howto Log it? Looking for a generic method that checks objects fields (compare 2 obj)

[Peter Primrose]

hi all,

I'm trying to build a logging mechanism that logs only the revisions made on an object (find the difference in the fields. think of an object that the user just got from the db, now he makes revisions - I wish to find those revisions)

I wonder if anyone knows any *generic* method that loops through 2 instances of the same class and find the difference in terms of their fields. In other words loop through 2 object's fields and compare them (must be the same class).

Below is a code example.
thanks for any thoughts.

  
public class LogIt 
{
public static void main(String args[])
{
Empl john = new Empl("John", "Jabber", 45);
Empl johnBackup = (Empl) john.clone();

john.setFname("James");

System.out.println(john);
System.out.println(johnBackup);

                //James Jabber (45)
                //John Jabber (45
 
}

public static void diff(Object o1, Object o2)
{
//here the diff should find the different between the 2 objects (the must be the same)
//the method must be generic for all kinds of classes
                // in the example above this method should show: John-->James
}
}  

 
public class Empl implements Cloneable
{
private String fname;
private String lname;
private int age;

public Empl (String fn, String ln, int age)
{
this.fname=fn;
this.lname=ln;
this.age=age;
}

public String getFname() {
return fname;
}
public void setFname(String fname) {
this.fname = fname;
}
public String getLname() {
return lname;
}
public void setLname(String lname) {
this.lname = lname;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}

public String toString(){
return fname+" "+lname+" ("+age+")";
}

public Object clone() {
        try {
            return super.clone();
        }
        catch (CloneNotSupportedException e) {     
            throw new InternalError(e.toString());
        }
 }
}   

[Jeff Rummings]

I wonder if anyone knows any *generic* method that loops through 2 instances of the same class and find the difference in terms of their fields.

What would such a method return? The diff(...) method in your example is void. You could have an equals method that uses reflection (I think EqualsBuilder from Jakarta commmons does something like that).

[Nitesh Kant]

Peter:I wonder if anyone knows any *generic* method that loops through 2 instances of the same class and find the difference in terms of their fields. In other words loop through 2 object's fields and compare them

Have you tried anything using reflection?
If you are comparing based on public fields/getters(java bean convention), then this must be straight forward.
All you have to do is list the getters, execute them for both the instances. Compare the values using equals. If not equal then the field has changed.
The trick is that if the field is an object other than the primitive type then you must call the diff generator recursively.