Question

我有一个类Logger，它使用3个数组作为xhared变量数组在构造函数中初始化但是当在课程的任何其他方法中访问它们时，我得到了一个的NullPointerException。

我需要知道原因和解决方案。

请参阅代码中的注释。

file Logger.java
package logger_010.standard;

import java.io.FileOutputStream;
import java.io.PrintStream;

public class Logger {

    // declaration
    private FileOutputStream[] files;
    private PrintStream[] pss;
    private String[] messages;

    public Logger() {
        // initialisation
        try {

            FileOutputStream[] files = { 
                    new FileOutputStream("G:\\Users\\TarekEZZAT\\Documents\\logs\\logger0.log"),
                    new FileOutputStream("G:\\Users\\TarekEZZAT\\Documents\\logs\\logger1.log"), 
                    new FileOutputStream("G:\\Users\\TarekEZZAT\\Documents\\logs\\logger2.log"), 
                    };

            PrintStream[] pss = { 
                    new PrintStream(files[0]), 
                    new PrintStream(files[1]), 
                    new PrintStream(files[2]),

            };

            String[] messages = {
                    new String ("Write error message to log file 0"),
                    new String ("Write error message to log file 1 + user"),
                    new String ("Write error message to log file 2 + user+ email"),
            };

            // Arrays instanciation is OK
            System.out.println(files[0].toString());
            System.out.println(files[1].toString());
            System.out.println(files[2].toString());
            System.out.println(pss[0].toString());
            System.out.println(pss[1].toString());
            System.out.println(pss[2].toString());
            System.out.println(messages[0].toString());
            System.out.println(messages[1].toString());
            System.out.println(messages[2].toString());
            System.out.println("++++++++++++");

        } catch (Exception e) {
            System.out.println("Exception " + e.getMessage());

        } finally {

        }
    }

    public void LogMessage(int level) {

        // Here I get a Null pointer exception

        System.out.println(files[0].toString());
        System.out.println(files[1].toString());
        System.out.println(files[2].toString());
        System.out.println(pss[0].toString());
        System.out.println(pss[1].toString());
        System.out.println(pss[2].toString());
        System.out.println(messages[0].toString());
        System.out.println(messages[1].toString());
        System.out.println(messages[2].toString());
        System.out.println("++++++++++++");

        // PrintStream[] files = OpenFiles();
        WriteLogMessage(this.getPss(), level);
        CloseFiles(pss);

    }

    private void CloseFiles(PrintStream[] pss2) {
        // TODO Auto-generated method stub

    }

    private PrintStream[] OpenFiles() {
        // TODO Auto-generated method stub
        return null;
    }

    private void WriteLogMessage(PrintStream[] files, int level) {

        this.getPss()[level].println(messages[level]);
        this.getPss()[level].flush();

    }

    public FileOutputStream[] getFiles() {
        return files;
    }

    public void setFiles(FileOutputStream[] files) {
        this.files = files;
    }

    public PrintStream[] getPss() {
        return pss;
    }

    public void setPss(PrintStream[] pss) {
        this.pss = pss;
    }

    public String[] getMessages() {
        return messages;
    }

    public void setMessages(String[] messages) {
        this.messages = messages;
    }

}

这是包含主要功能的文件

package logger_010.standard;

public class Start {

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        Logger l = new Logger();
        for (int i = 0; i < 15; i++) {
            int level = i % 2;
            l.LogMessage(level);
        }
    }

}

Answer 1

您在构造函数中声明了一个新文件，message，pss变量，而不是使用已经创建的class =＆gt;的变量;在LogMessage方法中使用时，它使用变量not init =＆gt;导致错误

Answer 2

您永远不会将您的类属性绑定到您在构造函数中定义的对象。

通过定义FileOutputStream[] files = ...而不是files = ...，这是您的对象属性，您只是创建一个局部变量，其范围仅在构造函数内。

你的构造函数应该是：

public Logger() {
    // initialisation
    try {

        files = { 
                new FileOutputStream("G:\\Users\\TarekEZZAT\\Documents\\logs\\logger0.log"),
                new FileOutputStream("G:\\Users\\TarekEZZAT\\Documents\\logs\\logger1.log"), 
                new FileOutputStream("G:\\Users\\TarekEZZAT\\Documents\\logs\\logger2.log"), 
                };

         pss = { 
                new PrintStream(files[0]), 
                new PrintStream(files[1]), 
                new PrintStream(files[2]),

        };

         messages = {
                new String ("Write error message to log file 0"),
                new String ("Write error message to log file 1 + user"),
                new String ("Write error message to log file 2 + user+ email"),
        };

        // Arrays instanciation is OK
        System.out.println(files[0].toString());
        System.out.println(files[1].toString());
        System.out.println(files[2].toString());
        System.out.println(pss[0].toString());
        System.out.println(pss[1].toString());
        System.out.println(pss[2].toString());
        System.out.println(messages[0].toString());
        System.out.println(messages[1].toString());
        System.out.println(messages[2].toString());
        System.out.println("++++++++++++");

    } catch (Exception e) {
        System.out.println("Exception " + e.getMessage());

    } finally {

    }
}

Answer 3

如果我使用FileOutputStream的子类和抛出FileNotFoundException的构造函数初始化FileOutputStream数组，我会收到此编译错误

＆＃34;默认构造函数无法处理隐式超级构造函数抛出的异常类型FileNotFoundException。必须定义一个显式的构造函数＆＃34;。

我终于使用函数（makeFileOutputStream）解决了这个问题，这个函数在try / catch块中调用FileOutputStream构造函数

这是我的Blogger课程的代码

import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.PrintStream;

public class Logger {

    // declarations

    private FileOutputStream[] files = { 
            makeFileOutputStream("G:\\Users\\TarekEZZAT\\Documents\\logs\\logger0.log"),
            makeFileOutputStream("G:\\Users\\TarekEZZAT\\Documents\\logs\\logger1.log"),
            makeFileOutputStream("G:\\Users\\TarekEZZAT\\Documents\\logs\\logger2.log"), 
            };

    private PrintStream[] pss = { 
            new PrintStream(files[0]), 
            new PrintStream(files[1]), 
            new PrintStream(files[2]),
    };

    private String[] messages = { 
            new String("Write error message to log file 0"),
            new String("Write error message to log file 1 + user"),
            new String("Write error message to log file 2 + user+ email"), 
            };





    private FileOutputStream makeFileOutputStream(String string) {
        // TODO Auto-generated method stub
        FileOutputStream fos = null;
        try {
            fos = new FileOutputStream(string);
        } catch (FileNotFoundException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

        return fos;
    }


    public void LogMessage(int level) {

        WriteLogMessage(this.getPss(), level);

    }


    public void CloseFile(PrintStream[] files, int level){
        // TODO Auto-generated method stub
        this.getPss()[level].close();

    }

    private void WriteLogMessage(PrintStream[] files, int level) {

        this.getPss()[level].println(messages[level]);
        this.getPss()[level].flush();
    }

    // Getters and Setters
    public FileOutputStream[] getFiles() {
        return files;
    }

    public void setFiles(FileOutputStream[] files) {
        this.files = files;
    }

    public PrintStream[] getPss() {
        return pss;
    }

    public void setPss(PrintStream[] pss) {
        this.pss = pss;
    }

    public String[] getMessages() {
        return messages;
    }

    public void setMessages(String[] messages) {
        this.messages = messages;
    }

}

Java数组的共享对象初始化错误

3 个答案: