viewPager+fragment

Mainactivity类

public class MainActivity extends AppCompatActivity {

    ViewPager vp;
   /* Fragment1 f1;
    Fragment2 f2;
    Fragment3 f3;*/
    List<Fragment> list ;

    RadioButton btn, btn2, btn3;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        vp = (ViewPager) findViewById(R.id.viewpager);

        btn = (RadioButton) findViewById(R.id.btn);
        btn2 = (RadioButton) findViewById(R.id.btn2);
        btn3 = (RadioButton) findViewById(R.id.btn3);

        list = new ArrayList<>();
        list.add(new Fragment1());
        list.add(new Fragment2());
        list.add(new Fragment3());
        ViewPagerAdapter adapter = new ViewPagerAdapter(getSupportFragmentManager(),list);
        vp.setAdapter(adapter);
        vp.setOnPageChangeListener(new ViewPager.OnPageChangeListener() {
            @Override
            public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {

            }

            @Override
            public void onPageSelected(int position) {

            }

            @Override
            public void onPageScrollStateChanged(int state) {

            }
        });
        btn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                vp.setCurrentItem(0);
            }
        });
        btn2.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                vp.setCurrentItem(1);
            }
        });
        btn3.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                vp.setCurrentItem(2);
            }
        });


    }

 class ViewPagerAdapter extends FragmentPagerAdapter {

        public ViewPagerAdapter(FragmentManager fm, List<Fragment> list) {
            super(fm);
        }

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

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

这里强调的是

Fragment 方法   要

List<Fragment> list ;

在oncreate方法里

list = new ArrayList<>();
list.add(new Fragment1());
list.add(new Fragment2());
list.add(new Fragment3());
ViewPagerAdapter adapter = new ViewPagerAdapter(getSupportFragmentManager(),list);
vp.setAdapter(adapter);

要这样new 与fragment new的方法不能相同 

之前fragment 是这样new 的

Fragment  f1 ；

fragment  f2；

oncreate里面是这样的

initView();

这是外面的方法

private void initView() {
    f1 = new fragment1();
    f2 = new fragment2();
    f3 = new fragment3();
    f4 = new fragment4();
}

Fragment类

public class Fragment3 extends Fragment {


    @Nullable
    @Override
    public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        View view =  inflater.inflate(R.layout.activity_fragment3,container,false);
        return view;
    }
}