1. Introduction
Execution of a Java program normally occurs from top to bottom, statement after statement. However, there are situations where you want to run a code block only if a condition is true. At the same time, you may want to keep executing a block of statements as long as another condition is verified. Flow control statements are the solution to this problem. There are three main types of flow control statements: Selection Statements, Iteration Statements, and Transfer Statements. In this article, we will cover all the details about each category.
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2. Selection Statements
Java provides statements that allow your program to choose between multiple paths as it executes. The choice of which path to execute depends on the condition specified in the selection statement.
2.1. The Simple if Statement
This structure allows a statement or a bloc of statements to be executed if a condition is verified. Its semantics is as follows:
if(condition)
if_body
Where condition is a boolean expression and if_body is a single statement or a block statement.
int i = 0;
if(i < 5)
System.out.println("i is less than 5");
It is a good practice to always use curly braces even for a single statement. Consider the following code:
int i = 0;
int j = 5;
if(i > 0)
System.out.println("i is greater than 0");
System.out.println("Division by zero will be performed: " +j /i);//is not part of the if statement
The code above will throw an ArithmeticException because the second statement is not part of the if body. This is why you should always use curly braces.
int i = 0;
int j = 5;
if(i > 0) {//start of if body
System.out.println("i is greater than 0");
System.out.println("Division by zero will NOT be performed: " + j / i);
}//end of if body
2.2. The if-else Statement
Use the if-else if you need to choose between two or more execution paths. It has the following syntax:
if(condition)
if_body
else
else_body
The else_body block is executed if the condition is evaluated to false. Just like for the simple if, it is recommended to always use curly braces to improve readability and prevent unexpected errors.
int i = 0;
if(i < 5) {
System.out.println("i is less than 5");
}else{
System.out.println("i is greater than 5");
}
You may have more than two branches:
int i = 0;
if(i < 5) {
System.out.println("i is less than 5");
}else if(i < 10){
System.out.println("i is less than 10");
}else{
System.out.println("i is greater than 10");
}
It is also possible to have nested if-else:
int i = 0;
if(i < 5) {
System.out.println("i is less than 5");
if(i>3){
System.out.println("i is less than 5 and greater than 3");
}else{
System.out.println("i is less than 5 and less than or equal to 3");
}
}
2.3. The switch Statement
You can read about the switch construct in this dedicated article.
3. Iteration Statements
These Statements are used when you want to repeat the same processing as long as a certain condition is verified.
3.1. the while loop Statement
The while loop is used when you do not know a priori the number of iterations to do. Its syntax is as follows:
while(loop_condition)
loop_body
Where loop_condition is a boolean expression and loop_body can be any valid expression statement.
Example with a Single Statement
int i = 0;
while(i < 5)
i++;
There is no semicolon(;) after the condition expression while(condition). Doing so will result in an empty loop body.
int i = 0;
while(i < 5);//This semicolon creates an empty loop body
i++;//This is not part of the loop body
It is a good practice to always use curly braces to improve readability.
int i = 0;
while(i < 5) {
i++;
}
Example with a Block Statement
int i = 0;
while(i<5) {//start of while loop
System.out.println("Value of i : "+i);
i++;
}//end of while loop
The while Statement will start by evaluating the condition expression. if it is true, it will start executing the loop body. It will keep executing it as long as the condition is met.
As a consequence:
The loop body will be executed zero to many times.
3.2. the do-while loop Statement
The do-while loop is similar to the while loop with a few differences (See below). Below are the semantics of the do-while:
do
loop_body
while(loop_condition)
Example with a Simple Statement
int i = 0;
do
i++;
while(i < 5);
Note that there is a semicolon (;) after the while(condition).
You cannot add multiple statements without using the curly braces. The following code will not compile:
int i = 0;
do
System.out.println("Value of i : "+i);
i++;
while(i < 5);
Example with a Block Statement
int i = 0;
do {//start of do-while loop
System.out.println("Value of i : " + i);
i++;
}while(i < 5);//end of do-while loop
Unlike the while statement, the do-while examines the loop condition after executing the loop body. It starts by executing the loop body and keeps executing it until the loop condition becomes false.
The
do-whilestatement is executed at least once.
3.3. the for(;;) loop Statement
The for loop is the most common loop. It is used when the number of iterations is known in advance. Its syntax is as follows:
for(initialization; loop_condition; update_expression)
loop_body
The initialization block is generally used to declare the variables which will control the execution of the loop. The Condition allows you to limit the number of iterations by indicating when to stop. It must be a Boolean expression. The update block allows you to update the loop variables. This ensures that the loop evolves and does not lead to an infinite loop. As with the while and do-while loops, the loop_body must be a [valid expression]().
