package Inventaris;

import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectOutputStream;

public class Saving  {
	public Object save(){
		
		RekapRuang RR= new RekapRuang();
		try
	      {
	         FileOutputStream fileOut =
	         new FileOutputStream("/tmp/employee.ser");
	         ObjectOutputStream out = new ObjectOutputStream(fileOut);
	         out.writeObject(RR);
	         out.close();
	         fileOut.close();
	         System.out.printf("Serialized data is saved in /tmp/employee.ser");
	      }catch(IOException i)
	      {
	          i.printStackTrace();
	      }

		return null;
	}

}

package Inventaris;

import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectOutputStream;

public class Saving  {
	public Object save(){
		
		RekapRuang RR= new RekapRuang();
		try
	      {
	         FileOutputStream fileOut =
	         new FileOutputStream("/tmp/employee.ser");
	         ObjectOutputStream out = new ObjectOutputStream(fileOut);
	         out.writeObject(RR);
	         out.close();
	         fileOut.close();
	         System.out.printf("Serialized data is saved in /tmp/employee.ser");
	      }catch(IOException i)
	      {
	          i.printStackTrace();
	      }

		return save();
	}
}

Update Saving.java