Java array of shared objects initialization error

Question

I have a class Logger that uses 3 arraysas xhared variables The arrays are initialized in the contructor but when accessing them in any other method of the class, I get a NullPointerException.

I need to know the reason and the solution.

Please see comments in the code.

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;
    }

}

this is the file containing the main function

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);
        }
    }

}

I solved this. We don't need an explicit constructor, because we get this error in the déclaration; see here stackmirror.cn/page/rhajqg5g6983 — Tarek EZZAT
– Tarek EZZAT, Commented Aug 6, 2017 at 23:10

TuyenNTA · Accepted Answer · 2017-08-06 00:48:47Z

1

You are declare a new files, message, pss variable inside constructor instead of using the variable already created of class => when using in the LogMessage method, it use the variable not init => cause the error

answered Aug 6, 2017 at 0:48

TuyenNTA

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Comments

TheBakker · Accepted Answer · 2017-08-06 00:49:16Z

You never actualy bind your class attribut with the object you define in your constructor.

By defining FileOutputStream[] files = ... instead of files = ..., which is your object attribut, you are just making a local variable whose scope is only inside the constructor.

Your constructor should be :

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 {

    }
}

Tarek EZZAT · Accepted Answer · 2017-08-06 23:21:06Z

If I initialize the FileOutputStream array with a sub class of FileOutputStream and a constructor that throws a FileNotFoundException I get this compilation error

"Default constructor cannot handle exception type FileNotFoundException thrown by implicit super constructor. Must define an explicit constructor".

I finally solved the problem by using a function (makeFileOutputStream) and this function call the FileOutputStream constructor in a try/catch block

here is the code for my class Blogger

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;
    }

}

Thanks to Tuyen Nguyen and TheBakker who put me on the right track

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