如何在MainActivity中获取随导航栏切换的多个fragment的java文件的实例

您可以通过在MainActivity中使用FragmentManager来获取随导航栏切换的多个Fragment的实例。以下是一个示例代码：

public class MainActivity extends AppCompatActivity {

    private Fragment1 fragment1;
    private Fragment2 fragment2;

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

        // 获取fragment实例
        fragment1 = new Fragment1();
        fragment2 = new Fragment2();

        // 将fragment添加到FragmentManager中
        FragmentManager fragmentManager = getSupportFragmentManager();
        FragmentTransaction transaction = fragmentManager.beginTransaction();
        transaction.add(R.id.container, fragment1);
        transaction.add(R.id.container, fragment2);
        transaction.hide(fragment1);
        transaction.hide(fragment2);
        transaction.commit();

        // 设置导航栏切换监听
        BottomNavigationView navigation = findViewById(R.id.navigation);
        navigation.setOnNavigationItemSelectedListener(mOnNavigationItemSelectedListener);
    }

    // 导航栏切换监听
    private BottomNavigationView.OnNavigationItemSelectedListener mOnNavigationItemSelectedListener
            = new BottomNavigationView.OnNavigationItemSelectedListener() {

        @Override
        public boolean onNavigationItemSelected(@NonNull MenuItem item) {
            switch (item.getItemId()) {
                case R.id.navigation_fragment1:
                    showFragment(fragment1);
                    return true;
                case R.id.navigation_fragment2:
                    showFragment(fragment2);
                    return true;
            }
            return false;
        }
    };

    // 显示指定的fragment
    private void showFragment(Fragment fragment) {
        FragmentManager fragmentManager = getSupportFragmentManager();
        FragmentTransaction transaction = fragmentManager.beginTransaction();
        // 隐藏所有fragment
        transaction.hide(fragment1);
        transaction.hide(fragment2);
        // 显示指定的fragment
        transaction.show(fragment);
        transaction.commit();
    }
}

在这个示例中，我们创建了两个Fragment实例并将它们添加到FragmentManager中。然后，我们设置导航栏切换监听，当用户点击不同的导航栏选项时，我们调用showFragment方法来显示相应的Fragment。在showFragment方法中，我们使用FragmentManager来隐藏所有Fragment并显示指定的Fragment。