Loops and Iteration

You have already seen a couple of examples of iteration and looping in Java. So this section will just serve as a reference for the differences in Syntax.

Definite Loop

In Python the easiest way to write a definite loop is using the for loop in conjunction with the range function. For example:

for i in range(10):
   print(i)

In Java we would write this as:

for (Integer i = 0; i < 10; i++ ) {
    System.out.println(i);
}

Recall that the range function provides you with a wide variety of options for controlling the value of the loop variable.

range(stop)
range(start,stop)
range(start,stop,step)

The Java for loop is really analogous to the last option giving you explicit control over the starting, stopping, and stepping in the three clauses inside the parenthesis. You can think of it this way:

for (start clause; stop clause; step clause) {
    statement1
    statement2
    ...
}

If you want to start at 100, stop at 0 and count backward by 5 the Python loop would be written as:

for i in range(100,-1,-5):
    print(i)

In Java we would write this as:

for (Integer i = 100; i >= 0; i -= 5)
    System.out.println(i);

In Python the for loop can also iterate over any sequence such as a list, a string, or a tuple. Java also provides a variation of its for loop that provides the same functionality in its so called for each loop.

In Python we can iterate over a list as follows:

l = [1, 1, 2, 3, 5, 8, 13, 21]
for fib in l:
   print(fib)

In Java we can iterate over an ArrayList of integers too:

ArrayList<Integer> l = new ArrayList<Integer>();
l.add(1); l.add(1); l.add(2); l.add(3);
for (Integer i : l) {
    System.out.println(i)
}

This example stretches the imagination a bit, and in fact points out one area where Java’ s primitive arrays are easier to use than an array list. In fact all primitive arrays can be used in a for each loop.

int l[] = {1,1,2,3,5,8,13,21};
for(int i : l) {
    System.out.println(i);
}

To iterate over the characters in a string in Java do the following:

String t = "Hello World";
for (char c : t.toCharArray()) {
    System.out.println(c);
}

Indefinite Loops

Both Python and Java support the while loop. Recall that in Python the while loop is written as:

while  condition:
   statement1
   statement2
   ...

In Java we add parenthesis and curly braces to get:

while (condition) {
    statement1
    statement2
    ...
}

Java adds an additional, if seldom used variation of the while loop called the do loop. The do loop is very similar to while except that the condition is evaluated at the end of the loop rather than the beginning. This ensures that a loop will be executed at least one time. Some programmers prefer this loop in some situations because it avoids an additional assignment prior to the loop. For example:

do {
    statement1
    statement2
    ...
} while (condition);
Next Section - Defining Classes in Java