Execute code from string with defined variables in Java

Question

I've got the following class

class ListNode
{
    int val;
    ListNode next;
    ListNode() {}
    ListNode(int val) { this.val = val; }
    ListNode(int val, ListNode next) { this.val = val; this.next = next; }
}

My task is to write a method that:

Takes int[] as a parameter.
Reverses the array.
Returns ListNode which has value of the current element in the array, has .next of a new created ListNode with the corresponding value in the array and so on.

Given the input of new int[]{0, 8, 7}; I expect to get a ListNode where value is 7, .next is a new node with the value of 8 and finally .next.next is a new node with the value of 0.

I wrote the following methods to accomplish such behaviour

public static ListNode reverseAndLinkToListNode(int [] arrayToReverse)
{
    ScriptEngineManager manager = new ScriptEngineManager();
    ScriptEngine engine = manager.getEngineByName("js");

    int finalDigitsArrayLength = arrayToReverse.length;

    ListNode finalNode = new ListNode();

    if (finalDigitsArrayLength > 0)
    {
        finalNode.val = arrayToReverse[finalDigitsArrayLength - 1];
        int j = 1;

        for (int i = finalDigitsArrayLength - 2; i >= 0; i--)
        {
            try
            {
                // create a new list node
                ListNode newlyCreatedNode = new ListNode(arrayToReverse[i]);
                // calculate how many 'nexts' I need
                String codeToExecute = returnNumberOfNexts(j);
                // set the next field with the list node I just created
                engine.eval(codeToExecute);
            }
            catch (Exception e)
            {
                System.out.println(e);
            }

            j++;
        }
    }

    return finalNode;
}


public static String returnNumberOfNexts(int nextCounter)
{
    String finalDotNextString = "finalNode";

    if (nextCounter > 0)
    {
        for (int i = 0; i < nextCounter; i++)
        {
            finalDotNextString += ".next";
        }
    }

    finalDotNextString += " = newlyCreatedNode;";

    return finalDotNextString;
}

The solution above returns the right code to execute, but when it comes to execution, I get:

javax.script.ScriptException: ReferenceError: "finalNode" is not defined in <eval> at line number 1

How do I define finalNode and newlyCreatedNode variables in the engine?

Why do you need ScriptEngine for this? Just call .next() in a loop by yourself. — dan1st
– dan1st, Commented Sep 19, 2021 at 16:26
This will simply overwrite .next member of finalNode. I need to link them all — Joe D
– Joe D, Commented Sep 19, 2021 at 17:56
I meant using a loop to traverse the nodes and set .next of the last element. You can assign the result of .next to a variable and call .next on that variable in a loop (setting the variable to the result every time) — dan1st
– dan1st, Commented Sep 19, 2021 at 18:34

queeg · Accepted Answer · 2021-09-20 20:40:43Z

1

Your scriptengine fails when it tries to resolve the 'finalNode' variable. The reason is that finalNode is known in Java but not in the Javascript engine. So you need to tell the scriptengine how to resolve the variable. I did not test the code but it might look somewhat like this:

ListNode finalNode = new ListNode();
ScriptEngine engine = manager.getEngineByName("js");
Bindings bindings = engine.getBindings(ScriptContext.GLOBAL_SCOPE);
if (bindings==null) {
    bindings = engine.createBindings();
    engine.setBindings(ScriptContext.GLOBAL_SCOPE);
}
bindings.put("finalNode", finalNode);
engine.eval(codeToExecute);

answered Sep 20, 2021 at 20:40

queeg

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