Using Debuggers

Debuggers provide the means to follow the execution of programs.

Download the ‘debug.java’ program, and issue the following commands in the given order.

javac -g debug.java
The -g flag provides for extra debugging information.
jdb debug
This command activates the debugger on the program.
stop in debug.main
This command requests a breakpoint in execution when the ‘main’ command is reached.
stop at debug:7
This command requests a breakpoint in execution when line 8 is reached
run
The command gets the execution started.
cont
This command asks the resumption of the execution resumed at breakpoints.
list
Lists a segment of the program and shows where the control is.
next
Execute the next line
print k
Shows the value of ‘k’.
dump a
A variant of the ‘print’ instruction, useful mainly for variables holding an object.
clear
Erase the existing breakpoints
stop in Cls.cmethod
This command requests a breakpoint in execution when the ‘main’ command is reached.
where
Shows the execution stack.
up

where

down

where

help

exit

debug.java
class debug {
   public static void main(String args[]){
     Cls a;
     int k;
     a = new Cls();
     k = a.imethod();
     for( ; k > 0; k=k-1 ){
        if( k == 5 ){ break; }
     }
     Cls.cmethod( k );
   }
}
class Cls{
   int i=1;
   static int j=2;
   int imethod(){
     i =  i + j;
     return i;
   }
   static void cmethod( int x ){
     j = j + x;
   }

}
Try figuring out the execution of the following program.
class fact {
   public static void main(String [] args ){
     System.out.println( f(6) );               // 3
   }
   static int f (int n ){
     int v;
     if( n < 2 ){ v = 1; }                     // 7
     else       { v = n * f(n-1); }            // 8
     return v;                                 // 9
   }
}