Telling which class called your method
Does anybody know a way you can tell which class is calling your method? I mean:
class A {
public static void main(String args[]) {
new A.doIt();
void doIt() {
B b = new B();
b.callMe();
class B {
public void callMe() {
String whoDidIt;
whoDidIt=[stuff i'm asking for];
System.out.println("I was called from an instance of class " + whoDidIt);
How can I get that code to print "I was called from an instance of class A"???
Lots of thanks in advance
Pass the calling object to callMe as a parameter, egB b = new B();
b.callMe(this);callMe becomes:
public void callMe(Object caller)
System.out.println("I was called from an instance of class " + caller.getClass());
}Of course, there is nothing to stop anyone who calls the method passing a different object and confusing callMe.
Similar Messages
-
Which object called the method
Suppose there is a class A which has three objects a1,a2,a3 and a method m. Now I'd like to know which particular object has called the method m.
>
which particular object has called the method
What class called my method
http://developer.java.sun.com/developer/qow/archive/104
a.The advice in the posted link is unreliable as the format of printStackTrace() is not standardized. A more reliable way to find out the calling class is to use SecurityManager.getClassContext() or Throwable.getStackTrace() [in JDK 1.4+].
However, it is not clear whether the original poster wanted to know the calling class or the calling object. In the latter case there is no easy way except for modify the signature of method m to accept the reference to the calling object.
Vlad. -
Hi,
If you have numerous methods in a class, can you just call them like so:
// in the .fla file
var john:Person = new Person(63,150); // creates a new person named john 63" and 150lbs.
john.weight(180); // changes johns weight to 180lbs.
// in the .as file
package {
import flash.display.MovieClip;
public class Person extends MovieClip {
public var _perHeight:Number;
public var _weight:Number;
public function Person(perHeight:Number, weight:Number) {
_perHeight=perHeight;
_weight=weight;
this.perHeight=_perHeight;
this.weight=_weight;
public function changeWeight(newWeight) {
this.weight=newWeight;
I must be calling the method completely wrong? I get this error:
1061: Call to a possibly undefined method weight through a reference with static type Person.That is because the method is called "changeWeight" and you are using "weight."
But what you really want is probably not a method but rather a getter/setter that acts like a property -- or maybe you just want a property.
So in your class you would have this:
package {
import flash.display.MovieClip;
public class Person extends MovieClip {
private var _perHeight:Number;
private var _weight:Number;
private var _age:Number;
public function Person(perHeight:Number, weight:Number) {
_perHeight=perHeight;
_weight=weight;
public function set weight(num:Number):void{
_weight=num;
public function get weight():Number{
return _weight;
Then in your code (after creating a person instance):
john.weight=180;// changes john's weight to 180.
The cool thing about getters and setters is that they act just like a property when you use them, but they allow for a whole function call at the other end and can do various things based on the inputs.
Also variable that start with an underscore are usually private variables. So in this case _weight, _age, and _perHeight would be private -- you couldn't access them directly through the john instance for example. And you would have appropriate getters and setters for them. For example you might make an age setter that didn't allow the number to be set as smaller than the current value or something. -
I have tried using the StringWriter and the printStackTrace() methodology for finding out the name of the method that actually called my class and it works fine. But the problem is that the execution time is too high (abt 70msec). I need a better and more efficient way of finding this out.
Does someone have an idea of how to do this?
Regards,
Manoj Rathod.Ok, this was my test code:
public class MethodTrace {
public static void main (String[] args) {
method3(false);
public static void method1 (boolean stop) {
StackTraceElement e[] = (new Throwable()).getStackTrace();
System.out.println("method1: called by " + e[1].getMethodName());
if (!stop) method3(false);
public static void method2 (boolean stop) {
StackTraceElement e[] = (new Throwable()).getStackTrace();
System.out.println("method2: called by " + e[1].getMethodName());
if (!stop) method1(true);
public static void method3 (boolean stop) {
StackTraceElement e[] = (new Throwable()).getStackTrace();
System.out.println("method3: called by " + e[1].getMethodName());
if (!stop) method2(false);
};And this was the output:
method3: called by main
method2: called by method3
method1: called by method2As for the speed, I don't know. It's cleaner than using printStackTrace(), and I would think it would be faster. But I'm not sure.
Why do you need to know the calling method, though? Perhaps there is some other, faster way to do what you want to do that doesn't have to use any stack tracing stuff at all.
