try / catch语句的自定义类，NPE内不工作 [英] Try/Catch not working inside a custom class, NPE
问题描述
非常简单。我有一个大类我的主要项目，并成为笨拙。所以今天我决定尝试让我的自己的类，这样我就可以简化在某些 code 大类。
所以这是我的小类，我用了数据存储。在的try / catch 语句的工作时，浮动本身在大类。虽然在小类，它抛出一个NPE。看来，当我让我的自己的类中的try / catch系统无法工作。为什么有什么想法？
file_in = openFileInput（array_saved）;
是logcat的叫了一声。
...这是类：
包com.eai.thepicker;进口java.io.FileInputStream中;
进口java.io.FileNotFoundException;
进口java.io.FileOutputStream中;
进口java.io.IOException异常;
进口java.io.ObjectInputStream中;
进口java.io.ObjectOutputStream中;
进口的java.util.ArrayList;
进口android.app.Activity;
公共类DataHandler的延伸活动{ FileOutputStream中file_out;
的FileInputStream file_in;
ObjectOutputStream的obj_out;
ObjectInputStream的obj_in;
ArrayList的＆LT;串GT; retrieved_data; 公众的DataHandler（）{
} @燮pressWarnings（未登记）
公众的ArrayList＆LT;串GT; retrieveData（）{ 尝试{
file_in = openFileInput（array_saved）;
obj_in =新的ObjectInputStream（obj_in）;
如果（obj_in.available（）大于0）{
retrieved_data =（ArrayList的＆LT;串GT;）obj_in.readObject（）;
}
其他{
retrieved_data =新的ArrayList＆LT;串GT;（）;
} obj_in.close（）;
file_in.close（）;
}赶上（FileNotFoundException异常五）{
e.printStackTrace（）;
}赶上（IOException异常五）{
e.printStackTrace（）;
}赶上（ClassNotFoundException的E）{
e.printStackTrace（）;
} 返回retrieved_data; } 公共无效SAVEDATA（ArrayList的＆LT;串GT; DATA_OUT）{
尝试{
file_out = openFileOutput（array_saved，0）;
obj_out =新的ObjectOutputStream（file_out）;
obj_out.writeObject（DATA_OUT）;
obj_out.close（）;
file_out.close（）;
}赶上（FileNotFoundException异常五）{
e.printStackTrace（）;
}赶上（IOException异常五）{
e.printStackTrace（）;
}
}
}
我认为问题是，你试图调用此方法
openFileInput（）
没有给它背景它需要。我仍然不认为你需要延长活动而是通过去除使这个普通班延伸活动。创建这个类的一个实例，当你需要它在调用活动像
DataHandler的数据=的DataHandler（本）; //提供上下文
data.retrieveData（）;
类似的东西要调用的方法。然后创建像
类的构造 ublic类DataHandler的延伸活动{FileOutputStream中file_out;
的FileInputStream file_in;
ObjectOutputStream的obj_out;
ObjectInputStream的obj_in;
ArrayList的＆LT;串GT; retrieved_data;
上下文mContext; //添加上下文变量公众的DataHandler（上下文的背景下）{
mContext =背景;分配上下文
}
然后在你的方法使用
公开的ArrayList＆LT;串GT; retrieveData（）{ 尝试{
file_in = mContext.openFileInput（array_saved）;
obj_in =新的ObjectInputStream（obj_in）;
如果（obj_in.available（）大于0）{
retrieved_data =（ArrayList的＆LT;串GT;）obj_in.readObject（）;
}
其他{
retrieved_data =新的ArrayList＆LT;串GT;（）;
} obj_in.close（）;
file_in.close（）;
}赶上（FileNotFoundException异常五）{
e.printStackTrace（）;
}赶上（IOException异常五）{
e.printStackTrace（）;
}赶上（ClassNotFoundException的E）{
e.printStackTrace（）;
} 返回retrieved_data;}
我相信这样的事情应该有所帮助。该方法 openFileInput（）期待一个上下文是空在这一点上，因此 NPE 。传递一个上下文的类可以解决这个问题，因为它会产生电流活动上下文
<一个href=\"http://developer.android.com/reference/android/content/Context.html#openFileInput%28java.lang.String%29\"相对=nofollow>文档
Fairly simple. I have a BIG class for my main project, and it became "unwieldy". SO today I decided to try making my own class, so I could simplify some of the code in the big class.
So this is my "little" class, I used for the Data storage. The TRY/CATCH statement works when floating by itself in the BIG class. In the little class though, it throws an NPE. It seems the Try/Catch system isn't working when I make my own class. Any thoughts on why?
file_in = openFileInput("array_saved");
Is called out by Logcat.
...and this is the class:
package com.eai.thepicker;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.util.ArrayList;
import android.app.Activity;
public class DataHandler extends Activity {
FileOutputStream file_out;
FileInputStream file_in;
ObjectOutputStream obj_out;
ObjectInputStream obj_in;
ArrayList<String> retrieved_data;
public DataHandler(){
}
@SuppressWarnings("unchecked")
public ArrayList<String> retrieveData(){
try {
file_in = openFileInput("array_saved");
obj_in = new ObjectInputStream(obj_in);
if(obj_in.available() > 0){
retrieved_data = (ArrayList<String>) obj_in.readObject();
}
else{
retrieved_data = new ArrayList<String>();
}
obj_in.close();
file_in.close();
} catch (FileNotFoundException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
} catch (ClassNotFoundException e) {
e.printStackTrace();
}
return retrieved_data;
}
public void saveData(ArrayList<String> data_out){
try {
file_out = openFileOutput("array_saved", 0);
obj_out = new ObjectOutputStream(file_out);
obj_out.writeObject(data_out);
obj_out.close();
file_out.close();
} catch (FileNotFoundException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
}
}
I believe the problem is that you are trying to call this method
openFileInput()
without giving it context which it needs. I still don't think you need to extend Activity but make this a regular class by removing extends Activity. Create an instance of this class when you need it in the calling Activity like
DataHandler data = DataHandler (this); // give context
data.retrieveData();
something like that to call the method. Then create a constructor in the class like
ublic class DataHandler extends Activity {
FileOutputStream file_out;
FileInputStream file_in;
ObjectOutputStream obj_out;
ObjectInputStream obj_in;
ArrayList<String> retrieved_data;
Context mContext; // add Context variable
public DataHandler(Context context){
mContext = context; assign context
}
Then in your method use
public ArrayList<String> retrieveData(){
try {
file_in = mContext.openFileInput("array_saved");
obj_in = new ObjectInputStream(obj_in);
if(obj_in.available() > 0){
retrieved_data = (ArrayList<String>) obj_in.readObject();
}
else{
retrieved_data = new ArrayList<String>();
}
obj_in.close();
file_in.close();
} catch (FileNotFoundException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
} catch (ClassNotFoundException e) {
e.printStackTrace();
}
return retrieved_data;
}
I believe something like this should help. The method openFileInput() is expecting a Context which is null at this point, hence the NPE. Passing a Context to the class will solve this problem as it will have the current Activity Context
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