Java flow control
last modified February 22, 2024
In this article we cover program flow control in Java. We use several keywords that enable us to control the flow of a Java program.
Control flow statements
In Java language there are several keywords that are used to alter the flow of
the program. Statements can be executed multiple times or only under a specific
condition. The if, else, and switch
statements are used for testing conditions, the while and
for statements to create cycles, and the
break and continue statements to alter a loop.
When the program is run, the statements are executed from the top of the source file to the bottom. One by one.
The if statement
The if statement has the following general form:
if (expression) {
statement;
}
The if keyword is used to check if an expression is true. If it is
true, a statement is then executed. The statement can be a single statement or a
compound statement. A compound statement consists of multiple statements
enclosed by a block. A block is code enclosed by curly brackets. The brackets
are optional if we have only one statement in the body.
import java.util.Random;
void main() {
var r = new Random();
int num = r.nextInt();
if (num > 0) {
System.out.println("The number is positive");
}
}
A random number is generated. If the number is greater than zero, we print a message to the terminal.
var r = new Random(); int num = r.nextInt();
These two lines generate a random integer. The number can be positive or negative.
if (num > 0) {
System.out.println("The number is positive");
}
Using the if keyword, we check if the generated number is greater
than zero. The if keyword is followed by a pair of round brackets.
Inside the brackets, we place an expression. The expression results in a boolean
value. If the boolean value is true, then the block enclosed by two curly
brackets is executed. In our case, the string "The number is positive" is
printed to the terminal. If the random value is negative, nothing is done. The
curly brackets are optional if we have only one expression.
The else keyword
We can use the else keyword to create a simple branch.
If the expression inside the square brackets following the if
keyword evaluates to false, the statement following the else
keyword is automatically executed.
import java.util.Random;
void main() {
var r = new Random();
int num = r.nextInt();
if (num > 0) {
System.out.println("The number is positive");
} else {
System.out.println("The number is negative");
}
}
Either the block following the if keyword or the block following
the else keyword is executed.
if (num > 0) {
System.out.println("The number is positive");
} else {
System.out.println("The number is negative");
}
The else keyword follows the right curly bracket of the
if block. It has its own block enclosed by a pair of curly
brackets.
$ java Main.java The number is positive $ java Main.java The number is negative $ java Main.java The number is negative
Multiple branches with if else
We can create multiple branches using the else if keyword. The
else if keyword tests for another condition if and only if the
previous condition was not met. Note that we can use multiple
else if keywords in our tests.
The previous program had a slight issue. Zero was given to negative values. The following program will fix this.
import java.util.Scanner;
void main() {
System.out.print("Enter an integer:");
try (var sc = new Scanner(System.in)) {
int num = sc.nextInt();
if (num < 0) {
System.out.println("The integer is negative");
} else if (num == 0) {
System.out.println("The integer equals to zero");
} else {
System.out.println("The integer is positive");
}
}
}
We receive a value from the user test it if it is a negative number or positive, or if it equals to zero.
System.out.print("Enter an integer:");
A prompt to enter an integer is written to the standard output.
Scanner sc = new Scanner(System.in); int num = sc.nextInt();
Using the Scanner class of the java.util package, we
read an integer value from the standard input.
if (num < 0) {
System.out.println("The integer is negative");
} else if (num == 0) {
System.out.println("The integer equals to zero");
} else {
System.out.println("The integer is positive");
}
If the first condition evaluates to true, e.g. the entered value is less than
zero, the first block is executed and the remaining two blocks are skipped. If
the first condition is not met, then the second condition following the if
else
keywords is checked. If the second condition evaluates to true, the second block
is executed. If not, the third block following the else keyword is
executed. The else block is always executed if the previous
conditions were not met.
$ java Main.java Enter an integer:4 The integer is positive $ java Main.java Enter an integer:0 The integer equals to zero $ java Main.java Enter an integer:-3 The integer is negative
The switch statement
The switch statement is a selection control flow statement. It
allows the value of a variable or expression to control the flow of a program
execution via a multi-way branch. It creates multiple branches in a simpler way
than using the combination of if and else if
statements. Each branch is ended with the break keyword.
We use a variable or an expression. The switch keyword is used
to test a value from the variable or the expression against a list of values.
The list of values is presented with the case keyword.
If the values match, the statement following the case is executed.
