how to create a tree of object

There are three classes which name Plus, Mult and Constant. How can i make in such a way that the Mult class has a method that holds the 5 and 4 object which is created earlier and the Plus class has a method that holds the 3 object and also Mult object which is holding the 5 and 4 object ?? This means that the higher hierarchy the tree is, the more object the method will be holding. I would like to know if it is possible for me to find online tutorial regarding such cases. Thanks

Code:

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Constant x = new Constant(3);
Constant y = new Constant(5);
Constant z = new Constant(4);

3+5*4

        +
      |-----|
    3      *
          |----|
          5    4

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Each operator points to two other objects - either a constant, or another operator

So an object containing operator + would point to a '3' and a '*'
The '*' in turn would point to a '5' and a '4'