Pager 사용법

* activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    tools:context=".MainActivity" >

    <Button
        android:id="@+id/button"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="버튼" />

    <androidx.viewpager.widget.ViewPager
        android:id="@+id/pager"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />
        
</LinearLayout>

* MainActivity.java

public class MainActivity extends AppCompatActivity {

    ViewPager pager;

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

        pager = findViewById(R.id.pager);
        pager.setOffscreenPageLimit(3);

        MyPagerAdapter adapter = new MyPagerAdapter(getSupportFragmentManager());

        Fragment1 fragment1 = new Fragment1();
        Fragment2 fragment2 = new Fragment2();
        Fragment3 fragment3 = new Fragment3();

        adapter.addItem(fragment1);
        adapter.addItem(fragment2);
        adapter.addItem(fragment3);

        pager.setAdapter(adapter);

        Button button = findViewById(R.id.button);
        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                pager.setCurrentItem(1);
            }
        });
    }

    class MyPagerAdapter extends FragmentStatePagerAdapter {
        ArrayList<Fragment> items = new ArrayList<Fragment>();

        public MyPagerAdapter(FragmentManager fm) {
            super(fm);
        }

        public void addItem(Fragment item) {
            items.add(item);
        }

        @NonNull
        @Override
        public Fragment getItem(int position) {
            return items.get(position);
        }

        @Override
        public int getCount() {
            return items.size();
        }
    }

}

* Fragment1.java

public class Fragment1 extends Fragment {

    @Nullable
    @Override
    public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        return inflater.inflate(R.layout.fragment1, container, false);
    }
    
}