fragment底部导航栏切换实现代码

以下一个简单的实现底部导航栏切换的示例代码：

public class MainActivity extends AppCompatActivity {
    private Fragment[] fragments = {new HomeFragment(), new MessageFragment(), new MineFragment()};
    private int[] icons = {R.drawable.ic_home, R.drawable.ic_message, R.drawable.ic_mine};
    private String[] titles = {"首页", "消息", "我的"};
    private int lastIndex = 0;

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

        BottomNavigationView bottomNavigationView = findViewById(R.id.bottom_navigation);
        bottomNavigationView.setOnNavigationItemSelectedListener(new BottomNavigationView.OnNavigationItemSelectedListener() {
            @Override
            public boolean onNavigationItemSelected(@NonNull MenuItem item) {
                int index = 0;
                switch (item.getItemId()) {
                    case R.id.action_home:
                        index = 0;
                        break;
                    case R.id.action_message:
                        index = 1;
                        break;
                    case R.id.action_mine:
                        index = 2;
                        break;
                }

                // 判断是否需要切换 Fragment
                if (index != lastIndex) {
                    FragmentTransaction transaction = getSupportFragmentManager().beginTransaction();
                    transaction.hide(fragments[lastIndex]);
                    if (!fragments[index].isAdded()) {
                        transaction.add(R.id.container, fragments[index]);
                    }
                    transaction.show(fragments[index]).commit();
                    lastIndex = index;
                }
                return true;
            }
        });

        // 初始化 Fragment
        FragmentTransaction transaction = getSupportFragmentManager().beginTransaction();
        for (int i = 0; i < fragments.length; i++) {
            transaction.add(R.id.container, fragments[i]);
            if (i != 0) {
                transaction.hide(fragments[i]);
            }
        }
        transaction.commit();

        // 设置底部导航栏图标和文字
        for (int i = 0; i < bottomNavigationView.getMenu().size(); i++) {
            bottomNavigationView.getMenu().getItem(i).setIcon(icons[i]);
            bottomNavigationView.getMenu().getItem(i).setTitle(titles[i]);
        }
    }
}

这里使用了 `BottomNavigationView` 控件来作为底部导航栏。在 `onCreate` 方法中，首先初始化了 `BottomNavigationView`，并设置了其点击事件。当点击不同的菜单项时，会根据当前的索引和点击的索引判断是否需要切换 Fragment。在初始化 Fragment 时，将所有 Fragment 都添加到了 `container` 容器中，并隐藏了除第一个 Fragment 外的其他 Fragment。最后，设置了底部导航栏的图标和文字。