Evaluate groovy expression in java class

Question

I am looking to evaluate math expressions from String values using groovy in java code. I have created groovy script like below:

def sum(List<MyObject> myObjList) {
    int sum =0
    myObjList.each {it -> sum += it.grade}
    return sum
}

The class MyObject is defined as:

public class MyObject
{
    private String name;
    private String description;
    private double grade;

    public String getName()
    {
        return name;
    }

    public void setName(String name)
    {
        this.name = name;
    }

    public String getDescription()
    {
        return description;
    }

    public void setDescription(String description)
    {
        this.description = description;
    }

    public double getGrade()
    {
        return grade;
    }

    public void setGrade(double grade)
    {
        this.grade = grade;
    }
}

The code to evaluate the expression is:

public class Test
{

    public static void main(String[] args) throws IOException
    {
        List<MyObject> objects = new ArrayList<>();
        MyObject myObject = new MyObject();
        myObject.setGrade(2.0);
        objects.add(myObject);
        myObject = new MyObject();
        myObject.setGrade(1.0);
        objects.add(myObject);
        ClassPathResource resource = new ClassPathResource("/groovy-scripts/functions.groovy",Test.class);
        File file = resource.getFile();
        Binding binding = new Binding();
        GroovyShell shell = new GroovyShell(binding);
        Script script = shell.parse(file);
        binding.setProperty("objects",objects);
        String formula = "sum(objects)";
        Object result = script.evaluate(formula);
        System.out.println(result);
    }
}

I keep getting the below exception when I try to run the code:

signature of method: Script1.sum() is applicable for argument types:
(java.util.ArrayList) values: [[com.example.MyObject@3081f72c,
com.example.MyObject@3148f668]] Possible solutions: run(), run(),
dump(), use([Ljava.lang.Object;), any(), use(java.lang.Class,
groovy.lang.Closure)  at
org.codehaus.groovy.runtime.ScriptBytecodeAdapter.unwrap(ScriptBytecodeAdapter.java:71)
  at
org.codehaus.groovy.runtime.callsite.PogoMetaClassSite.callCurrent(PogoMetaClassSite.java:80)
  at
org.codehaus.groovy.runtime.callsite.CallSiteArray.defaultCallCurrent(CallSiteArray.java:51)
  at
org.codehaus.groovy.runtime.callsite.AbstractCallSite.callCurrent(AbstractCallSite.java:157)
  at
org.codehaus.groovy.runtime.callsite.AbstractCallSite.callCurrent(AbstractCallSite.java:169)
  at Script1.run(Script1.groovy:1)    at
groovy.lang.GroovyShell.evaluate(GroovyShell.java:574)    at
groovy.lang.GroovyShell.evaluate(GroovyShell.java:612)    at
groovy.lang.GroovyShell.evaluate(GroovyShell.java:583)    at
groovy.lang.Script.evaluate(Script.java:210)  at
com.example.Test.main(Test.java:37) ```

Please suggest the correct approach to achieve this

Answer 1

i'm using groovy to implement main but it's not far from java.

---

option 1 : use closures instead of functions

List objects = [ [grade: 111], [grade: 222] ];

GroovyShell shell = new GroovyShell();

//use undeclared closure variables 
Script functions = shell.parse('''
    sum = {List myObjList ->
        int sum =0
        myObjList.each {it -> sum += it.grade}
        return sum
    }
''');

functions.run(); // all closure variables assigned into binding
functions.getBinding().setProperty("objects", objects);
Object result = functions.evaluate("sum(objects)");

println("result="+result);

---

option 2: use script inheritance

List objects = [ [grade: 111], [grade: 222] ];

GroovyShell shell_1 = new GroovyShell();

shell_1.parse('''
    def sum (List myObjList) {
        int sum =0
        myObjList.each {it -> sum += it.grade}
        return sum
    }
''', "FUNCTIONS"); // give a class name to parsed script


def cc = new org.codehaus.groovy.control.CompilerConfiguration();
cc.setScriptBaseClass("FUNCTIONS"); // any parsed scripts will extend FUNCTIONS script class
//use shell_1 class loader because it contains FUNCTIONS class definition
GroovyShell shell_2 = new GroovyShell(shell_1.getClassLoader(), cc);

Binding binding = new Binding();
binding.setProperty("objects", objects);

Script formula = shell_2.parse("sum(objects)");
formula.setBinding(binding);

Object result = formula.run();

println("result="+result);

---

option 3: groovy itself could be much shorter and probably you don't need over-complicated solution

List objects = [ [grade: 111], [grade: 222] ];

Object result = groovy.util.Eval.me("objects", objects, "objects.sum{it.grade}");

println("result="+result);