tablayout下仅显示第一个片段

<android.support.design.widget.TabLayout
    android:id="@+id/tabLayout"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    app:tabMode="fixed"
    app:tabGravity="fill"/>

<android.support.v4.view.ViewPager
    android:id="@+id/viewPager"
    android:layout_width="match_parent"
    android:layout_height="match_parent"/>

ViewPager viewPager = findViewById(R.id.viewPager);
TabLayout tabLayout = findViewById(R.id.tabLayout);

FragmentPagerAdapter adapter = new FragmentPagerAdapter(getSupportFragmentManager()) {
    @Override
    public Fragment getItem(int position) {
        // 返回对应位置的片段
        return fragments.get(position);
    }

    @Override
    public int getCount() {
        // 返回片段的数量
        return fragments.size();
    }

    @Override
    public CharSequence getPageTitle(int position) {
        // 返回选项卡的标题
        return tabTitles.get(position);
    }
};

viewPager.setAdapter(adapter);
tabLayout.setupWithViewPager(viewPager);

List<Fragment> fragments = new ArrayList<>();
fragments.add(new FirstFragment());
// fragments.add(new SecondFragment());
// fragments.add(new ThirdFragment());
// ...

// 设置选项卡的标题
List<String> tabTitles = new ArrayList<>();
tabTitles.add("First");
// tabTitles.add("Second");
// tabTitles.add("Third");
// ...