从MainActivity以外的类隐藏浮动操作按钮

我想在另一个类中隐藏或更改浮动操作按钮,但未能成功。有人能帮我吗?

public class MainActivity extends AppCompatActivity {
    private WebView mywebview;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                // Some onclick action
            }
        });

    }

WebAppInterface。JAVA

public class WebAppInterface {
    Context mContext;

    /** Instantiate the interface and set the context */
    WebAppInterface(Context c) {
        mContext = c;
    }

    /** Show a toast from the web page */
    @JavascriptInterface
    public void showToast(String toast) {
        if (toast=="hide") {
            fab.setVisibility(View.GONE);
        } else if (toast=="change") {
        // update src.Compat command ?
        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
            // new onclick action
            }
    }
    }
}