAdapter(fm) {
    override fun getItem(position: Int): Fragment {
        return PlaceholderFragment.newInstance(position + 1)
    }

    override fun getCount(): Int = 3
}

2. Private Inner Class (Only for Use Within the Host Class) #

If the adapter is exclusively used by one Activity/Fragment, make it a private inner class of that host. This keeps it fully encapsulated and only accessible to the host class:

public class MainActivity extends AppCompatActivity {
    // Private inner class—only MainActivity can access this
    private class SectionsPagerAdapter extends FragmentPagerAdapter {
        public SectionsPagerAdapter(FragmentManager fm) {
            super(fm);
        }

        @Override
        public Fragment getItem(int position) {
            return PlaceholderFragment.newInstance(position + 1);
        }

        @Override
        public int getCount() {
            return 3;
        }
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        // Host class can instantiate the private inner class without issues
        SectionsPagerAdapter pagerAdapter = new SectionsPagerAdapter(getSupportFragmentManager());
        ViewPager viewPager = findViewById(R.id.view_pager);
        viewPager.setAdapter(pagerAdapter);
    }
}

3. Public Top-Level Class (For Cross-Module/Reusable Use Cases) #

If you need to reuse the adapter across multiple classes or modules, mark it as public and pass necessary dependencies (like FragmentManager) via its constructor to keep it flexible.

Yes, you do need to pass the required parameters to the parent class! SectionsPagerAdapter typically extends either FragmentPagerAdapter or FragmentStatePagerAdapter—both require a FragmentManager in their constructors (use getSupportFragmentManager() from your Activity for AndroidX).

If you're using the newer ViewPager2, the corresponding adapter is FragmentStateAdapter, which requires a FragmentActivity or Fragment instance instead:

public class SectionsPagerAdapter extends FragmentStateAdapter {
    public SectionsPagerAdapter(FragmentActivity activity) {
        super(activity);
    }

    // Implement necessary methods here
}

In short: Whatever parameters the parent class requires must be passed via super() in your adapter's constructor—otherwise you'll get a compile error.

内容的提问来源于stack exchange，提问作者M. Warren