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 ReflectionToStringBuilder does this for you.

import org.apache.commons.lang3.builder.ReflectionToStringBuilder

// your code goes here

public String toString() {
   return ReflectionToStringBuilder.toString(this);
}

Answer 2

Another option, if you are ok with JSON, is Google's GSON library.

public String toString() {
    return new GsonBuilder().setPrettyPrinting().create().toJson(this);
}

It's going to do the reflection for you. This produces a nice, easy to read JSON file. Easy-to-read being relative, non tech folks might find the JSON intimidating.

You could make the GSONBuilder a member variable too, if you don't want to new it up every time.

If you have data that can't be printed (like a stream) or data you just don't want to print, you can just add @Expose tags to the attributes you want to print and then use the following line.

 new GsonBuilder()
.setPrettyPrinting()
.excludeFieldsWithoutExposeAnnotation()
.create()
.toJson(this);

Answer 3

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 4

You can use already implemented libraries, as ReflectionToStringBuilder from Apache commons-lang. As was mentioned.

Or write smt similar by yourself with reflection API.

Here is some example:

class UniversalAnalyzer {

   private ArrayList<Object> visited = new ArrayList<Object>();

   /**
    * Converts an object to a string representation that lists all fields.
    * @param obj an object
    * @return a string with the object's class name and all field names and
    * values
    */
   public String toString(Object obj) {
      if (obj == null) return "null";
      if (visited.contains(obj)) return "...";
      visited.add(obj);
      Class cl = obj.getClass();
      if (cl == String.class) return (String) obj;
      if (cl.isArray()) {
         String r = cl.getComponentType() + "[]{";
         for (int i = 0; i < Array.getLength(obj); i++) {
            if (i > 0) r += ",";
            Object val = Array.get(obj, i);
            if (cl.getComponentType().isPrimitive()) r += val;
            else r += toString(val);
         }
         return r + "}";
      }

      String r = cl.getName();
      // inspect the fields of this class and all superclasses
      do {
         r += "[";
         Field[] fields = cl.getDeclaredFields();
         AccessibleObject.setAccessible(fields, true);
         // get the names and values of all fields
         for (Field f : fields) {
            if (!Modifier.isStatic(f.getModifiers())) {
               if (!r.endsWith("[")) r += ",";
               r += f.getName() + "=";
               try {
                  Class t = f.getType();
                  Object val = f.get(obj);
                  if (t.isPrimitive()) r += val;
                  else r += toString(val);
               } catch (Exception e) {
                  e.printStackTrace();
               }
            }
         }
         r += "]";
         cl = cl.getSuperclass();
      } while (cl != null);

      return r;
   }    
}

Answer 5

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 6

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 7

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