java 中错误 attempt to invoke virtual method on a null object reference-ag捕鱼王app官网

本篇文章介绍如何解决 java 中的 attempt to invoke virtual method on a null object reference 错误。

java 中 attempt to invoke virtual method on a null object reference 错误

在处理 android 应用程序时，可能会出现错误“attempt to invoke virtual method on a null object reference”，这是一种 nullpointerexception 类型。 每当我们尝试使用 null 对象调用方法时，它都会抛出此错误。

这是一个基于android平台的系统，当尝试在空对象引用描述上调用虚拟方法时，会抛出nullpointerexception。

此示例具有三个不同的 java 类。

main_player.java：

package jiyik;
import javax.naming.context;
public class main_player {
    private context appcontext;
    private sharedpreferences sharedpreferencessettingsu;
    private sharedpreferences.editor preferenceseditoru;
    private static final int preference_mode = 0;
    private static final string unique_preferences_file = "demoapp";
    public player(context appcontext, string player_name) {
        this.appcontext = appcontext;
        save_name(player_name);
    }
    public void save_name(string nvalue) {
        sharedpreferencessettingsu = appcontext.getsharedpreferences(unique_preferences_file, preference_mode);
        preferenceseditoru = sharedpreferencessettingsu.edit();
        preferenceseditoru.putstring("keyname", nvalue);
        preferenceseditoru.commit();
    }
    public string get_name(context democontext) {
        sharedpreferencessettingsu = democontext.getsharedpreferences(unique_preferences_file, preference_mode);
        string playername = sharedpreferencessettingsu.getstring("keyname", "anonymous");
        return playername;
    }
}

player_name.java:

package jiyik;
import javax.naming.context;
public class player_name extends activity {
private edittext player_name;
private player m_player;
public static context appcontext;
@override
protected void oncreate(bundle savedinstancestate) {
    super.oncreate(savedinstancestate);
    setcontentview(r.layout.player_name);
    appcontext = this;
    player_name = (edittext) findviewbyid (r.id.etname);
}
public void onc_confirm(view btnclick) {
    m_player = new player(appcontext, string.valueof(player_name.gettext()));
    //m_player.savename();
    intent intent = new intent(player_name.this, game_play.class);
    startactivity(intent);
}
public void onc_testshpref(view btnclick) {
    intent intent = new intent(player_name.this, game_play.class);
    startactivity(intent);
}

game_play.java:

public class game_play extends activity {
    private textview welcome_player;
    private listview games_created;
    private player m_player;
    @override
    protected void oncreate(bundle savedinstancestate) {
        super.oncreate(savedinstancestate);
        setcontentview(r.layout.play_game);
        welcome_player = (textview) findviewbyid (r.id.tvplayer_name);
        welcome_player.settext("welcome back player, "   string.valueof(m_player.getname(this))   " !");
        games_created = (listview) findviewbyid (r.id.listcreatedgames);
        games_created.setemptyview(findviewbyid (r.id.tvnogames));
    }
}

我们在android平台上运行上述系统，会抛出java.lang.nullpointerexception: attempt to invoke virtual method导致的错误。

caused by: java.lang.nullpointerexception: attempt to invoke virtual method 'java.lang.string plp.cs4b.thesis.drawitapp.main_player.getname()' on a null object reference
    at plp.cs4b.thesis.drawitapp.game_play.oncreate(game_play.java:20)
    at android.app.activity.performcreate(activity.java:5933)
    at android.app.instrumentation.callactivityoncreate(instrumentation.java:1105)
    at android.app.activitythread.performlaunchactivity(activitythread.java:2251)
    ... 10 more

该错误发生在 game_play.java 类中的以下代码行中。

welcome_player.settext("welcome back player, "   string.valueof(m_player.getname(this))   " !");

出现此错误是因为 m_player 为 null。 毕竟还没有初始化。 通过初始化，我们可以解决这个问题。

查看ag捕鱼王app官网的解决方案：

game_play.java:

package jiyik;
public class game_play extends activity {
    private textview welcome_player;
    private listview games_created;
    //initialize the m_player
    private player m_player = new main_player(appcontext,"");
    @override
    protected void oncreate(bundle savedinstancestate) {
        super.oncreate(savedinstancestate);
        setcontentview(r.layout.play_game);
        welcome_player = (textview) findviewbyid (r.id.tvplayer_name);
        welcome_player.settext("welcome back player, "   string.valueof(m_player.getname(this))   " !");
        games_created = (listview) findviewbyid (r.id.listcreatedgames);
        games_created.setemptyview(findviewbyid (r.id.tvnogames));
    }
}

因此，每当在这样的系统上工作时，您必须确保方法没有调用任何空值； 否则，会抛出此异常。