Question

我正在尝试将对象保存在内部存储中，并且可以正常工作，我可以保存数据，检索数据，但是在官方站点上说，仅当您卸载应用程序时，才会删除存储在内部存储中的文件，因此，如果我退出应用程序，然后重新启动它，它应该在该位置，但是当我尝试检索数据时遇到null object reference exception时，我可能做错了。任何人都可以让我回到正确的轨道上。

public class UserData implements Serializable {

public static User user;
static UserData instance=null;

public static UserData getInstance(){
    if( instance == null )
        instance = new UserData();
    return instance;
}
public static void setUser(User usr){
    user = new User(usr.getNume(),usr.getPhone(),usr.getEmail());
}

public static void saveData(UserData instance,Context ctx,User usr){
    setUser(usr);
        String filename = "User.data";
       ObjectOutput out;
        try {
            File outFile = new File(ctx.getFilesDir(), filename);
            out = new ObjectOutputStream(new FileOutputStream(outFile));
            out.writeObject(instance);
            out.close();
        } catch (Exception e) {
            e.printStackTrace();
        }

}

public static UserData loadData(Context ctx){
    ObjectInput in;
    String filename = "User.data";
    UserData ss=null;
    try {
        File outFile = new File(ctx.getFilesDir(), filename);
        in = new ObjectInputStream(new FileInputStream(outFile));
        ss=(UserData) in.readObject();
        in.close();
    } catch (Exception e) {e.printStackTrace();}
    return ss;
}
}

这是User类的实现方式

  public class User {
    private String nume;
    private String phone;
    private String email;

    public User(String nume, String phone, String email) {
        this.nume = nume;
        this.phone = phone;
        this.email = email;
    }

    public String getNume() {
        return nume;
    }

    public String getPhone() {
        return phone;
    }

    public String getEmail() {
        return email;
    }
    }

下面是检索数据的调用，但是包含params的对象是null

 UserData user = UserData.getInstance();
    user.loadData(Login.this);
    if(user.user != null){ // commenting this line will lead to the inner if and accesing user.user.getNume() will throw a null pointer exception
        if(user.user.getNume() != "null"){
            Log.d("STATE", user.user.toString());
            Intent intent = new Intent(Login.this,MenuRest.class);
            this.startActivity(intent);
        }
    }

编辑，如下所示是错误的堆栈跟踪：

E/AndroidRuntime: FATAL EXCEPTION: main
Process: gabriel.androidf, PID: 30318
java.lang.RuntimeException: Unable to start activity ComponentInfo{gabriel.androidf/gabriel.androidf.activities.Login}: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String gabriel.androidf.models.User.getNume()' on a null object reference
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2416)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
    at android.app.ActivityThread.-wrap11(ActivityThread.java)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)
    at android.os.Handler.dispatchMessage(Handler.java:102)
    at android.os.Looper.loop(Looper.java:148)
    at android.app.ActivityThread.main(ActivityThread.java:5417)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
 Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String gabriel.androidf.models.User.getNume()' on a null object reference
    at gabriel.androidf.activities.Login.onCreate(Login.java:42)
    at android.app.Activity.performCreate(Activity.java:6237)
    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1107)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2369)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476) 
    at android.app.ActivityThread.-wrap11(ActivityThread.java) 
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344) 
    at android.os.Handler.dispatchMessage(Handler.java:102) 
    at android.os.Looper.loop(Looper.java:148) 
    at android.app.ActivityThread.main(ActivityThread.java:5417) 
    at java.lang.reflect.Method.invoke(Native Method) 
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) 
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616) 
Application terminated.

Answer 1

使用此代码即可：

public class UserData implements Serializable {

public  User user;
static UserData instance=null;
private static File myfolder;

public static UserData getInstance(){
    if( instance == null )
        instance = new UserData();
    return instance;
}
public  void setUser(User usr){
    user = new User(usr.getNume(),usr.getPhone(),usr.getEmail());
}
public User getUser(){
    return user;
}

public  void saveData(Activity pContext){
    if(myfolder == null){
        myfolder = pContext.getExternalFilesDir(null);
    }
    ObjectOutput out;
    try {
        File outFile = new File(myfolder,
                "someRandom.data");
        out = new ObjectOutputStream(new FileOutputStream(outFile));
        out.writeObject(instance);
        out.close();
    } catch (Exception e) {
        e.printStackTrace();
    }

}

public UserData loadData(Context pContext){
    ObjectInput in;
    if(myfolder == null){
        myfolder = pContext.getExternalFilesDir(null);
    }
    instance = null;
    try {
        FileInputStream fileIn = new FileInputStream(myfolder.getPath() + File.separator + "someRandom.data");
        in = new ObjectInputStream(fileIn);
        instance = (UserData) in.readObject();
        in.close();
    } catch (Exception e) {
        e.printStackTrace();
    }
    return instance;
}
}

以及通过使User类实现可序列化来进行设置

从内部存储中检索数据

1 个答案: