目的： 我有一个Object（自定义类名为Students）的列表，我想更新每个人的出勤率（改变Students类中的Map字段）。所以，我想退出应用程序，当我重新启动它时，我希望所有的学生都是最新的。因此，我们可以说我想将作为（学生）列表或数组的数据保存到文件中，并在应用重新启动时读取它。
尝试： 我的学生班：
public class Students implements Serializable{
private int ID;
private int imageId;
private String firstName;
private String lastName;
private Map<Date, Boolean> Attendance;
public Students(int ID, String firstName, String lastName) {
this.ID = ID;
imageId = -1;//-1 indicates that there is no image
this.firstName = firstName;
this.lastName = lastName;
Attendance = new HashMap<>();
}
public Students(int ID, int imageId, String firstName, String lastName) {
this.ID = ID;
this.imageId = imageId;
this.firstName = firstName;
this.lastName = lastName;
Attendance = new HashMap<>();
}
public void putAttendance(Date d, Boolean b){
Attendance.put(d,b);
}
我的读取/写入数据的活动:(请注意，我创建此活动作为测试活动。因此，如果设备是新的，我想首先有8名学生。所以如果保存文件没有＆＃39;存在，我将把8名学生留在并保存他们）。不过，让我们回到我的代码：
public class studentsAttendance extends AppCompatActivity {
private final String fileName = "data.bin";
private Teachers teacher;
private Students[] students = new Students[8];//sample of 8 students
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_students_attendance);
Bundle extras = getIntent().getExtras();
if (extras != null) {
teacher = (Teachers) getIntent().getSerializableExtra("Teachers"); //Obtaining the teacher
}
TextView tvWelcomeNote = (TextView) findViewById(R.id.tvWelcomeNote);
tvWelcomeNote.setText("Welcome " + teacher.getFirstName() + " " + teacher.getLastName());
File f = new File(this.getFilesDir().toString()+"/"+fileName);
if (!f.exists()) {
Toast.makeText(this, "File doesn't exist!", Toast.LENGTH_SHORT).show();
students = new Students[]{new Students(0, 0, "Barak", "Obama"),
new Students(1, 1, "Donald", "Trump"),
new Students(2, 2, "Sarah", "Franklin"),
new Students(3, 3, "Jack", "Johns"),
new Students(4, 4, "Aston", "Martin"),
new Students(5, 5, "Kaleb", "Philips"),
new Students(6, 6, "John", "Cena"),
new Students(7, 7, "Randy", "Orton")};
for(Students s: students){
this.saveStudent(s);
}
Toast.makeText(this, "Saved!", Toast.LENGTH_SHORT).show();
} else {
Toast.makeText(this, "File Exists", Toast.LENGTH_SHORT).show();
Students[] o = readStudent();
Log.d("", Arrays.deepToString(students));//->giving me [null,null,null.....]
Toast.LENGTH_SHORT).show();Toast.LENGTH_SHORT).show();
}
}
public Boolean saveStudent(Students s) {//saves student into fileName (data.bin)
ObjectOutputStream oos = null;
try {
File file = new File(this.getFilesDir().toString(), fileName);
file.createNewFile();
FileOutputStream fos = this.openFileOutput(fileName, this.MODE_PRIVATE);
oos = new ObjectOutputStream(fos);
oos.writeObject(s);
oos.close();
fos.close();
return true;
} catch (FileNotFoundException err) {
Toast.makeText(this, "Something went wrong while saving", Toast.LENGTH_SHORT).show();
return false;
} catch (Exception abcd) {
Toast.makeText(this, "Something went wrong while saving 2.0", Toast.LENGTH_SHORT).show();
return false;
}
finally {//makes sure to close the ObjectOutputStream
if (oos != null) {
try {
oos.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}
}
public Students[] readStudent() {//reads Student[] object from(data.bin) and returns it.
ObjectInputStream ois = null;
try {
ois = new ObjectInputStream(new FileInputStream(new File(this.getFilesDir().toString()+"/"+fileName)));
Students[] temp = new Students[8];
for(int i=0;i<8;i++){
temp[i]=(Students) ois.readObject();
}
ois.close();
return temp;
} catch (IOException e) {
e.printStackTrace();
return null;
} catch (ClassNotFoundException e) {
e.printStackTrace();
return null;
} finally {
if (ois != null) {
try {
ois.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}
}
}
日志：
05-21 22:46:02.941 11424-11424/? I/art: Not late-enabling -Xcheck:jni (already on)
05-21 22:46:02.941 11424-11424/? W/art: Unexpected CPU variant for X86 using defaults: x86
05-21 22:46:02.955 11424-11431/? I/art: Debugger is no longer active
05-21 22:46:02.955 11424-11431/? I/art: Starting a blocking GC Instrumentation
05-21 22:46:03.029 11424-11424/? W/System: ClassLoader referenced unknown path: /data/app/com.example.mohammadel_ghali.attendance-1/lib/x86
05-21 22:46:03.033 11424-11424/? I/InstantRun: starting instant run server: is main process
05-21 22:46:03.034 11424-11424/? V/InstantRun: Starting server socket listening for package com.example.mohammadel_ghali.attendance on android.net.LocalSocketAddress@6c32846
05-21 22:46:03.034 11424-11424/? V/InstantRun: Started server for package com.example.mohammadel_ghali.attendance
05-21 22:46:03.475 11424-11441/? V/InstantRun: Received connection from IDE: spawning connection thread
05-21 22:46:03.478 11424-11441/? V/InstantRun: Received Ping message from the IDE; returned active = false
05-21 22:46:03.479 11424-11441/? V/InstantRun: Received EOF from the IDE
05-21 22:46:05.289 11424-11424/com.example.mohammadel_ghali.attendance W/art: Before Android 4.1, method android.graphics.PorterDuffColorFilter android.support.graphics.drawable.VectorDrawableCompat.updateTintFilter(android.graphics.PorterDuffColorFilter, android.content.res.ColorStateList, android.graphics.PorterDuff$Mode) would have incorrectly overridden the package-private method in android.graphics.drawable.Drawable
05-21 22:46:05.452 11424-11534/com.example.mohammadel_ghali.attendance I/OpenGLRenderer: Initialized EGL, version 1.4
05-21 22:46:05.452 11424-11534/com.example.mohammadel_ghali.attendance D/OpenGLRenderer: Swap behavior 1
05-21 22:46:05.452 11424-11534/com.example.mohammadel_ghali.attendance W/OpenGLRenderer: Failed to choose config with EGL_SWAP_BEHAVIOR_PRESERVED, retrying without...
05-21 22:46:05.452 11424-11534/com.example.mohammadel_ghali.attendance D/OpenGLRenderer: Swap behavior 0
05-21 22:46:05.536 11424-11424/com.example.mohammadel_ghali.attendance W/art: Before Android 4.1, method int android.support.v7.widget.ListViewCompat.lookForSelectablePosition(int, boolean) would have incorrectly overridden the package-private method in android.widget.ListView
05-21 22:46:11.215 11424-11424/com.example.mohammadel_ghali.attendance W/IInputConnectionWrapper: finishComposingText on inactive InputConnection
05-21 22:46:13.978 11424-11424/com.example.mohammadel_ghali.attendance W/System.err: java.io.EOFException
05-21 22:46:13.978 11424-11424/com.example.mohammadel_ghali.attendance W/System.err: at java.io.ObjectInputStream$BlockDataInputStream.peekByte(ObjectInputStream.java:2604)
05-21 22:46:13.978 11424-11424/com.example.mohammadel_ghali.attendance W/System.err: at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1319)
05-21 22:46:13.978 11424-11424/com.example.mohammadel_ghali.attendance W/System.err: at java.io.ObjectInputStream.readObject(ObjectInputStream.java:373)
05-21 22:46:13.978 11424-11424/com.example.mohammadel_ghali.attendance W/System.err: at com.example.mohammadel_ghali.attendance.studentsAttendance.readStudent(studentsAttendance.java:119)
05-21 22:46:13.978 11424-11424/com.example.mohammadel_ghali.attendance W/System.err: at com.example.mohammadel_ghali.attendance.studentsAttendance.onCreate(studentsAttendance.java:69)
