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

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Iteration in Java

An iteration statement can result in a statement (or set of statements in a compound statement) being executed never, once or many times. What determines whether the statement(s) will be executed or not is the result of a test that is applied either before or after an execution of the loop statements. If the result of the test is true then loop will continue, if the result is false the loop will exit and the first statement following the end of the loop will be the next to be executed..

There are three forms of iteration statement implemented in Java:

while
for
do...while

In this module we shall concentrate on the first two of these iteration statements. They are all technically equivalent; the choice as to which form of iteration statement to implement in a program is made by each programmer on the grounds of style or convenience or expressive power. (The expressive power of a programming language statement is how clearly and effectively it communicates to a reader the effect the programmer wants to achieve).

Iteration: WHILE

A while statement works in a very similar way to an if statement, except that if the test is true, and the statements executed, the test is repeated, and the statements repeated until the test is false. Using pseudocode code we can illustrate how a while statement works. Consider the following pseudocode for a system to process bank transactions

while ( more transactions to process for account 00710 )

process the next transaction for account 00710

This illustrates the general form of an while statement, i.e.:

while (test)

action to perform if test is true

To implement our fire alarm pseudocode as a valid Java statement we could write the following:

while ( moreTransactions( 00710) )

processNextTransaction( 00710 );

(of course we are assuming the class has methods moreTransactions() and processNextTransaction()).
The way a while statement can cause deviation from the default sequential flow of execution can be illustrated as follows. Consider this pseudocode design:

statementA;

while( moreTransaction(00710) )

statementB;

statementC;

First statementA is executed. Then the test is made. Let us assume that the method moreTransaction(00710)returns false. Then the loop statement is skipped, and the next statement to be executed is statementC:

statementA

statementC

However, consider the situation where after statementA has been executed, the test (i.e. method moreTransaction(00710) ) returns true the first time. Then statementB is executed. The test is conducted again. Let us assume that this time method moreTransaction(00710)returns false. Then the loop statement is then skipped, and the next statement to be executed is statementC:

statementA

statementB

statementC

Now consider the situation where after statementA has been executed, the test (i.e. method moreTransaction(00710) ) first returns true. Then statementB is executed. The test is conducted again. Let us assume that moreTransaction(00710)returns true again (i.e. there were 2 transactions for this account waiting to be processed). Then statementB is executed a second time. The test is conducted again. Let us assume that this time method moreTransaction(00710)returns false Then the loop statement is then skipped, and the next statement to be executed is statementC:

statementA

statementB

statementC

Thus the result of an iteration statement means that some statements may not be executed, or may be executed once, or be executed many times.

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