Calculate similarity between TreeKernelNode

I have to calculate the similarity between two TreeKernelNode structures. Where the TreeKernelNode class is of this type:

private String instructionClass;
private String instruction;
private String contextClass;
private List <String> lexemes = new ArrayList <String> ();

private List <TreeKernelNode> children = new ArrayList <TreeKernelNode> ();

public TreeKernelNode (String instructionClass, String instruction) {
super();
this.instructionClass = instructionClass;
this.instruction = instruction;
this.contextClass = "";
}

`

So I have to compare and get a similarity index between two TreeKernelNode structures. How can I do?