Or maybe you could just call the method, passing the name of the calling method as a String parameter:
public void excellent (String caller) {
System.out.println("Called by " + caller);
public void superb () {
excellent("superb");
public void hard () {
excellent("hard");
};Lemme know if you find out anything interesting.
Jason Cipriani
[email protected]
[email protected] -
If there is a method in a class that could be called by any one of, lets say, one hundred other classes, in that method is there a way of printing out the name of the class that called it ?
So if class Xyz calls the method I would like to just do a println in the method saying that class Xyz called it ?Yup... Thread.currentThread().getStackTrace();
package krc.utilz;
import java.io.PrintStream;
public class Tracer
public static PrintStream out = null;
public boolean enabled = true;
public enum Format { LONG, SHORT }
public Format format;
public Tracer() {
this(System.err, Tracer.Format.LONG);
public Tracer(PrintStream out) {
this(out, Tracer.Format.LONG);
public Tracer(Format format) {
this(System.err, format);
public Tracer(PrintStream out, Format format) {
this.out = out;
this.format = format;
public String toString() {
return get(5);
public String get() {
return get(2);
public String get(int i) {
StackTraceElement[] st = Thread.currentThread().getStackTrace();
if (i>=st.length) i = st.length-1; //don't go past top of stack
StackTraceElement t = st;
String s = t.getFileName()+":"+t.getLineNumber()+":";
if (this.format == Format.LONG) {
s += t.getClassName()+"."+t.getMethodName();
return(s);
public void debug(String msg) {
if(!enabled)return;
out.println("DEBUG: "+msg);
public void print(String msg) {
if(!enabled)return;
if (out==null) return;
out.println(get(3)+" : "+msg);
public void print() {
if(!enabled)return;
if (out==null) return;
out.println(get(3));
public void print(int i) {
if(!enabled)return;
if (out==null) return;
out.println(get(i));
public static String getCurrentMethodName() {
StackTraceElement[] st = Thread.currentThread().getStackTrace();
return(st[2].getMethodName()); -
Is knowing of Class Name, which called current method, possible?
Hallo everyone,
My method is called from inside some method of some class. Can I know which class called my method.
In other words I would like to know is that possible to have a name of class from which my current method is called? It's sometrhing like I need an access to the call stack.
thanks!You can do the same thing before 1.4 as well:
Throwable t;
PrintStream ps;
PrintWriter pw;
String str;
StringBuffer sb;
StringWriter st;
t = new Throwable();
st = new StringWriter();
pw = new PrintWriter(st);
t.printStackTrace(pw);
sb = st.getBuffer();
str = sb.toString();You now have the stack as a String in the variable str.
The problem is that you have to parse the String to separate methods from classes and so on. And different platform may produce slightly different string representations of the stack.
So, it is easier in Java 1.4 -
Calling a method within an application?
Hi I'm a student taking an intro Java class and right now I have to develop and application that prompts for someone to input a character of the alphabet and it will display the corresponding number on a telephone that it is listed under. The main method gets the input from the user and we are required to retrieve the corresponding digit to the character from a separate method using either a switch statement or a nested if-then-else statement to determine what the digit is. I'm having trouble calling that method in the main class which is another requirement. any suggestions?
public class Hmwk08
* Main driver begins program execution.
* @param args program arguments
public static void main(String[] args) throws Exception
char letter;
char ch;
int relatesTo;
PrintStream win = new PrintStream(System.out);
Scanner in = new Scanner(System.in);
String newLine = System.getProperty("line.separator");
win.println("Given a letter of the alphabet, this program" + newLine
+ "will tell you the corresponding digit on a" + newLine
+ "telephone keypad.");
win.println();
win.print("Enter a letter: ");
letter = in.nextLine().charAt(0);
letter = Character.toUpperCase(letter);
* Get the digit on a telephone keypad that corresponds
* to the specified character. If the character is neither
* a letter nor a digit, this method will retun the
* asterisk character: '*'
* @param letter the specified character
* @return the corresponding digit
private static char getPhoneDigit(char letter)
char digit;
digit = '*';
if(letter == 'A' || letter == 'B' || letter == 'C')
digit = '1';
return digit;
}Your method is static so you can call your method without creating a Hmwk08 object.
char result = getPhoneDigit(letter);Would suffice in your main method
Mel -
How to Call C++ Method from Java
I need to call C++ method from Java.
I have gone through the JNI tuorial , but was not able to pin point things.
I read that :
You have to write JNI c functions which then call your C++ member functions.You need to write a JNI function which will call new on your C++ class.
