java - 如何从 Android 应用程序中的另一个类调用非静态方法

Question

所以我试图在 Android 应用程序中点击某个项目后调用一个方法。

这是我要调用的方法 getFavourites();

public class Favourites extends Activity {
    public void getFavourites(){
        ViewFlipper viewFlipper = (ViewFlipper) findViewById(R.id.viewFlipper);
        viewFlipper.setDisplayedChild(0);
        setTitle("Favourites");
    }
}

这里是我要调用方法getFavourtites();

 public boolean onNavigationItemSelected(MenuItem item) {
        // Handle navigation view item clicks here.
        int id = item.getItemId();

    if (id == R.id.nav_favourite) {
        Favourites getFav = new Favourites();
        getFav.getFavourites();
    } else if (id == R.id.nav_search) {
        ViewFlipper viewFlipper = (ViewFlipper) findViewById(R.id.viewFlipper);
        viewFlipper.setDisplayedChild(1);
        setTitle("Search");
    } else if (id == R.id.nav_share) {

    } else if (id == R.id.nav_sendFeedback) {

    }

    DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
    drawer.closeDrawer(GravityCompat.START);
    return true;
}

我真的不知道我做错了什么，但每次运行它时，应用程序都会崩溃。

编辑:这是 Collection 夹的完整堆栈

package com.ctt.mrdan.croydontramtime2;

import android.app.Activity;
import android.widget.ViewFlipper;

/**
 * Created by MrDan on 16/07/2016.
 */
public class Favourites extends Activity {
    public void getFavourites(){
        ViewFlipper viewFlipper = (ViewFlipper) findViewById(R.id.viewFlipper);
        viewFlipper.setDisplayedChild(0);
        setTitle("Favourites");
    }
}

这是主程序的完整堆栈

package com.ctt.mrdan.croydontramtime2;

import android.content.Intent;
import android.os.Bundle;
import android.support.design.widget.FloatingActionButton;
import android.support.design.widget.Snackbar;
import android.view.View;
import android.support.design.widget.NavigationView;
import android.support.v4.view.GravityCompat;
import android.support.v4.widget.DrawerLayout;
import android.support.v7.app.ActionBarDrawerToggle;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.ViewFlipper;

public class MainActivity extends AppCompatActivity
        implements NavigationView.OnNavigationItemSelectedListener {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

        FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                        .setAction("Action", null).show();
            }
        });

        DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
        ActionBarDrawerToggle toggle = new ActionBarDrawerToggle(
                this, drawer, toolbar, R.string.navigation_drawer_open, R.string.navigation_drawer_close);
        drawer.setDrawerListener(toggle);
        toggle.syncState();

        NavigationView navigationView = (NavigationView) findViewById(R.id.nav_view);
        navigationView.setNavigationItemSelectedListener(this);
    }

    @Override
    public void onBackPressed() {
        DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
        if (drawer.isDrawerOpen(GravityCompat.START)) {
            drawer.closeDrawer(GravityCompat.START);
        } else {
            super.onBackPressed();
        }
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }

    @SuppressWarnings("StatementWithEmptyBody")
    @Override
    public boolean onNavigationItemSelected(MenuItem item) {
        // Handle navigation view item clicks here.
        int id = item.getItemId();

        if (id == R.id.nav_favourite) {
            Favourites getFav = new Favourites();
            getFav.getFavourites();
        } else if (id == R.id.nav_search) {
            ViewFlipper viewFlipper = (ViewFlipper) findViewById(R.id.viewFlipper);
            viewFlipper.setDisplayedChild(1);
            setTitle("Search");
        } else if (id == R.id.nav_share) {

        } else if (id == R.id.nav_sendFeedback) {

        }

        DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
        drawer.closeDrawer(GravityCompat.START);
        return true;
    }
}

Answer 1

您正在实例化 Favourites，然后在新实例上调用 getFavourites()。 getFavourites() 调用 findViewById()，后者隐式调用 getWindow().findViewById()，但 Activity没有内容 View ，导致 NullPointerException。

Collection 夹根本没有任何目的。

您可以将代码从 getFavourites() 移动到 onNavigationItemSelected():

viewFlipper = (ViewFlipper) findViewById(R.id.viewFlipper);

// ...

if (id == R.id.nav_favourite) {
    viewFlipper.setDisplayedChild(0);
    setTitle("Favourites");
} else if (id == R.id.nav_search) {
    viewFlipper.setDisplayedChild(1);
    setTitle("Search");
}

此外，您永远不要使用 new 关键字实例化 Activity。使用 Intent 并将实例化留给 ActivityManager。