Java Reference
In-Depth Information
int count ;
for ( count = 0 ; count < 10 ; count ++)
System . out . println ( count );
Notice how this syntax places all the important information about the loop variable
on a single line, making it very clear how the loop executes. Placing the update
expression in the for statement itself also simplifies the body of the loop to a single
statement; we don't even need to use curly braces to produce a statement block.
The for loop supports some additional syntax that makes it even more convenient
to use. Because many loops use their loop variables only within the loop, the for
loop allows the initialize expression to be a full variable declaration, so that the
variable is scoped to the body of the loop and is not visible outside of it. For
for ( int count = 0 ; count < 10 ; count ++)
System . out . println ( count );
Furthermore, the for loop syntax does not restrict you to writing loops that use
only a single variable. Both the initialize and update expressions of a for loop
can use a comma to separate multiple initializations and update expressions. For
for ( int i = 0 , j = 10 ; i < 10 ; i ++, j --)
sum += i * j ;
Even though all the examples so far have counted numbers, for loops are not
restricted to loops that count numbers. For example, you might use a for loop to
iterate through the elements of a linked list:
for ( Node n = listHead ; n != null ; n = n . nextNode ())
process ( n );
The initialize , test , and update expressions of a for loop are all optional; only
the semicolons that separate the expressions are required. If the test expression is
omitted, it is assumed to be true . Thus, you can write an infinite loop as for(;;) .
The foreach Statement
Java's for loop works well for primitive types, but it is needlessly clunky for han‐
dling collections of objects. Instead, an alternative syntax known as a foreach loop is
used for handling collections of objects that need to be looped over.
The foreach loop uses the keyword for followed by an opening parenthesis, a vari‐
able declaration (without initializer), a colon, an expression, a closing parenthesis,
and finally the statement (or block) that forms the body of the loop:
for ( declaration : expression )
Despite its name, the foreach loop does not have a keyword foreach —instead, it is
common to read the colon as “in”—as in “foreach name in studentNames.”
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