Serialize

saveVar_.idea_misc.xml

<?xml version="1.0" encoding="UTF-8"?>
<project version="4">
  <component name="ProjectRootManager" version="2" languageLevel="JDK_1_8" default="true" project-jdk-name="1.8" project-jdk-type="JavaSDK">
    <output url="file://$PROJECT_DIR$/out" />
  </component>
</project>

saveVar_.idea_modules.xml

<?xml version="1.0" encoding="UTF-8"?>
<project version="4">
  <component name="ProjectModuleManager">
    <modules>
      <module fileurl="file://$PROJECT_DIR$/saveVar.iml" filepath="$PROJECT_DIR$/saveVar.iml" />
    </modules>
  </component>
</project>

saveVar_saveVar.iml

<?xml version="1.0" encoding="UTF-8"?>
<module type="JAVA_MODULE" version="4">
  <component name="NewModuleRootManager" inherit-compiler-output="true">
    <exclude-output />
    <content url="file://$MODULE_DIR$">
      <sourceFolder url="file://$MODULE_DIR$/src" isTestSource="false" />
    </content>
    <orderEntry type="inheritedJdk" />
    <orderEntry type="sourceFolder" forTests="false" />
  </component>
</module>

saveVar_src_File

s=Hello;i=10;

saveVar_src_Run.java

import java.io.IOException;

public class Run {
    public static void main (String[] args) throws IllegalAccessException, IOException, NoSuchFieldException, InstantiationException {
        Saver saver = new Saver();
        Serialize ser = new Serialize();
        ToFileFromFile toFileFromFile = new ToFileFromFile();

        saver.setS("Hello");
        saver.setI(10);
        saver.setD(50.3);

        toFileFromFile.save(ser.serialize(saver));

        String fromFile = toFileFromFile.read();
        ser.deserialize(fromFile, saver.getClass());

        
    }
}

saveVar_src_Save.java

import java.lang.annotation.*;

@Target(value = ElementType.FIELD)
@Retention(value = RetentionPolicy.RUNTIME)
public @interface Save {
}

saveVar_src_Saver.java

public class Saver {

    @Save
    public String s;
    @Save
    private int i;
    public double d;

    public void setS(String s) {
        this.s = s;
    }

    public void setI(int i) {
        this.i = i;
    }

    public void setD(double d) {
        this.d = d;
    }

    public String getS() {
        return s;
    }

    public int getI() {
        return i;
    }

    public double getD() {
        return d;
    }
}

saveVar_src_Serialize.java

import java.io.FileReader;
import java.lang.reflect.Field;
import java.lang.reflect.Modifier;
import java.security.InvalidParameterException;

public class Serialize {
    public static String serialize (Object o) throws IllegalAccessException {
        Class<?> cls = o.getClass();
        StringBuilder sb = new StringBuilder();

        Field[] fields = cls.getDeclaredFields();

        for (Field field : fields) {
            if (!field.isAnnotationPresent(Save.class)) {
                continue;
            }
            if (Modifier.isPrivate(field.getModifiers())) {
                field.setAccessible(true);
            }
            sb.append(field.getName() + "=");

            if (field.getType()==int.class) {
                sb.append(field.getInt(o));
            } else {
                if (field.getType() == String.class) {
                    sb.append((String) field.get(o));
                } else {
                    if (field.getType() == long.class) {
                        sb.append(field.getDouble(o));
                    }
                }
            }
            sb.append(";");
        }
        return sb.toString();
    }

    public static <T> T deserialize (String s, Class<T> cls) throws IllegalAccessException, InstantiationException, NoSuchFieldException {
        T res = (T)cls.newInstance();
        String[] pairs = s.split(";");

        for (String pair : pairs) {
            String[] nv = pair.split("=");
            if (nv.length!=2) {
                throw new InvalidParameterException();
            }
            String name = nv[0];
            String value = nv[1];
            Field field = cls.getDeclaredField(name);
            if (Modifier.isPrivate(field.getModifiers())) {
                field.setAccessible(true);
                if (field.isAnnotationPresent(Save.class)) {
                    if (field.getType() == int.class) {
                        field.setInt(res, Integer.parseInt(value));
                    } else {
                        if (field.getType() == String.class) {
                            field.set(res, value);
                        } else {
                            if (field.getType() == Double.class) {
                                field.setDouble(res, Double.parseDouble(value));
                            }
                        }
                    }
                }
            }
        }
        return res;
    }
}

saveVar_src_ToFileFromFile.java

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

public class ToFileFromFile {
    String path = "C:\\Users\\admin\\IdeaProjects\\saveVar\\src\\File";

    public  void save (String s) throws IOException {
        FileWriter fileWriter = new FileWriter(path);
        try {
            fileWriter.write(s);
        } finally {
            fileWriter.close();
        }
    }

    public String read () throws FileNotFoundException {
        FileReader fileReader = new FileReader(path);
        String res = fileReader.toString();
        return res;
    }
}