Java: What is the difference between checked and unchecked exceptions?

Checked — compile-time enforced. Unchecked — runtime only.

Analogy: Checked = required safety inspections. Unchecked = unexpected accidents.

Checked:

• Extend Exception (not RuntimeException).
• Must catch or declare throws.
• Recoverable errors.
• Examples: IOException, SQLException.

1public void readFile(String path) throws IOException {
2    FileInputStream fis = new FileInputStream(path);
3}
4
5public void loadConfig() {
6    try {
7        FileInputStream fis = new FileInputStream("config.txt");
8    } catch (FileNotFoundException e) {
9        System.err.println("Not found");
10    } catch (IOException e) {
11        System.err.println("Error");
12    }
13}

Unchecked:

• Extend RuntimeException.
• No compile-time enforcement.
• Programming bugs.
• Examples: NullPointerException, ArithmeticException.

1String str = null;
2// str.length(); // NullPointerException
3
4int[] arr = new int[5];
5// arr[10] = 1; // ArrayIndexOutOfBoundsException
6
7int result = 10 / 0; // ArithmeticException
8
9// Custom unchecked:
10public void setAge(int age) {
11    if (age < 0) throw new IllegalArgumentException("Invalid");
12    this.age = age;
13}

Hierarchy:

Differences:

• Checked — compile-time. Unchecked — runtime.
• Checked — Exception. Unchecked — RuntimeException.
• Checked — recoverable. Unchecked — bugs.

Best practices:

• Handle checked exceptions properly.
• Use unchecked for programming errors.
• Never catch Error.
• Create custom checked for recoverable errors.
• Create custom unchecked for programming errors.