为什么我打开应用程序时没有出现默认选项卡？ [复制]

Question

我只想在这个应用程序中将“Tab1”设置为默认选项卡，为此我在这段代码中设置了true' tabLayout.addTab(tabLayout.newTab().setText("Tab1"), true);但是，这不起作用。

public class MainActivity extends AppCompatActivity implements TabLayout.OnTabSelectedListener {

    Toolbar toolbar;
    Fragment fragment;
    TabLayout tabLayout;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        tabLayout = findViewById(R.id.tablayout);
        toolbar = findViewById(R.id.toolbar);
        toolbar.setTitle("lalit kumar");
        setSupportActionBar(toolbar);
        tabLayout.addTab(tabLayout.newTab().setText("Tab1"), true);
        tabLayout.addTab(tabLayout.newTab().setText("Tab2"));
        tabLayout.addTab(tabLayout.newTab().setText("Tab3"));
        tabLayout.addOnTabSelectedListener(this);
    }


    @Override
    public void onTabSelected(TabLayout.Tab tab) {
        fragmentimplement(tab.getPosition());
    }

    @Override
    public void onTabUnselected(TabLayout.Tab tab) {

    }

    @Override
    public void onTabReselected(TabLayout.Tab tab) {

    }

    public void fragmentimplement(int position) {
        switch (position) {
            case 0:
                fragment = new AFragment();
                break;
            case 1:
                fragment = new BFragment();
                break;
            case 2:
                fragment = new CFragment();
                break;
        }
        getSupportFragmentManager().beginTransaction().replace(R.id.frame, fragment).commit();
    }
}

Answer 1

将以下行添加到 Oncreate 方法的代码末尾

@Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        tabLayout = findViewById(R.id.tablayout);
        toolbar = findViewById(R.id.toolbar);
        toolbar.setTitle("lalit kumar");
        setSupportActionBar(toolbar);

tabLayout.setOnTabSelectedListener(new TabLayout.OnTabSelectedListener() {
        @Override
        public void onTabSelected(TabLayout.Tab tab) {
            setCurrentTabFragment(tab.getPosition());
        }

        @Override
        public void onTabUnselected(TabLayout.Tab tab) {
        }

        @Override
        public void onTabReselected(TabLayout.Tab tab) {
        }
    });

    tabLayout.addTab(tabLayout.newTab().setText("Tab1"), true);
    tabLayout.addTab(tabLayout.newTab().setText("Tab2"));
    tabLayout.addTab(tabLayout.newTab().setText("Tab3"));
}



private void setCurrentTabFragment(int tabPosition)
    {
        switch (tabPosition)
        {
            case 0 :
                replaceFragment(fragmentOne);
                break;
            case 1 :
                replaceFragment(fragmentTwo);
                break;
            case 3 :
                replaceFragment(fragmentTwo);
                break;
        }
    }


public void replaceFragment(Fragment fragment) {
                     getSupportFragmentManager()
                    .beginTransaction()
                    .replace(R.id.fragment_container, fragment)
                    .commit();
    }