Traverse ArrayList To Search For Integer Key

I am very new to coding and I am struggling with one of my intro to Java Projects. This project involves creating a GUI that has a list of trees and their possible heights, giving each tree a range for their possible minimum and maximum heights.

I am having trouble creating this method for the TreeDatabase (TreeDB class):

public String queryByPossibleHeight(int key) Instructions for this method are as follows: this method creates and returns a single string that contains the trees that can be found such that the value key is in the range for possible height. Traverse the entire ArrayList of Trees and for each one of the trees contained in the ArrayList invoke the method inRange() from the class Tree. Pass to the method inRange() the parameter key. If inRange() returns true, invoke the method toString() from the class Tree. Append the value returned by invoking the method toString() to the String result, adding a “” at the end. Once the entire ArrayList has been traversed, return the String result.

My 3 classes so far (I haven't gotten to the actual GUI yet):

public class Range {
private int low;
private int high;
private int value;
public Range(int plow,int phigh){
low = plow;
high = phigh;

[Code]...

As you can see in the last TreeDB class, in the public String queryByPossibleHeight(int key) method I have something wrote in, but I was just testing random things.