Throw keyword in Java - Step-by-Step Execution

Concept Flow - Throw keyword

Start

↓

Check condition

Yes|No↓

Throw Exception

↓

Exception Propagates

↓

Catch or Program Ends

The program checks a condition, and if true, it throws an exception which propagates until caught or program ends.

Execution Sample

Java

public class Test {
  public static void main(String[] args) {
    int age = 15;
    if (age < 18) {
      throw new IllegalArgumentException("Age must be 18 or older");
    }
  }
}

This code throws an exception if age is less than 18.

Execution Table

Step	Action	Condition	Result	Exception Thrown
1	Set age = 15	-	age = 15	No
2	Check if age < 18	15 < 18	True	No
3	Throw IllegalArgumentException	-	-	Yes: "Age must be 18 or older"
4	Program stops or exception propagates	-	-	Exception propagates

💡 Exception thrown at step 3 stops normal execution

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
age	undefined	15	15	15

Key Moments - 2 Insights

Why does the program stop after the throw statement?

Because at step 3 in the execution_table, the throw keyword creates an exception that interrupts normal flow and propagates unless caught.

Is the exception thrown even if the condition is false?

No, at step 2 the condition is checked; if false, the throw statement is skipped and no exception is thrown.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'age' at step 2?

A 15

B undefined

C 0

D null

💡 Hint

Check variable_tracker row for 'age' after Step 2

At which step does the exception get thrown?

A Step 2

B Step 3

C Step 1

D Step 4

💡 Hint

See execution_table 'Exception Thrown' column

If age was 20, how would the execution_table change?

A Exception thrown at step 3

B Program throws exception at step 1

C Condition at step 2 would be false, no exception thrown

D Program stops at step 2 with exception

💡 Hint

Refer to step 2 condition and exception thrown columns

Concept Snapshot

Throw keyword in Java:
- Used to throw an exception explicitly
- Syntax: throw new ExceptionType("message");
- Stops normal flow and propagates exception
- Usually inside condition checks
- Must be caught or declared to avoid program crash

Full Transcript

This example shows how the throw keyword works in Java. The program sets an integer variable age to 15. It then checks if age is less than 18. Since 15 is less than 18, the condition is true. The program then throws an IllegalArgumentException with a message. Throwing the exception stops the normal program flow and the exception propagates unless caught. The variable age remains 15 throughout. If the age was 20, the condition would be false and no exception would be thrown. This shows how throw interrupts execution when a condition requires it.