Well-behaved objects - Objects First with Java: A Practical Introduction Using BlueJ - page 261

Code 7.6
A state-reporting
method

/**
* Print the values of this object's fields.
* @param where Where this state occurs.
*/
public void reportState(String where)
{

System.out.println( "displayValue: " + displayValue +
" leftOperand: " + leftOperand +
" previousOperator: " +
previousOperator + " at " + where);

}

If each method of CalcEngine contained a print statement at its entry point and a call to
reportState at its end, Figure 7.8 shows the output that might result from a call to the tester's
testPlus method. (This was generated from a version of the calculator engine that can be
found in the calculator-engine-print project.) Such output allows us to build up a picture of how
control flows between different methods. For instance, we can see from the order in which the
state values are reported that a call to plus contains a nested call to applyPreviousOperator .

Figure 7.8
Debugging output
from a call to
testPlus

clear called
displayValue: 0 leftOperand: 0 previousOperator: at end of clear
numberPressed called with: 3
displayValue: 3 leftOperand: 0 previousOperator: at end of number...
plus called
applyPreviousOperator called
displayValue: 3 leftOperand: 3 previousOperator: at end of apply...
displayValue: 0 leftOperand: 3 previousOperator: + at end of plus
numberPressed called with: 4
displayValue: 4 leftOperand: 3 previousOperator: + at end of number...
equals called
displayValue: 7 leftOperand: 0 previousOperator: + at end of equals

Print statements can be very effective in helping us understand programs or locate errors, but
there are a number of disadvantages:

■ It is not usually practical to add print statements to every method in a class. So they are only
fully effective if the right methods have been annotated.

■ Adding too many print statements can lead to information overload. A large amount of out-
put can make it difficult to identify what you need to see. Print statements inside loops are a
particular source of this problem.
■ Once their purpose has been served, it can be tedious to remove them.

■ There is also the chance that, having removed them, they will be needed again later. It can be
very frustrating to have to put them back in again!