Java toString() using reflection?

Question

I was writing a toString() for a class in Java the other day by manually writing out each element of the class to a String and it occurred to me that using reflection it might be possible to create a generic toString() method that could work on ALL classes. I.E. it would figure out the field names and values and send them out to a String.

Getting the field names is fairly simple, here is what a co-worker came up with:

public static List initFieldArray(String className) throws ClassNotFoundException {

    Class c = Class.forName(className);
    Field field[] = c.getFields();
    List<String> classFields = new ArrayList(field.length);

    for (int i = 0; i < field.length; i++) {
        String cf = field[i].toString();
        classFields.add(cf.substring(cf.lastIndexOf(".") + 1));
    }

    return classFields;
}

Using a factory I could reduce the performance overhead by storing the fields once, the first time the toString() is called. However finding the values could be a lot more expensive.

Due to the performance of reflection this may be more hypothetical then practical. But I am interested in the idea of reflection and how I can use it to improve my everyday programming.

Answer 1

Apache commons-lang ToStringBuilder does this for you.

Answer 2

If you're using Eclipse, you may also have a look at JUtils toString generator, which does it statically (generating the method in your source code).

Answer 3

Not reflection, but I had a look at generating the toString method (along with equals/hashCode) as a post-compilation step using bytecode manipulation. Results were mixed.

Answer 4

W/reflection, as I hadn't been aware of the apache library:

(be aware that if you do this you'll probably need to deal with subobjects and make sure they print properly - in particular, arrays won't show you anything useful)

@Override
public String toString()
{
 StringBuilder b = new StringBuilder("[");
 for (Field f : getClass().getFields())
 {
  if (!isStaticField(f))
  {
   try
   {
    b.append(f.getName() + "=" + f.get(this) + " ");
   } catch (IllegalAccessException e)
   {
    // pass, don't print
   }
  }
 }
 b.append(']');
 return b.toString();
}


private boolean isStaticField(Field f)
{
 return Modifier.isStatic(f.getModifiers());
}

Answer 5

Here is the Netbeans equivalent to Olivier's answer; smart-codegen plugin for Netbeans.