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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。