Interesting Question.

[Rahul Singhai]

While going through my notes. I came across a very good question. I havent remember from where i got it.

public class StaticBase
{
static
{
run();
}
public static void run()
{
System.out.println("Base from " + Thread.currentThread().getStackTrace()[2]);
}

public static class Derived extends StaticBase
{
static
{
run();
StaticBase.run();
Derived.run();
}
public static void run()
{
System.out.println("Derived from " + Thread.currentThread().getStackTrace()[2]);
}
}

public static final void main(String[] args)
{
new StaticBase().run();
System.out.println("Here.....!");
new Derived().run();
}
}


can you guess the output?

[Ankit Garg]

the question generates an exception on my JVM

[sweety sinha]

i got the output on my system.
the output is:-

Base from StaticBase.run(StaticBase.java:9)
Base from StaticBase.run(StaticBase.java:9)
Here......!
Derived from StaticBase$Derived.run(StaticBase.java:22)
Base from StaticBase.run(StaticBase.java:9)
Derived from StaticBase$Derived.run(StaticBase.java:22)
Derived from StaticBase$Derived.run(StaticBase.java:22)

but how it is working?

thanks

[ramesh maredu]

It is producing fillowing output

Base from com.StaticBase.<clinit>(StaticBase.java:5)
Base from com.StaticBase.main(StaticBase.java:27)
Here.....!
Derived from com.StaticBase$Derived.<clinit>(StaticBase.java:15)
Base from com.StaticBase$Derived.<clinit>(StaticBase.java:16)
Derived from com.StaticBase$Derived.<clinit>(StaticBase.java:17)
Derived from com.StaticBase.main(StaticBase.java:29)

it is producing sys out in run method and from where that method is been called i mean line number of caller.

Thread.currentThread().getStackTrace() returns an array of StackTraceElement, StackTraceElement object is used to represent single stack frame
[ August 28, 2008: Message edited by: ramesh maredu ]

[Ankit Garg]

OK now I get the same output....I actually didn't see that it has an inner class.I was actually running the code with the name of the inner class i.e. Derived...and I was getting this error

Exception in thread "main" java.lang.NoClassDefFoundError: Base (wrong name base)
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClass(Unknown Source)
at java.security.SecureClassLoader.defineClass(Unknown Source)
at java.net.URLClassLoader.defineClass(Unknown Source)
at java.net.URLClassLoader.access$000(Unknown Source)
at java.net.URLClassLoader$1.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClassInternal(Unknown Source)
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClass(Unknown Source)
at java.security.SecureClassLoader.defineClass(Unknown Source)
at java.net.URLClassLoader.defineClass(Unknown Source)
at java.net.URLClassLoader.access$000(Unknown Source)
at java.net.URLClassLoader$1.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClassInternal(Unknown Source)

but I don't understand how I was getting this exception???
[ August 28, 2008: Message edited by: Ankit Garg ]

[ramesh maredu]

try this

 
 
public class StackTrace {
public static void main(String[] args) {
test1();
}
private static void test1() {
test2();
}
private static void test2() {
StackTraceElement[] stackTraces = Thread.currentThread().getStackTrace();
for(StackTraceElement element : stackTraces){
System.out.println(element);
}
}
}
 

see the API of StackTraceElement it says answer

"An element in a stack trace, as returned by Throwable.getStackTrace(). Each element represents a single stack frame. All stack frames except for the one at the top of the stack represent a method invocation. The frame at the top of the stack represents the execution point at which the stack trace was generated. Typically, this is the point at which the throwable corresponding to the stack trace was created."