Execution of the program by means of a debugger

A debugger allows us to:

• execute the statements of a program one at a time,
• execute the program until the execution reaches certain points defined by the user (called breakpoints),
• examine the content of variables and objects at any time during the execution.

Debuggers are very useful tools to detect the causes of errors in programs.

Example 1:

int a, b, x;
a = 5;
b = Integer.parseInt(kb.readLine()); // reading of b
...         // statements that do not change b
x = a/b;

By means of a debugger we can verify the value of the variable b before executing the statement that generates the error.

Example 2:

String s, t;
s = null;
...
t = s.concat("a");

The assignment statement for t generates an exception of type NullPointerException. Such an error depends on the fact that, when the assignment statement is executed, the value of s is null. To check this error, we can use a debugger and observe the value of the variable s before executing the statement that generates the error.