[Java] Remember to add the object to the file in java – Append object to file exist in java

December 17, 2014 nguyenvanquan7826 EN Java 6 responses


After all Read the objects from the file in java, there are many issues that you need to write more data on file. Usually we just declare

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FileOutputStream f = new FileOutputStream(fileName,true);

But after many attempts I still do not. Search for a while, apart from the extra stuff 2 such is true, then we need to override the method writeStreamHeader as follows:

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ObjectOutputStream oStream = new ObjectOutputStream(f) {
    protected void writeStreamHeader() throws IOException {
        reset();
    }
};

1. Quick Code to test

code by nguyenvanquan7826 - ReadWriteObject.java
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package vietSource.net.IOFile;
 
import java.io.File;
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.io.Serializable;
 
public class ReadWriteObject {
 
    public static void main(String[] args) {
 
        File f = new File("student.dat");
        FileOutputStream fo;
        ObjectOutputStream oStream = null;
 
        // ghi file lan 1
        try {
            fo = new FileOutputStream(f);
            oStream = new ObjectOutputStream(fo);
 
            oStream.writeObject(new MyStudent("pham ha", 22));
            oStream.close();
        } catch (FileNotFoundException e1) {
            e1.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
 
        // ghi file lan 2
        try {
            fo = new FileOutputStream(f, true);
            oStream = new ObjectOutputStream(fo) {
                protected void writeStreamHeader() throws IOException {
                    reset();
                }
            };
            oStream.writeObject(new MyStudent("nguyenvanquan7826", 22));
            oStream.close();
        } catch (FileNotFoundException e1) {
            e1.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
 
        // doc file
        try {
            FileInputStream fis = new FileInputStream(f);
            ObjectInputStream inStream = new ObjectInputStream(fis);
 
            System.out.println(inStream.readObject());
            System.out.println(inStream.readObject());
 
            inStream.close();
        } catch (ClassNotFoundException e) {
            System.out.println("Class not found");
            e.printStackTrace();
        } catch (IOException e) {
            System.out.println("Error Read file");
            e.printStackTrace();
        }
    }
}
 
class MyStudent implements Serializable {
    String name;
    int age;
 
    public MyStudent(String name, int age) {
        super();
        this.name = name;
        this.age = age;
    }
 
    public String toString() {
        return "MyStudent [name=" + name + ", age=" + age + "]";
    }
}

2. Complete code should be used

Do not exist if no object file or files will be added error, therefore to ensure that all cases you see the following code:

code by nguyenvanquan7826 - ReadWriteObject.java
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package vietSource.net.IOFile;
 
import java.io.File;
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.io.Serializable;
 
public class ReadWriteObject {
 
    // kiem tra file co object hay khong
    public static boolean hasObject(File f) {
        // thu doc xem co object nao chua
        FileInputStream fi;
        boolean check = true;
        try {
            fi = new FileInputStream(f);
            ObjectInputStream inStream = new ObjectInputStream(fi);
            if (inStream.readObject() == null) {
                check = false;
            }
            inStream.close();
 
        } catch (FileNotFoundException e) {
            check = false;
        } catch (IOException e) {
            check = false;
        } catch (ClassNotFoundException e) {
            check = false;
            e.printStackTrace();
        }
        return check;
    }
 
    public static void write(MyStudent s) {
        try {
 
            File f = new File("student.dat");
            FileOutputStream fo;
            ObjectOutputStream oStream = null;
 
            // neu file chu ton tai thi tao file va ghi binh thuong
            if (!f.exists()) {
                fo = new FileOutputStream(f);
                oStream = new ObjectOutputStream(fo);
            } else { // neu file ton tai
 
                // neu chua co thi ghi binh thuong
                if (!hasObject(f)) {
                    fo = new FileOutputStream(f);
                    oStream = new ObjectOutputStream(fo);
                } else { // neu co roi thi ghi them vao
 
                    fo = new FileOutputStream(f, true);
 
                    oStream = new ObjectOutputStream(fo) {
                        protected void writeStreamHeader() throws IOException {
                            reset();
                        }
                    };
                }
            }
 
            oStream.writeObject(s);
            oStream.close();
 
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
 
    public static void read() {
        try {
            File f = new File("student.dat");
            FileInputStream fis = new FileInputStream(f);
            ObjectInputStream inStream = new ObjectInputStream(fis);
            Object s;
            int i = 0;
            while (true) {
                s = inStream.readObject();
                System.out.println(++i + ":" + s.toString());
            }
        } catch (ClassNotFoundException e) {
        } catch (IOException e) {
        }
    }
 
    public static void main(String[] args) {
 
        // ghi sinh vien 1
        write(new MyStudent("nguyenvanquan7826", 22));
 
        // ghi tiep sinh vien 2
        write(new MyStudent("phamha", 22));
 
        // doc file
        read();
 
    }
}
 
class MyStudent implements Serializable {
    String name;
    int age;
 
    public MyStudent(String name, int age) {
        super();
        this.name = name;
        this.age = age;
    }
 
    public String toString() {
        return "MyStudent [name=" + name + ", age=" + age + "]";
    }
}