Thanks for your advices. But actually i asked these questions for parameters 
file. How can do that i said in parameters file? 

in parameters file, i have to define pop.subpop.0.species.ind.numtrees field as 
I understand. But i have to define this parameter fixed number for example 
pop.subpop.0.species.ind.numtrees = 2. this parameter said that all 
individuals for subpop.0 must have 2 trees. Am i wrong? but i don't want to do 
this. I want to define all individuals with different number of trees in the same 
subpopulation. For example i wrote this sentences to parameters file

pop.subpop.0.species.ind.numtrees = 2
pop.subpop.0.species.ind.tree.0 = ec.gp.GPTree
pop.subpop.0.species.ind.tree.0.tc = tc0
pop.subpop.0.species.ind.tree.1 = ec.gp.GPTree
pop.subpop.0.species.ind.tree.1.tc = tc0

and when i read subpop.0 from file like this;

Number of Individuals: i2|
Individual Number: i0|
Evaluated: F
Fitness: f1143268979|659.60077|i0|
Tree 0:
 (- (+ l0 l0) (- h1 l0))
Tree 1:
 (- (+ (- l0 l0) (- l0 h1)) (- (+ h1 l0) (- h1 l0)))
Individual Number: i1|
Evaluated: F
Fitness: Standardized=832.0 Adjusted=0.0012004802 Hits=0
Tree 0:
 (+ (+ (+ (+ (- l0 h1) (- l0 h1)) (+ (- l0
     h1) (- l0 h1))) (- (+ (+ h1 (- (+ l0 l0)
     (+ h1 h1))) (- l0 h1)) (+ (+ l0 l0) (- h1
     l0)))) (- (+ (- (- (- (- l0 h1) (+ l0 h1))
     (- (+ h1 h1) (+ l0 l0))) (+ h1 l0)) (- (-
     l0 l0) (- l0 l0))) (- (- (+ l0 l0) (+ l0
     l0)) (+ (- l0 h1) (+ h1 l0)))))
Tree 1:
 (+ (- (- h1 h1) (+ h1 l0)) (- (- l0 l0) (-
     l0 l0)))
Tree 2:
 (+ (- h1 l0) (+ h1 h1))

and 

pop.subpop.0.extra-behavior = fill



of course, ecj didn't read Individual1 tree2. But i want to read tree2 too. How 
can do this in parameters file? is not this possible for parameters file? or i just 
can do that with java code programmatically. Thanks.

Emre