Now i have java class :
Java Code JavaClass.java ---->
class JavaClass{
public native void nativeMethod();
static
System.loadLibrary("NativeCppCode");
private void callCppMethod()
//call C++ method
JavaClass jvc = new JavaClass();
jvc.nativeMethod()
}Cpp Code:
NativeCppCode.h---->
class NativeCppCode
public:
getValue();
setValue();
private:
int a;
JNIEXPORT void JNICALL Java_JavaClass_nativeMethod(JNIEnv *env
,jobject obj);NativeCppCode.C---->
NativeCppCode::getValue()
return a;
NativeCppCode::setValue()
a = 1;
JNIEXPORT void JNICALL Java_JavaClass_nativeMethod(JNIEnv *env
,jobject obj)
NativeCppCode* nativeInstabce = new NativeCppCode();
NativeCppCode.setValue();
}Is this the correct way to do it.
Any suggestion would be a great help to metryit wrote:
I need to call C++ method from Java.Not possible.
JNI uses C methods.
Is this the correct way to do it.Same way you would do it in any C/C++ method (not java)
MyClass* p = ....
p->doit();
Common idiom for the pointer in the above is to pass it back and forth to your java code as a java long. You cast it it and from your class pointer. Provide an explicit java method to free it when done. Besides providing the explicit method also implement a finalizer to free it as well (however that is a fail safe and should not be relied upon.) -
Hi All,
I have never worked with EJBs so I am kind of stuck with one problem.
I have to make use of one existing EJB project through my external standard java class. The existing EJB project makes use of some other packages which are also EJB projects and I am not allowed to change any functionality of the EJB project but to use them as it is.
I am just initilizing the class object of the EJB project and calling its method. Within the EJB project there is a method which is calling another method of another EJB class.
The problem is that I dont see this other EJB class object getting initilized first and then calling its method as it happens in a normal scenario, hence when that method gets called i get a null pointer execption.
Let me give you some idea by posting some code snippet.
@WebService(targetNamespace = "http://www.ttt.de/ota")
@SOAPBinding(style = SOAPBinding.Style.DOCUMENT, use = SOAPBinding.Use.LITERAL, parameterStyle = SOAPBinding.ParameterStyle.BARE)
// standard EJB3 annotations
@Local(AMSAgencyWSPortType.class)
@Stateless
@TransactionAttribute(TransactionAttributeType.REQUIRES_NEW)
// jboss propriatary annotations
@WebContext(transportGuarantee = "NONE", secureWSDLAccess = false)
public class AMSProfileServiceBean extends AbstractProfileBuilder implements AMSAgencyWSPortType
private static Log log = LogFactory.getLog(AMSMappingServiceBean.class);
@EJB(mappedName = "PotsdamProfileService/local")
private PotsdamProfileService profileService;
private Security security;
@EJB
private OTAProfileBuilder otaProfileBuilder;
@Override
public OTATTProfileReadRS otaTTReadRQ(Holder<Security> wsseHeader, OTATTReadRQ request)
security = wsseHeader.value;
BigDecimal requestVersion = request.getVersion();
OTATTProfileReadRS otaProfileReadRS = new OTATTProfileReadRS();
otaProfileReadRS.setVersion(requestVersion);
PotsdamInstanceKey profileIntanceKey;
try
profileIntanceKey = getKeyHandler(requestVersion).buildUserKey(wsseHeader);
PotsdamInstance userInstance = loadProfileFromPotsdam(profileIntanceKey);
private PotsdamInstance loadProfileFromPotsdam(PotsdamInstanceKey userCredentials) throws PotsdamException
PotsdamInstance potsdamInstance = null;
try{
Holder<com.ttt.potsdam.common.messages.Security> wsseHeader = buildWsseHeader(security);
ProfileResponse potsdamInstanceRS = profileService.loadProfile(wsseHeader, userCredentials);
...I get an error message before the sequence reaches loadProfile(..), which in my sense is due to the fact that 'profileService' never was initilized in the EJB project.