There is an optional default statement. It is executed if no other
match is found.
import java.util.Scanner;
void main() {
System.out.print("Enter a domain:");
try (var sc = new Scanner(System.in)) {
String domain = sc.nextLine();
domain = domain.trim().toLowerCase();
switch (domain) {
case "us":
System.out.println("United States");
break;
case "de":
System.out.println("Germany");
break;
case "sk":
System.out.println("Slovakia");
break;
case "hu":
System.out.println("Hungary");
break;
default:
System.out.println("Unknown");
break;
}
}
}
The user is requested to enter a domain name. The domain name is read
and stored in a variable. The variable is tested with the switch
keyword against a list of options. In our program, we have a domain variable. We
read a value for the variable from the command line. We use the
case statement to test for the value of the variable. There are
several options. If the value equals for example to "us", the "United States"
string is printed to the console.
try (var sc = new Scanner(System.in)) {
String domain = sc.nextLine();
The input from the user is read from the console.
domain = domain.trim().toLowerCase();
The trim method strips the variable from potential leading and
trailing white spaces. The toLowerCase converts the characters to
lowercase. Now the "us", "US", or "us " are viable options for the us domain
name.
switch (domain) {
...
}
In the round brackets, the switch keyword takes an input which is
going to be tested. The input can be of byte, short,
char, int, enum, or String
data type. The body of the switch keyword is placed inside a pair
or curly brackets. Inside the body, we can place multiple case
options. Each option is ended with the break keyword.
case "us":
System.out.println("United States");
break;
In this case option, we test if the domain variable is equal to "us" string. If
true, we print a message to the console. The option is ended with the
break keyword. If one of the options is successfully evaluated,
the break keyword terminates the switch block.
default:
System.out.println("Unknown");
break;
The default keyword is optional. If none of the case
options is evaluated, then the default section is executed.
$ java Main.java Enter a domain:us United States
The switch expression
Java switch expression simplifies the original switch statement. It allows using multiple case labels called arms.
import java.util.Scanner;
void main() {
System.out.print("Enter a domain: ");
try (var sc = new Scanner(System.in)) {
String domain = sc.nextLine();
domain = domain.trim().toLowerCase();
switch (domain) {
case "us" -> System.out.println("United States");
case "de" -> System.out.println("Germany");
case "sk" -> System.out.println("Slovakia");
case "hu" -> System.out.println("Hungary");
default -> System.out.println("Unknown");
}
}
}
The previous switch statement example is rewritten using switch expression.
case "us" -> System.out.println("United States");
The switch expression the -> to separate label from the
statement. The break statement is not needed, once an option is
matched, other options are not evaluated.
Switch expressions can return values.
import java.util.Scanner;
void main() {
System.out.print("Enter a domain: ");
try (var sc = new Scanner(System.in)) {
String domain = sc.nextLine();
domain = domain.trim().toLowerCase();
String res = switch (domain) {
case "us" -> "United States";
case "de" -> "Germany";
case "sk" -> "Slovakia";
case "hu" -> "Hungary";
default -> "Unknown";
};
System.out.println(res);
}
}
This is a modified version of the previous example. The arms now return a string value, which is printed later.
The while statement
The while statement is a control flow statement
that allows code to be executed repeatedly based on a given boolean
condition.
while (expression) {
statement;
}
The while keyword executes the statements inside the block enclosed
by the curly brackets. The statements are executed each time the expression is
evaluated to true.
void main() {
int i = 0;
int sum = 0;
while (i < 10) {
i++;
sum += i;
}
System.out.println(sum);
}
In the code example, we calculate the sum of values from a range of numbers.
The while loop has three parts: initialization, testing, and
updating. Each execution of the statement is called a cycle.
int i = 0;
We initiate the i variable. It is used as a counter.
while (i < 10) {
...
}
The expression inside the round brackets following the while
keyword is the second phase, the testing. The statements in the body are
executed until the expression is evaluated to false.
i++;
The last phase of the while loop is the updating. We increment
the counter. Note that improper handling of the while loops may lead
to endless cycles.
$ Main.java 55
The program calculated the sum of 0, 1, ..., 9 values.
There is a modified version of the while statement. It is the
do while statement. It is guaranteed that the statements
inside the block are run at least once, even if the condition is not met.
void main() {
int count = 0;
do {
System.out.println(count);
} while (count != 0);
}
First the block is executed and then the truth expression is evaluated. In our
case, the condition is not met and the do while statement
terminates.
The for statement
When the number of cycles is know before the loop is initiated, we can use the
for statement. In this construct we declare a counter variable,
which is automatically increased or decreased in value during each repetition of
the loop.
void main() {
for (int i = 0; i < 10; i++) {
System.out.println(i);
}
}
In this example, we print numbers 0..9 to the console.
for (int i = 0; i < 10; i++) {
System.out.println(i);
}
There are three phases in a for loop. First, we initiate the counter
i to zero. This phase is done only once. Next comes the condition.
If the condition is met, the statement inside the for block is executed. Then
comes the third phase: the counter is increased. Now we repeat 2 and 3 phases
until the condition is not met and the for loop is terminated. In our case, when
the counter i is equal to 10, the for loop stops executing.