Any for(;;) loop can be converted into a whileby proceeding as follows:
initialization;
while(loop_condition){
loop_body;
update_expression
}
Example with a Simple Statement
for(int i=0; i < 5; i++)
System.out.println("Value of i : " + i);
Just like with the while loop, it is advised to always use curly braces for readability and to prevent any unexpected behavior.
for(int i=0; i < 5; i++) {
System.out.println("Value of i : " + i);
}
Example with a Block Statement
for(int i=0; i < 5; i++) {//start of for loop
System.out.println("Value of i : " + i);
System.out.println("Another Statement in the same block");
}//end of for loop
Example with Multiple Initialization Expressions
You may add multiple expressions in the initialization section:
for(int i=0, j=1, k=2; i < 5; i++) {
System.out.println("Value of i : " + i);
}
Example with Multiple Update Expressions
You can also add multiple expressions in the update_expression section:
for(int i=0, j=1, k=2; i < 5; i++,j--,k++) {
System.out.println("Value of i : " + i);
}
Example of an Infinite Loop
All the sections of the for(;;) loop are optional. Leaving all of them empty will result in an infinite loop:
for(;;) {
System.out.println("This is an infinite loop");
}
3.4. the for(:) loop Statement
This loop is also called an enhanced for loop. It makes it easier to iterate on array or Collection objects. Its syntax is as follows:
for(itemType item : items){
loop_body
}
itemType is the data type of the elements in the items structure. The object items must either be an array or an object that implements the Iterable interface. An example with an array of integers is shown below:
int[] items = {1,2,3,4,5};
int sum = 0;
for(int item : items){
sum += item;
}
This for(:) loop is more compact than the for(;;) loop we saw earlier . You may consider using this enhanced form when you don’t need to update the elements in the array (or the collection).
4. Transfer Statements
4.1. Label Statements
A statement may have a label:
label: statement;
label can be any valid Java identifier. Labels have their own namespace, so they cannot conflict with the names of packages, classes, methods, variables, etc.
Below is an example of a label:
myLabel:{
int i = 0;
System.out.println(i + 1);
}
A statement can have multiple labels:
labelA:
labelB:
System.out.println("I belong to labelA and labelB");
A declaration statement cannot have a label:
myLabel:
int i = 0;//Compile-time error
The label doesn’t change anything to the behaviour of a statement. A label statement is executed just as if it were unlabeled.
4.2. The break Statement
There are two forms of break: the labeled version and the unlabeled version.
The unlabeled break
This form terminates the execution of the following statements: switch, while, do-while, for(;;), and for(:). Here is the syntax:
break;
Below is an example with a forloop:
for(int i=0; i < 5; i++) {
if(i==2)
break;//stop the loop and go to (1)
System.out.println("Value of i : " + i);
}
System.out.println("I'm out of the loop");//1
In case of nested loops, the unlabeled
breakapplies to the innermost loop.
The labeled break
The labeled breakstatement can be used to terminate any labeled that contains the break statement.
its semantic is as follows:
break myLabel;
Here is an example:
int i = 0;
myLabel:{
//
if(i==2)
break myLabel;//get out of the label myLabel and go to (1)
System.out.println("Value of i : " + i);
i++;
//
}
System.out.println("I'm out of the label");//1
4.3. The continue Statement
Just like the break statements, there are two forms of continue: the labeled version and the unlabeled version.
The unlabeled continue
The unlabeled continue works similarly to the unlabeled break. However, instead of stopping the execution, it will move the execution to the next iteration in case of for(;;) and for(::) loops. For while and do-while loops, it will move the execution to the loop condition evaluation.
An example with the while loop is shown below:
int i = 0;
while(i<5) {//start of while loop
if(i==2) {
i++;
continue;
}
System.out.println("Value of i : "+i);//This will not be printed for i == 2
i++;
}//end of while loop
In case of nested loops, the unlabeled
continueapplies to the innermost loop.
The labeled continue
The labeled continue statement skips the current iteration of an outer loop marked with the given label.
myLabel:
for (int i=0; i< 5; i++){
System.out.println("In the first loop i="+i);
for(int j=0; j< 5; j++){
if(j==2)
continue myLabel;
System.out.println("In the second loop j="+j);
}
}
Without the label, this continue statement will apply to the innermost statement.
4.4. The return Statement
The return statement is used to stop the execution of a method and hand over to the caller. The syntax depends on the return type of the method.
A Method without a Return Value
When the return type of the method is void, you don’t need to use the return statement. You may however use it if you want to end the method execution when a specific condition is met.
public void displayArray(int[] arr){
if(arr == null)
return;
for(int item : arr){
System.out.println(item);
}
}
In the code above, we are using the return statement to stop the execution of the displayArray() method if the provided array is null. This check prevents us from having a NullPointerException at runtime.
A Method with a Return Value
The return statement is mandatory in this case if the method is not terminated explicitly.
public int performSum(int[] arr){
if(arr == null)
return 0;//Removing the 0 will lead to a compile-time error
int sum = 0;
for(int item : arr){
sum += item;
}
return sum;
}
Throwing an exception is a valid way of terminating a program. While doing that you don’t have to use the
returnstatement.
5. Conclusion
In conclusion, mastering control flow statements in Java is crucial for you to run your programs smoothly. In this article, you learned the different statements that allow you to control the execution flow of your Java program.
6. References
1) OCP Oracle Certified Professional Java SE 17 by Khalil A. Mughal and Vasily A. Strelnikov
2) Oracle Java Documentation
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