profileService is object of another EJB project, which looks somethign like this:
@Stateless
@LocalBinding(jndiBinding = "PotsdamProfileService/local")
// jboss 4.2
@org.jboss.annotation.ejb.LocalBinding(jndiBinding = "PotsdamProfileService/local")
public class PotsdamProfileServiceBean implements PotsdamProfileService {
private static Log log = LogFactory.getLog(PotsdamProfileServiceBean.class);
@EJB
private CacheProfileService cacheProfileService;
@EJB(mappedName = "/PersistenceUnitsManagerBean/local")
private PersistenceUnitsManager persistenceUnitsManager;
@EJB
private PotsdamService potsdamService;
public ProfileResponse loadProfile(Holder<Security> wsseHeader,
PotsdamInstanceKey cacheInstanceKey) {
log.debug("inside loadProfile");
...Now I am calling the main EJB like this:
PotsdamotaTTReadRQ();
public OTATTProfileReadRS PotsdamotaTTReadRQ()
AMSProfileServiceBean a = new AMSProfileServiceBean();
Holder<Security> wsseHeader = new Holder<Security>();
OTATTProfileReadRS response = a.otaTTReadRQ(wsseHeader, request);
return response;
}So my confusion is how can I make use of profileService.loadProfile(wsseHeader, userCredentials); ??
I tried to initilize: PotsdamProfileServiceBean profileService = new PotsdamProfileServiceBean();but I get jndi binding and lots of other exception.
Please help.
Edited by: 925515 on 04.07.2012 06:40Thanks for your pointer. Could you please help me out with a small code snippet example?
I want to make use of this existing bean (AMSProfileServiceBean ):
@WebService(targetNamespace = "http://www.ttt.de/ota")
@SOAPBinding(style = SOAPBinding.Style.DOCUMENT, use = SOAPBinding.Use.LITERAL, parameterStyle = SOAPBinding.ParameterStyle.BARE)
// standard EJB3 annotations
@Local(AMSAgencyWSPortType.class)
@Stateless
@TransactionAttribute(TransactionAttributeType.REQUIRES_NEW)
// jboss propriatary annotations
@WebContext(transportGuarantee = "NONE", secureWSDLAccess = false)
public class AMSProfileServiceBean extends AbstractProfileBuilder implements AMSAgencyWSPortType
private static Log log = LogFactory.getLog(AMSMappingServiceBean.class);
@EJB(mappedName = "PotsdamProfileService/local")
private PotsdamProfileService profileService;
private Security security;
@EJB
private OTAProfileBuilder otaProfileBuilder;
@EJB
private OTAProfileUpdater otaProfileUpdater;
@Override
public OTATTProfileReadRS otaTTReadRQ(Holder<Security> wsseHeader, OTATTReadRQ request)
...in my soap-connector :
@WebServiceProvider(wsdlLocation = "WEB-INF/wsdl/PotsdamAMSAgencyWS.wsdl", targetNamespace = "http://www.ttt.de/ota", serviceName = "PotsdamAMSAgencyWS", portName = "PotsdamAMSAgencyWSPortType")
@ServiceMode(value = Service.Mode.MESSAGE)
public class PotsdamAgencyWSProvider extends WSProvider implements Provider<SOAPMessage>
@Resource
WebServiceContext wsCtx;
...A small example would be really appreciated.
Thanks. -
To print which class is calling a method??
Hello people
Is there any way it could be known which class is calling a particular method from the method itself?
ThanksTry this:
* @author Malcolm McMahon
* @version $Revision: $
public class TestTrace {
static {
printCall();
private static void c() {
printCall();
private static void printCall() {
String name = Thread.currentThread().getStackTrace()[3].getMethodName();
System.out.println("Call: " + name);
public static void main(String[] args) {
printCall();
c();
}Only think is, I'm not confident that the index 3 will remain unchanged in different releases (there are two levels within getStackTrace() and there's no guarantee that will remain true). -
How to load a class dynamically and then a call a method?
Hi
I want to call a method from a class,which class is loaded dynamically.
Consider a classA and ClassB..
ClassB contains a method showvalue() which returns a String value.
I want to load a ClassB dynamically in ClassA,and call the method showvalue() and print the returned value of that method (showvalue).
How to do this?
ThanksSince you found your way to Reflections and Reference Objects, I can only assume you know that reflection is the answer. Since the reflection tutorial on this site, and indeed, the many others on the web, can explain this a whole lot better and more consisely than can be done in a forum, I'll point you in that direction instead. As a starting point, and to show I'm not just fobbing you off, you're interested in the classes java.lang.Class, java.lang.reflect.Method, and the method Class.getMethod(String, Class[])
-
Calling a method from another class. Please Help!!
