Checked vs unchecked exceptions in Java - Visual Side-by-Side Comparison

Concept Flow - Checked vs unchecked exceptions

Start

↓

Code runs

↓

Exception occurs?

No→Continue normal flow

Yes↓

Is exception checked?

Yes→Must handle or declare

↓

Handle or declare

↓

Unchecked exception

↓

Can be caught but not required

↓

Program may crash if uncaught

↓

End

The program runs and may throw exceptions. Checked exceptions must be handled or declared, while unchecked exceptions can be caught but are not required to be handled.

Execution Sample

Java

public void readFile() throws IOException {
  FileReader file = new FileReader("file.txt");
  int data = file.read();
  file.close();
}

This method reads a file and declares it throws IOException, a checked exception that must be handled or declared.

Execution Table

Step	Action	Exception Thrown?	Exception Type	Handling Required?	Result
1	Start method readFile()	No	-	No	Method starts normally
2	Create FileReader object	No	-	No	FileReader created
3	Call read() method	Yes	IOException (checked)	Yes	Exception must be handled or declared
4	Method declares throws IOException	N/A	N/A	Handled by declaration	Compilation succeeds
5	If exception occurs at runtime and not caught	Yes	IOException	Yes	Program throws exception at runtime
6	If exception caught in try-catch	Yes	IOException	Handled	Program continues safely
7	Throw unchecked exception (e.g., NullPointerException)	Yes	NullPointerException (unchecked)	No	Handling optional, program may crash if uncaught
8	End of method	No	-	No	Method ends

💡 Execution stops normally or with exception if uncaught

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
file	null	FileReader object	FileReader object	FileReader object	Closed or null
data	undefined	undefined	int value or exception	int value or exception	int value or exception

Key Moments - 3 Insights

Why must checked exceptions be declared or handled?

Checked exceptions like IOException are checked by the compiler to ensure the programmer handles possible errors, as shown in execution_table row 3 and 4.

Can unchecked exceptions be ignored without handling?

Yes, unchecked exceptions like NullPointerException do not require handling, but if uncaught, they can crash the program (see execution_table row 7).

What happens if a checked exception is not declared or caught?

The code will not compile because the compiler enforces handling or declaration of checked exceptions, as shown in execution_table row 3 and 4.

Visual Quiz - 3 Questions

Look at the execution_table, at which step is a checked exception first thrown?

A Step 2

B Step 3

C Step 7

D Step 5

💡 Hint

Check the 'Exception Thrown?' and 'Exception Type' columns in execution_table rows.

According to variable_tracker, what is the state of 'file' after step 2?

A null

B Closed

C FileReader object

D undefined

💡 Hint

Look at the 'file' row and 'After Step 2' column in variable_tracker.

If the IOException is not declared or caught, what happens according to the key_moments and execution_table?

A Program compiles and runs normally

B Compiler error, code does not compile

C Program crashes at runtime

D Exception is ignored silently

💡 Hint

Refer to key_moments about checked exceptions and execution_table rows 3 and 4.

Concept Snapshot

Checked exceptions must be declared or handled in code.
Unchecked exceptions do not require declaration or handling.
Checked exceptions are checked at compile time.
Unchecked exceptions occur at runtime and may crash the program if uncaught.
Use try-catch or throws declaration for checked exceptions.

Full Transcript

This visual execution shows how Java handles checked and unchecked exceptions. The program starts and runs code. If an exception occurs, it checks if it is checked or unchecked. Checked exceptions like IOException must be declared or handled, or the code will not compile. Unchecked exceptions like NullPointerException do not require handling but may crash the program if uncaught. The variable tracker shows how variables like the FileReader object change during execution. Key moments clarify why checked exceptions require handling and what happens if they are ignored. The quiz tests understanding of when exceptions occur and how they affect program flow.