将 Java IO 类转换为 Android IO

Question

我想将我的 IO 类从 Eclipse 上的 java 转换为 Android API。出于某种原因，它在 android 上不起作用！它在我的 Println 方法上给了我一个 NullPointerException。此类用于创建、写入、读取和打开文本文件。我的目标是让所有这些方法在 android 上可读。

import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;
import java.io.PrintStream;
import java.io.PrintWriter;
public class IO {
    private static PrintWriter fileOut;
    private static BufferedReader fileIn;

    public static void createOutputFile(String fileName) {
        try {
            fileOut = new PrintWriter(new BufferedWriter(new FileWriter(fileName)));
        } catch (IOException e) {
            System.out.println("*** Cannot create file: " + fileName + " ***");
        }
    }

    public static void openOutputFile(String fileName) {
        try {
            fileOut = new PrintWriter(new BufferedWriter(new FileWriter(fileName, true)));
        } catch (IOException e) {
            System.out.println("*** Cannot open file: " + fileName + " ***");
        }
    }

    public static void openOutputFile2(String fileName) {
        try {
            fileOut = new PrintWriter(new BufferedWriter(new FileWriter(fileName, false)));
        } catch (IOException e) {
            System.out.println("*** Cannot open file: " + fileName + " ***");
        }
    }



    public static void print(String text) {
        fileOut.print(text);
    }

    public static void println(String text) {
        fileOut.println(text);
        //System.out.println(text);
    }

    public static void closeOutputFile() {
        fileOut.close();
    }

    public static void openInputFile(String fileName) {
        try {
            fileIn = new BufferedReader(new FileReader(fileName));
            //System.out.println("opening " + fileName);
        } catch (FileNotFoundException e) {
            System.out.println("***Cannot open " + fileName + "***");
        }
    }

    public static String readLine()
    // throws IOException
    // Note: if there's an error in this method it will return IOException
    {
        try {

            return fileIn.readLine();

        } catch (IOException e) {
            e.printStackTrace();
            return "errors";
        }

    }

    public static void closeInputFile() {// throws IOException
        // Note: if there's an error in this method it will return IOException
        try {
            fileIn.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

Answer 1

如果您在 println 中收到 NullPointerException，那是因为 'fileOut' 为空。

public static void println(String text) {
    fileOut.println(text);
    //System.out.println(text);
}

您实际上是在对第二个错误做出反应，因为您忽略了第一个错误。在您设置fileOut 的所有情况下，您都在吞下（有效隐藏）错误。例如

public static void openOutputFile(String fileName) {
    try {
        fileOut = new PrintWriter(new BufferedWriter(new FileWriter(fileName, true)));
    } catch (IOException e) {
        //Don't do this, it makes debugging much more difficult.
        //Because the root problem is hidden.
        //So now you have 2 problems to solve.
        //And you've thrown away the information that might have
        //helped to solve the problem in the first place.
        System.out.println("*** Cannot open file: " + fileName + " ***");
    }
}

停止隐藏错误，如果出现问题，您需要找出它。因为总是忽略错误会导致以后出现更复杂的错误。

修复你糟糕的异常处理，你将能够找到根本问题（可能是文件未找到或权限错误）。