Here is my full program. Like I said in my last post. I am having trouble calling the baseMakesError1 method. This method is called in the subcallsbase1 method. Also, it would be of great help if you could look for any other errors with with my program. Thank you so much. I really appreciate it.
import java.io.BufferedReader;
import java.io.FileNotFoundException;
import java.io.FileReader;
public class ThomasR11
String sy;
public static void main(String[] args)
System.out.println("(main): Ryan Thomas, file ThomasR11: Exceptions");
SubRT obj3;
SubRT obj4;
String sX;
String x;
String s;
String sY;
try
ThomasR11 obj1=new ThomasR11();
catch(NullPointerException NPE)
System.err.println("Null Pointer Exception");
ThomasR11 obj2=new ThomasR11("Hello");
System.out.println("This is the last statement in main");
public ThomasR11()
SubRT obj3= new SubRT();
obj3.subCallsBase1();
public ThomasR11(String sY)
throws FileNotFoundException
SubRT obj4= new SubRT();
obj4.subError2();
obj4.subError3(sY);
private static class BaseRT
public BaseRT()
System.out.println("In BaseRT constructor= " + this);
public void baseMakesError1(int value0)
System.out.println("In BaseRT constructor, about to throw exception");
if (value0==0)
throw new NullPointerException("NullPointerException");
private class SubRT extends BaseRT
SubRT subNull;
public SubRT()
System.out.println("In SubRT constructor before call to aMakes an error= " +
"\n SubRT constructor= " + this);
public void subCallsBase1()
try
System.out.println("Calling the method baseMakeserror1");
baseMakesError1(subNull);<---------------------This is where I am getting the error it says "ThomasR11.java [79:1] baseMakesError1(int) in ThomasR11.BaseRT cannot be applied to (ThomasR11.SubRT)"
catch(NumberFormatException NFE)
System.err.println("\nNumber Format Exception");
catch (NullPointerException NPE)
System.err.println(NPE.getMessage() + "\n");
System.err.println(NPE.toString() + "\n");
throw NPE;
public void subError2()
try
BufferedReader in = new BufferedReader( new FileReader( "c:/xx/xx.xx" ) );
catch(NullPointerException NPE)
System.out.println("Null Pointer Exception");
catch (FileNotFoundException FNFE)
System.err.println("FileNotFoundException");
public void subError3( String sX)
try
System.out.println("Hello");
System.out.println (sX.length());
catch(NullPointerException NPE)
System.err.println("Null Pointer Exception");"ThomasR11.java [79:1] baseMakesError1(int) in ThomasR11.BaseRT cannot be applied to (ThomasR11.SubRT)"
...this error is telling out that you wrote baseMakesError1 to accept an "int" as an argument, and you are trying to pass it an instance of the ThomasR11.SubRT class. You either have to change baseMakesError1 to accept a SubRT as an argument, or change your call to pass an integer. -
Calling another method in the class from within the body of a method Im wri
Hello out there.
I have a question. I keep getting an error that reads as follows:
1 error found:
File: /Users/matthieubell/Academia/University of Waterloo/CS 125/L06/CreditCard.java [line: 80]
Error: double cannot be dereferenced
I think it is occuring because I am trying to call a method on an instance variable, which is not an object. But how do call a method I have already written, on another method I am writing in the general sense. I could make a particular object, but Im not shure that would get me the same result. I want to be able to call the method calcMinPayment on the instance variable currentBalance to wirte the method makePayment.
I have a class CreditCard
with the instance variables "private double currentBalance = 0; "
public double calcMinPayment()
// Add code here
double minimumPayment;
if (currentBalance < 50)
minimumPayment = currentBalance/10;
else
minimumPayment = 50;
return minimumPayment; // Replace this statement
* This method will decrease the current balance on the credit card if
* this payment meets or exceeds the minimum payment amount.
* pre: paymentAmt > 0
* post: The current balance should be decreased by paymentAmt if
* paymentAmt >= the minimum payment amount. Otherwise, the payment
* will not be recorded and an appropriate error message should be
* displayed.
public void makePayment(double paymentAmt)
// Add code here
double minimumPaymentAmount;
minimumPaymentAmount = currentBalance.calcMinPayment();
if (paymentAmt < minimumPaymentAmount)
System.out.println("Sorry, but your payment must exceed the minimum payment amount.");
System.out.println("This payment has not been recorded, please try again.");
else
this.currentBalance = currentBalance - paymentAmt;
thanks for youre help
-Matthieu'calcMinPayment' takes no arguments, uses a member variable (currentBalance) to compute a local variable 'minimumPayment' which it returns, ie, sends back to the caller. So you can call 'calcMinPayment' at any time.
minimumPaymentAmount = calcMinPayment(); -
Calling a method in Class from webdynpro
Hi All,
I have to perform some action when i click the button created in my webdynpro screen.
