import java.io.IOException;
import java.io.StringReader;

import org.codehaus.commons.compiler.CompileException;
import org.codehaus.janino.SimpleCompiler;

public class JaninoCompileFromString {
	
	private static SimpleCompiler compiler = new SimpleCompiler();
	
	public static void main(String[] args) throws CompileException, IOException, ClassNotFoundException, InstantiationException, IllegalAccessException {
		String source = "public class JaninoEvaluator implements Evaluator {\n"
		         +"\tpublic int evaluate(int i) {" + "\t\t"
		         +"\n" + "\t\treturn i-2;\n" + "\t}\n"
		         +"}\n";
		
		compiler.cook(new StringReader(source));
		
		Class clazz = compiler.getClassLoader().loadClass("JaninoEvaluator");
		
		Evaluator eval = (Evaluator)clazz.newInstance();
		
		System.out.println(source);
		System.out.println("\n\n\n evaluate(13): "+eval.evaluate(13));
	}
}

Exception in thread "main" java.lang.VerifyError: (class: JaninoEvaluator, method: evaluate signature: (I)D) Expecting to find object/array on stack
	at java.lang.Class.getDeclaredConstructors0(Native Method)
	at java.lang.Class.privateGetDeclaredConstructors(Unknown Source)
	at java.lang.Class.getConstructor0(Unknown Source)
	at java.lang.Class.newInstance0(Unknown Source)
	at java.lang.Class.newInstance(Unknown Source)
	at JaninoCompileFromString.main(JaninoCompileFromString.java:26)

public interface Evaluator {

	public double evaluate(int i);

}

public interface Evaluator {

	public int evaluate(int i);

}

		String source = "public class JaninoEvaluator implements Evaluator {\n"
		         +"public static String var;"
		         +"public JaninoEvaluator(){ this.var=\"private Variable\";}"
		         +"\tpublic int evaluate(int i) {" + "\t\t"
		         +"\n" + "\t\treturn i-2;\n" + "\t}\n"
		         +"}\n";