A for loop can be used for easy traversal of an array. From the
length property of the array, we know the size of the array.
void main() {
String[] planets = { "Mercury", "Venus", "Earth",
"Mars", "Jupiter", "Saturn", "Uranus", "Pluto" };
for (int i = 0; i < planets.length; i++) {
System.out.println(planets[i]);
}
System.out.println("In reverse:");
for (int i = planets.length - 1; i >= 0; i--) {
System.out.println(planets[i]);
}
}
We have an array holding the names of planets in our Solar System. Using two for loops, we print the values in ascending and descending orders.
for (int i = 0; i < planets.length; i++) {
System.out.println(planets[i]);
}
The arrays are accessed by zero-based indexing. The first item has index 0.
Therefore, the i variable is initialized to zero. The condition
checks if the i variable is less than the length of the array. In
the final phase, the i variable is incremented.
for (int i = planets.length - 1; i >= 0; i--) {
System.out.println(planets[i]);
}
This for loop prints the elements of the array in reverse order. The
i counter is initialized to array size. Since the indexing is zero
based, the last element has index array size-1. The condition ensures that the
counter is greater or equal to zero. (Array indexes cannot be negative). In the
third step, the i
counter is decremented by one.
More expressions can be placed in the initialization and iteration phase of the for loop.
import java.util.Arrays;
import java.util.Random;
void main() {
var r = new Random();
int[] values = new int[10];
int num;
int sum = 0;
for (int i = 0; i < 10; i++, sum += num) {
num = r.nextInt(10);
values[i] = num;
}
System.out.println(Arrays.toString(values));
System.out.println("The sum of the values is " + sum);
}
In our example, we create an array of ten random numbers. A sum of the numbers is calculated.
for (int i = 0; i < 10; i++, sum += num) {
num = r.nextInt(10);
values[i] = num;
}
In the third part of the for loop, we have two expressions separated by
a comma character. The i counter is incremented and the current
number is added to the sum variable.
$ java Main.java [1, 9, 2, 9, 0, 9, 8, 5, 5, 3] The sum of the values is 51
Enhanced for statement
The enhanced for statement simplifies traversing over collections
of data. It has no explicit counter. The statement goes through an array or a
collection one by one and the current value is copied to a variable defined in
the construct.
void main() {
String[] planets = {
"Mercury", "Venus", "Earth",
"Mars", "Jupiter", "Saturn", "Uranus", "Pluto"
};
for (String planet : planets) {
System.out.println(planet);
}
}
In this example, we use the enhanced for statement to go through an
array of planets.
for (String planet : planets) {
System.out.println(planet);
}
The usage of the for statement is straightforward.
The planets is the array that we iterate through. A planet
is the temporary variable that has the current value from the array. The
for statement goes through all the planets and prints them to the
console.
$ java Main.java Mercury Venus Earth Mars Jupiter Saturn Uranus Pluto
The break statement
The break statement can be used to terminate a block defined by
while, for, or switch statements.
import java.util.Random;
void main() {
var random = new Random();
while (true) {
int num = random.nextInt(30);
System.out.print(num + " ");
if (num == 22) {
break;
}
}
System.out.print('\n');
}
We define an endless while loop. We use the break
statement to get out of this loop. We choose a random value from 1 to 30 and
print it. If the value equals to 22, we finish the endless while loop.
while (true) {
...
}
Placing true in the brackets of the while statement creates an endless loop. We must terminate the loop ourselves. Note that such code is error-prone. We should be careful using such loops.
if (num == 22) {
break;
}
When the randomly chosen value is equal to 22, the break
statement is executed and the while loop is terminated.
$ java Main.java 23 12 0 4 13 16 6 12 11 9 24 23 23 19 15 26 3 3 27 28 25 3 3 25 6 22 $ java Main.java 23 19 29 27 3 28 2 2 26 0 0 24 17 4 7 12 8 20 22 $ java Main.java 15 20 10 25 2 19 26 4 13 21 15 21 21 24 3 22
The continue statement
The continue statement is used to skip a part of the loop and
continue with the next iteration of the loop. It can be used in combination with
for and while statements.
In the following example, we print a list of numbers that cannot be divided by 2 without a remainder.
void main() {
int num = 0;
while (num < 100) {
num++;
if (num % 2 == 0) {
continue;
}
System.out.print(num + " ");
}
System.out.print('\n');
}
We iterate through numbers 1..99 with the while loop.
if (num % 2 == 0) {
continue;
}
If the expression num % 2 returns 0, the number in question can be
divided by 2. The continue statement is executed and the rest of
the cycle is skipped. In our case, the last statement of the loop is skipped and
the number is not printed to the console. The next iteration is started.
Source
Java Control Flow statements - tutorial
In this article we were talking about control flow structures. We have covered
if, if else, else, while,
switch, for, break, continue
statements.
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