Objects made from objects - how to manipulate?

Question

Hello all - my first post here and new to OOP from procedural
I am trying to construct an object ("Box") from 2 Things. The Things have a colour and can be on or off.
I can construct the Box from a RED Thing and a BLUE Thing, and initialise them as being on, but....
How do I manipulate the Things in the Box once constructed?
I don't know how to address the Thing methods from the Box Class... please could someone help as I am sure this is really simple - I just can't find the answer in any of my books!
public class Thing {
     boolean on;
     String colour;
public Thing(String s) {
     colour = s;
     on = false;
// this main is just to test this class
public static void main (String[] args){
     Thing test = new Thing("TEST");
     test.turnOn();
     test.getState();
     try {
          System.out.println ("Pause.....");
          Thread.sleep(1000);
     catch (Exception e) {
          System.out.println ("CarCrash!");
     test.turnOff();
     test.getState();
public void turnOn() {
     on = true;
public void turnOff() {
     on = false;
public void getState() {
     if (on) {
          System.out.println (colour + ": ON");
     else {
          System.out.println (colour + ": OFF");
and here is the box class...
public class Box {
public Box() {
     Thing thingOne = new Thing ("RED");
     Thing thingTwo = new Thing ("BLUE");
     thingOne.turnOn();
     thingTwo.turnOn();
public void getBoxState() {
     System.out.println (thingOne.getState());
     System.out.println (thingTwo.getState());
public static void main (String[] args) {
     Box boxOne = new Box();
     boxOne.getBoxState(); // does not compile - this is the crux of the matter...
}

Answer 1

NickM999 wrote:
I've simplified Box to get to the root of the problem:
Nice.
public class Box {
    public Box() {
         Thing thingOne = new Thing ("RED");
         Thing thingTwo = new Thing ("BLUE");
         thingOne.turnOn();
         thingTwo.turnOn();
    public static void main (String[] args) {
         Box boxOne = new Box();
         thingOne.getState();   <--- this is the problem...(line 22)
}Box.java:22: cannot find symbol
symbol : variable thingOne
location: class Box
thingOne.getState();
^
1 error
You've got much worse problems than you think.
If you look at your constructor, you create two instances of Thing. The variables that point to them are local to the constructor, so as so as you leave that method those objects are eligible for garbage collection.
I'd like to create a number of Box instances (all with the same Things (Red/Blue) in them), then manipuate the Things within each box
example: Box 1 - Red Thing ON, Blue Thing OFF... Box 2 - Red Thing OFF, Blue Thing OFF... Box 2 - Red Thing ON, Blue Thing Off... and so on.
I also tried to address the methods in Thing as boxOne.thingOne.turnOff();
Is there a better design?
Thanks for your time!Here's what I'd recommend: don't expose those objects in public. Give clients an interface that they can use to get at them in a way that YOU can control internally.
You should follow the Java Beans conventions for Thing:
public class Thing
    boolean on;
    public boolean isOn() { return on; }
    public void setOn(boolean state) { on = state; }
}Since the state is really two flags, not one, I'd create a State object that would hold the variables. Less atomic that way. The more you can encapsulate in objects the better.
public class Box {
    private Thing thingOne;
    private Thing thingTwo;
    public Box() {
         thingOne = new Thing ("RED");
         thingTwo = new Thing ("BLUE");
         thingOne.turnOn();
         thingTwo.turnOn();
    public boolean getThingOne() { return thingOne.getState(); }
    // same thing for thing two.
    public setThingOne(boolean isOn)
        if (isOn)
            thingOne.turnOn();  //  I assume you have these methods.
        else
            thingOne.turnOff();
    public static void main (String[] args) {
         Box boxOne = new Box();
         thingOne.getState();   <--- this is the problem...(line 22)
}