Course:
Learn Java Programming
In this lesson, you will learn
assert keyword.
Assertions are used during the testing of a program. They enable you to test the assumptions made in a program during its execution.
The assert keyword is used to implement assertions. Here’s the basic syntax for using assertions in Java:
assert expression;
Or, with an additional detail message:
assert expression : "error message";
Here the expression is a boolean condition. During program execution, the expression with assert statement is tested.
If it returns true then “The assertion made in the program is true and execution continues uninterruptedly”
If the expression evaluates to false, the assertion fails, and an AssertionError is thrown.
Assertions are enabled or disabled with the -ea (enable assertions) or -da (disable assertions) flag when running the JVM. By default, assertions are disabled.
Before you can successfully implement assertions in your code, you need to enable
them in the Java environment.
Please follow the following steps after creating a program.
2. Arguments -> VM Arguments -> Apply -> Run
package assertions;
import java.util.Scanner;
public class ValidateAge {
//Validate age using 'assert'
public static void validateAge(String name,int age) {
assert age>0 && age<100: "Age must be between 1 and 100";
System.out.println("The age of "+name+" is: "+age);
}
public static void main(String args[]) {
@SuppressWarnings("resource")
Scanner scanner = new Scanner(System.in);
System.out.print("Enter name: ");
String name = scanner.next();
System.out.print("Enter age: ");
int age = scanner.nextInt();
validateAge(name, age);
}
}
Test Case 1:
Enter name: James
Enter age: 32
The age of James is: 32
Test Case 2:
Enter name: James
Enter age: -10
Exception in thread "main" java.lang.AssertionError:
Age must be between 1 and 100
at assertions.ValidateAge.validateAge(ValidateAge.java:8)
at assertions.ValidateAge.main(ValidateAge.java:18)
In this example, we use an assertion to ensure that a method always returns a non-negative value. This is particularly useful in methods where the business logic should guarantee certain outcomes.
package assertions;
public class InventoryAssertion {
private int itemsCount;
public InventoryAssertion(int initialItems) {
assert initialItems >= 0 : "Initial items count cannot be negative";
this.itemsCount = initialItems;
}
public void addItem(int count) {
assert count > 0 : "Must add a positive number of items";
itemsCount += count;
}
public void removeItem(int count) {
assert count > 0 && count <= itemsCount :
"Invalid count of items to remove";
itemsCount -= count;
}
public int getItemsCount() {
assert itemsCount >= 0 : "Items count should never be negative";
return itemsCount;
}
public static void main(String[] args) {
InventoryAssertion inventory = new InventoryAssertion(10);
inventory.addItem(5);
inventory.removeItem(3);
System.out.println("Current inventory count: " + inventory.getItemsCount());
inventory.removeItem(15);
// This will trigger an AssertionError if assertions are enabled.
}
}
Test Case 1:
Current inventory count: 12
Test Case 2: After enabling Assertion in VM Arguments
Current inventory count: 12
Exception in thread "main" java.lang.AssertionError: Invalid count of items to remove
at assertions.InventoryAssertion.removeItem(InventoryAssertion.java:17)
at assertions.InventoryAssertion.main(InventoryAssertion.java:31)
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