05-21 22:46:13.978 11424-11424/com.example.mohammadel_ghali.attendance W/System.err: at android.app.Activity.performCreate(Activity.java:6679)
05-21 22:46:13.978 11424-11424/com.example.mohammadel_ghali.attendance W/System.err: at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1118)
05-21 22:46:13.978 11424-11424/com.example.mohammadel_ghali.attendance W/System.err: at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2618)
05-21 22:46:13.978 11424-11424/com.example.mohammadel_ghali.attendance W/System.err: at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2726)
05-21 22:46:13.978 11424-11424/com.example.mohammadel_ghali.attendance W/System.err: at android.app.ActivityThread.-wrap12(ActivityThread.java)
05-21 22:46:13.978 11424-11424/com.example.mohammadel_ghali.attendance W/System.err: at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1477)
05-21 22:46:13.978 11424-11424/com.example.mohammadel_ghali.attendance W/System.err: at android.os.Handler.dispatchMessage(Handler.java:102)
05-21 22:46:13.978 11424-11424/com.example.mohammadel_ghali.attendance W/System.err: at android.os.Looper.loop(Looper.java:154)
05-21 22:46:13.978 11424-11424/com.example.mohammadel_ghali.attendance W/System.err: at android.app.ActivityThread.main(ActivityThread.java:6119)
05-21 22:46:13.978 11424-11424/com.example.mohammadel_ghali.attendance W/System.err: at java.lang.reflect.Method.invoke(Native Method)
05-21 22:46:13.978 11424-11424/com.example.mohammadel_ghali.attendance W/System.err: at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886)
05-21 22:46:13.978 11424-11424/com.example.mohammadel_ghali.attendance W/System.err: at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)
[ 05-21 22:46:13.978 11424:11424 D/ ]
[null, null, null, null, null, null, null, null]
05-21 22:46:14.089 11424-11424/com.example.mohammadel_ghali.attendance W/IInputConnectionWrapper: finishComposingText on inactive InputConnection
05-21 22:46:14.089 11424-11424/com.example.mohammadel_ghali.attendance W/IInputConnectionWrapper: finishComposingText on inactive InputConnection
05-21 22:46:14.106 11424-11534/com.example.mohammadel_ghali.attendance D/OpenGLRenderer: endAllActiveAnimators on 0x93e27f00 (RippleDrawable) with handle 0x92d26040
如果你能帮助我解决这个问题，或建议另一种更容易解决这个问题的方法，那就太好了。谢谢。
答案 0 :(得分：0)
每次保存一个学生时，你都会覆盖以前的学生。而是将整个数组保存到文件中，并立即读取所有数组。修改你的`saveStudent'方法如下：
public Boolean saveStudents(Students[] students) {//saves student into fileName (data.bin)
ObjectOutputStream oos = null;
try {
File file = new File(this.getFilesDir().toString(), fileName);
file.createNewFile();
FileOutputStream fos = this.openFileOutput(fileName, this.MODE_PRIVATE);
oos = new ObjectOutputStream(fos);
oos.writeObject(students);
oos.close();
fos.close();
return true;
} catch (FileNotFoundException err) {
Toast.makeText(this, "Something went wrong while saving", Toast.LENGTH_SHORT).show();
return false;
} catch (Exception abcd) {
Toast.makeText(this, "Something went wrong while saving 2.0", Toast.LENGTH_SHORT).show();
return false;
}
finally {//makes sure to close the ObjectOutputStream
if (oos != null) {
try {
oos.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}
}
您的readStudent方法看起来像这样：
public Students[] readStudents() {//reads Student[] object from(data.bin) and returns it.
ObjectInputStream ois = null;
try {
ois = new ObjectInputStream(new FileInputStream(new File(this.getFilesDir().toString()+"/"+fileName)));
Students[] temp = (Students[])ois.readObject();
ois.close();
return temp;
} catch (IOException e) {
e.printStackTrace();
return null;
} catch (ClassNotFoundException e) {
e.printStackTrace();
return null;
} finally {
if (ois != null) {
try {
ois.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}
}
然后你会这样写：
saveStudents(students);
然后像这样读回来：
Students[] o = readStudents();
不要忘记删除该应用以清除您现在拥有的内容。另外，我同意您应该使用本地数据库。