However I must not write the code for the button action inside the button event. But I have to create a class and inside the class i have to create the method for this button event.
Can anyone tell me how to call a method in a class created in SE24 from webdynpro on clicking the button.
Thanks in advance.hi..
When you want to perform some action after clicking a Button, then you have to write the code only in 'Events' - 'On Action' event. From your question, i assume that, you wanna use Assistance class in your webdynpro so as to increase better performance. Here is the link which will guide yuo with screen shot for using assistance class in WD-ABAP.
http://wiki.sdn.sap.com/wiki/display/WDABAP/AssistanceclassfunctionalityinWDA
thanks,
Padma -
Errors in a Java class when calling other methods
I am giving the code i have given the full class name . and now it is giving the following error :
Cannot reference a non-static method connectDB() in a static context.
I am also giving the code. Please do help me on this. i am a beginner in java.
import java.sql.*;
import java.util.*;
import DButil.*;
public class StudentManager {
/** Creates a new instance of StudentManager */
public StudentManager() {
Connection conn = null;
Statement cs = null;
public Vector getStudent(){
try{
dbutil.connectDB();
String Query = "Select St_Record, St_L_Name, St_F_Name, St_Major, St_Email_Address, St_SSN, Date, St_Company, St_Designation";
cs = conn.createStatement();
java.sql.ResultSet rs = cs.executeQuery(Query);
Vector Studentvector = new Vector();
while(rs.next()){
Studentinfo Student = new Studentinfo();
Student.setSt_Record(rs.getInt("St_Record"));
Student.setSt_L_Name(rs.getString("St_L_Name"));
Student.setSt_F_Name(rs.getString("St_F_Name"));
Student.setSt_Major(rs.getString("St_Major"));
Student.setSt_Email_Address(rs.getString("St_Email_Address"));
Student.setSt_Company(rs.getString("St_Company"));
Student.setSt_Designation(rs.getString("St_Designation"));
Student.setDate(rs.getInt("Date"));
Studentvector.add(Student);
if( cs != null)
cs.close();
if( conn != null && !conn.isClosed())
conn.close();
return Studentvector;
}catch(Exception ignore){
return null;
}finally {
dbutil.closeDB();
import java.sql.*;
import java.util.*;
public class dbutil {
/** Creates a new instance of dbutil */
public dbutil() {
Connection conn;
public void connectDB(){
conn = ConnectionManager.getConnection();
public void closeDB(){
try{
if(conn != null && !conn.isClosed())
conn.close();
}catch(Exception excep){
The main error is occuring at the following lines connectDB() and closeDB() in the class student manager. The class dbutil is in an another package.with an another file called connectionManager which establishes the connection with DB. The dbutil has the openconnection and close connection methods. I have not yet written the insert statements in StudentManager. PLease do Help meYou're doing quite a few things that will cause errors, I'm afraid. I'll see if I can help you.
The error you're asking about is caused by the following line:
dbutil.connectDB();
Now first of all please always ensure that your class names begin with capital letters and your instances of classes with lowercase letters. That's what everybody does, so it makes your code much easier to follow.
So re-write the couple of lines at the beginning of your dbutil class like this:
public class Dbutil {
/** Creates a new instance of Dbutil */
public Dbutil() {
Now you need to create an instance of dbutil before you can call connectDB() like this:
Dbutil dbutil = new Dbutil();
now you can call the method connectDB():
dbutil.connectDB();
The problem was that if you don't create an instance first then java assumes that you are calling a static method, because you don't need an instance of a class to call a static method. If it was static, the method code would have been:
public static void connectDB(){
You have a fine example of a static method call in your code:
ConnectionManager.getConnection();
If it wasn't a static method your code would have to look like this:
ConnectionManager connectionManager = new ConnectionManager();
connectionManager.getConnection();
See the difference? I also know that ConnectionManager.getConnection() is a call to a static method because it begins with a capital letter.
Anyway - now on to other things:
You have got two different Connection objects called conn. One is in StudentManager and the other is in Dbutils, and for the moment they have nothing to do with each other.
You call dbUtil.connectDb() and so if your connectDb method is working properly you have a live connection called conn in your dbUtil object. But the connection called conn in StudentManager is still null.
If your connection in the dbUtil object is working then you could just add a line after the call to connectDb() in StudentManager so that the StudentManager.conn object references the dbUtil.conn object like this:
dbutil.connectDB();
conn = dbUtil.conn;
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