Control Flow
In this section we examine the syntax of control flow statements in Java - conditionals and loops - comparing them to Python. We will find them to be very similar, with relatively predictable syntax variations based on the Java that we have learned to this point.
Conditionals
Conditional statements in Python and Java are very similar. In Python we have three patterns.
Simple if
Let's consider an if statement with no else clause. Here's the Python version.
if condition:
statement1
statement2
...In Java this same pattern is simply written as:
if (condition) {
statement1
statement2
...
}Once again, you can see that in Java the curly braces, rather than indentation, define a block. In Java, the parentheses around the condition are required.
if-else
Adding an else clause, we have:
if condition:
statement1
statement2
...
else:
statement1
statement2
...In Java this is written as:
if (condition) {
statement1
statement2
...
} else {
statement1
statement2
...
}elif / else if
In Java we can utilize the same behavior that elif provides in Python, with a slightly different syntax. Here is a simple example in both Python and Java.
grade = int(input('enter a grade'))
if grade < 60:
print 'F'
elif grade < 70:
print 'D'
elif grade < 80:
print 'C'
elif grade < 90:
print 'B'
else:
print 'A'In Java we would write this as follows:
import java.util.Scanner;
public class ElseIf {
public static void main(String args[]) {
Scanner in = new Scanner(System.in);
System.out.println('enter a grade');
int grade = in.nextInt();
if (grade < 60) {
System.out.println('F');
} else if (grade < 70) {
System.out.println('D');
} else if (grade < 80) {
System.out.println('C');
} else if (grade < 90) {
System.out.println('B');
} else {
System.out.println('A');
}
}
}switch
Java also supports a switch statement that acts something like the Python elifstatement under certain conditions. The switch statement is not used very often, and we generally recommend you avoid using it. It is not as powerful as the else if model because the switch variable can only be compared for equality with a very small class of types.
Here is a quick example of a switch statement:
import java.util.Scanner;
public class DayPrinter {
public static void main(String[] args) {
Scanner in = new Scanner(System.in);
System.out.println("Enter an integer: ");
int dayNum = in.nextInt();
String day;
switch (dayNum) {
case 0:
day = "Sunday";
break;
case 1:
day = "Monday";
break;
case 2:
day = "Tuesday";
break;
case 3:
day = "Wednesday";
break;
case 4:
day = "Thursday";
break;
case 5:
day = "Friday";
break;
case 6:
day = "Saturday";
break;
default:
// in this example, this block runs if none of the above blocks match
day = "Int does not correspond to a day of the week";
}
System.out.println(day);
}
}In the example above, if the user entered the number 4, "Thursday" is what would be printed. If the user entered the number 10, "Int does not correspond to a day of the week" would be printed.
Additionally, if break statements are omitted from the individual cases on accident, a behavior known as fallthrough is carried out. Fallthrough can be quite unintuitive, and is only desirable in very specific circumstances. We will discuss break statements in more detail in the loop section below, but for now just know that when used in a switch block they terminate the switch statement they are in, so the flow of control in your program moves to the next statement after the switch block.
Here's a quick example of how fallthrough works:
import java.util.Scanner;
public class DayPrinter {
public static void main(String[] args) {
System.out.println("Enter an integer: ");
Scanner in = new Scanner(System.in);
int dayNum = in.nextInt();
String day;
switch (dayNum) {
case 0:
day = "Sunday";
case 1:
day = "Monday";
case 2:
day = "Tuesday";
case 3:
day = "Wednesday";
case 4:
day = "Thursday";
case 5:
day = "Friday";
case 6:
day = "Saturday";
default:
// in this example, this block runs even if one of the above blocks match
day = "Int does not correspond to a day of the week";
}
System.out.println(day);
}
}This time, without the break statements in each case, if the user entered 4, "Int does not correspond to a day of the week" would be printed. This is because after the switch statement matches the case for 4 and assigns the value "Thursday" to the variable day, it proceeds to execute every statement in every case that follows, all the way through the default case. So the String that ends up being printed will reflect the last executed statement in the switch block.
As another variation on this example, if there were a break statement in case 6 after day = "Saturday";, then if the user entered 4, the execution would fallthrough until it reached that break statement and "Saturday" is what would be printed instead.
Iteration
At a conceptual level, loops in Java aren't that different from loops in Python, though the syntax varies significantly in some cases.
For Loops
In Python the easiest way to write a definite loop (aka a for loop) is using the for loop in conjunction with the range function. For example:
for i in range(10):
print iIn Java we would write this as:
for (int 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 analogous to the last option giving you explicit control over the starting, stopping, and stepping in the three clauses inside the parentheses. 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 iIn Java we would write this as:
for (int 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, or for-in loop.
In Python we can iterate over a list as follows:
k = [1, 1, 2, 3, 5, 8, 13, 21]
for i in k:
print iIn Java, this would look like:
int k[] = {1, 1, 2, 3, 5, 8, 13, 21};
for(int i : k) {
System.out.println(i);
}This version translates nicely to iterating through a string, where we can convert the string to an array of characters:
String msg = "Hello World";
for (char c : msg.toCharArray()) {
System.out.println(c);
}While Loops
Both Python and Java support the while loop, or indefinite loop. Recall that in Python the while loop is written as:
while condition:
statement1
statement2
...In Java we add parentheses and curly braces to get:
while (condition) {
statement1
statement2
...
}Java adds an additional, if seldom used, variation of the while loop called the do-while loop. The do-while 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);Break and Continue Statements
There are instances where you may want to terminate a loop if a given condition is met. In these instances, the break statement comes in handy. For example, if you want to loop through an array of integers searching for a given integer and you want to quit the loop once that number is found, you can do the following:
public class testBreak {
public static void main(String [] args) {
int[] someInts = {1, 10, 2, 3, 5, 8, 10};
int searchTerm = 10;
for(int oneInt : someInts) {
if (oneInt == searchTerm) {
System.out.println("Found it!");
break;
}
}
}In the code above, instead of the for loop iterating through all the integers in the array, it will stop after it finds the first matching instance. So once it finds the first 10 in the array, it prints "Found it!" and then terminates the loop. If the break statement weren't there, the loop would continue and when it found the second 10 it would print "Found it!" a second time.
Note that the break statement terminates the innermost loop that it is contained within. So if you have nested loops and use a break statement within the innermost loop, then it will only terminate that loop and not the outer one.
The continue statement is similar to, but importantly different from, the break statement. Like break, it interrupts the normal flow of control of the loop. But unlike break, the continue statement only terminates the current iteration of the loop. So the loop will continue to run from the top (as long as the boolean expression that controls the loop is still true) after a continue statement. Here is an example:
public class testContinue {
public static void main(String [] args) {
int[] someInts = {1, 10, 2, 3, 5, 8, 10};
int searchTerm = 10;
for(int oneInt : someInts) {
if (oneInt == searchTerm) {
System.out.println("Found it!");
continue;
}
System.out.println("Not here");
}
}
}The above program will print "Not here" on every iteration of the for loop except where the number has been found. So the output looks like this:
Not here
Found it!
Not here
Not here
Not here
Not here
Found it!Because of the continue statement, the final print statement in the for loop is skipped. If the continue statement weren't there, the output would look like this instead (notice the extra "Not here" printouts):
Not here
Found it!
Not here
Not here
Not here
Not here
Not here
Found it!
Not here