Question

I posted this this question, which basically amounts to an attempt to map Strings to functions in Java. I am trying to solve it by creating a HashMap <String, Method>, but every time I try to run Method.invoke (this, args), where args is an array of Objects corresponding to the parameters of Method, I get a NullPointerException. I declare the Methods in my HashMap as follows: mypackage.MyClass.class.getMethod ("methodName", args), where args is an array of Class object corresponding to the parameters of methodName. I’ve been working on this for a couple of hours and I’m starting to get really frustrated. If anyone has any suggestions I would really appreciate it!

HashMap <String, Method> myMethods = new HashMap <String, Method> (); 
Class [] args = {param1.class, param2.class, param3.class}; 
myMethods.put ("aKey", mypackage.MyClass.class.getMethod ("aMethod", args)); 
Method thisTest = myTests.get (thisKey);
Object [] args1 = {param1, param2, param3};
String thisResult = (String) thisTest.invoke (this, args1); // NPE occurs in this line

Aug 14, 2012 10:37:36 PM org.apache.catalina.core.StandardWrapperValve invoke
SEVERE: Servlet.service() for servlet jsp threw exception
java.lang.NullPointerException
        at mypackage.MyClass.myMethod (MyClass.java:61)
        at org.apache.jsp.myMethod_jsp._jspService(getZabbix_jsp.java:70)
        at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
        at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:393)
        at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:320)
        at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:266)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
        at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:230)
        at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:175)
        at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:128)
        at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:104)
        at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
        at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:261)
        at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:844)
        at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:581)
        at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:447)
        at java.lang.Thread.run(Unknown Source)

Answer 1

Although the population of myTests Map is not shown, it obviously doesn’t contain the test method thisTest resulting in an NPE.

The first thing to do here is to inspect the contents of this Map to ensure that it contains the requested key.

Also, it’s worth having a guard clause on thisTest before you attempt to invoke the method to prevent it blowing up.