java退出应用程序导致无限循环

Question

i have made a simple java app to print a text file then should terminate. I tried to use the return statemend but didn't work at all. So i used the system.exit(0) but instead of exit the app, it goes in an infinite loop... this is my code: can someone tell me what is wrong ?

package com.example.testprinterdemo;

import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.os.Handler;

import com.pda3505.Service.CaptureService;
import com.pda3505.printer.PrinterClassSerialPort;

import java.io.BufferedReader;
import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.IOException;


public class MainActivity extends Activity {

    public static PrinterClassSerialPort printerClass = null;

    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        init();

        printerClass.write(new byte[] { 0x1d, 0x21,0x00,0x00});
        printerClass.write(new byte[] { 0x1b, 0x26, 0x00 });


        BufferedReader reader = null;
        try {reader = new BufferedReader(new FileReader("/mnt/sdcard/temp.txt"));} 
        catch (FileNotFoundException e) {e.printStackTrace();}
        String line = null;
        try {line = reader.readLine();} 
        catch (IOException e) {e.printStackTrace();}
        while(line!=null) {
            printerClass.printText(line);
            printerClass.printText("\n");
            try {line = reader.readLine();} 
            catch (IOException e) {e.printStackTrace();}
        }
        system.exit(0);
    }


    private void init() {       

        printerClass = new PrinterClassSerialPort(new Handler());
        printerClass.open(this);

        Intent newIntent = new Intent(MainActivity.this, CaptureService.class);
        newIntent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
        startService(newIntent);

    }

}

Answer 1

    try{
        //never hard code the file path in Android.
        String filePath = Environment.getExternalStorageDirectory().getAbsolutePath()+File.separator+"temp.txt";
        BufferedReader reader = new BufferedReader(new FileReader(filePath));
        while (true){
            String line = reader.readLine();
            if(line.length() > 0){
                printerClass.printText(line);
                printerClass.printText("\n");
            }else{
                break;
                //you can use return too
            }
        }
    }catch (IOException e){
        e.printStackTrace();
    }

In your approach, line is never null, it is either "" or it is " " or "\\n" etc, but never null. so listen for length and break the loop

Please avoid multiple try catch blocks when you can have your code under single try block. And please scope the variables properly, do not declare variables outside the scope where it is not used, as it causes problems for JAVA Garbage collector/