Java 天地·Core Java·Lecture Notes·Unitt4 - Control of execution

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Content Page # 16

Layout convention: indentation

The use of indentation can be very helpful to the human reader when statements are being used to control program execution. Thus any statements that may never be executed (i.e. within a selection statement), or may be executed many times (i.e. within an iteration statement) can be indented to indicate the possible change from sequence execution of statements.

An example of a selection statement without indentation:

if( accountBalance < transactionTotal ){
showMessage("sorry no funds for transaction");
return false;}
else {
showMessage("transaction processed");
accountBalance -= transactionTotal;
return true;}

and now written with indentation of all statements that may or may not be executed:

if( accountBalance < transactionTotal ){
showMessage("sorry no funds for transaction");
return false;}
else {
showMessage("transaction processed");
accountBalance -= transactionTotal;
return true;}

In fact with the above example, we ought to also follow the convention for vertically aligning the braces forming the compound statement, and so write the code as follows:

if( accountBalance < transactionTotal )
{
showMessage("sorry no funds for transaction");
return false;
}
else

{

showMessage("transaction processed");
accountBalance -= transactionTotal;
return true;

}

This form of indentation has made this code much easier to understand.

However, there is a danger with the use of indentation. The use of TAB or space characters for indentation is completely ignored by the Java compiled, thus both if statements above are treated the same. A common programming error is to indent statements to indicate to a human reader that the statements are part of a selection or iteration statement, but to make an error in writing the statement so that not all the indented statements are actually effected by the control statement.

For example, in the following piece of code the programmer has forgotten to use braces to create a compound statement, so only the first indented statement is actually effected by the control statement:

for( int i = 0; i < balls.length; i++)
balls[i].paint( g );
balls[i].move(); // not part of loop !!

To make all the indented statements part of this loop, braces must be added to tell the compiler where the compound statement starts and finishes:

for( int i = 0; i < balls.length; i++)
{
balls[i].paint( g );
balls[i].move();
}

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