Multi-catch and Custom Exceptions

Multi-catch

A single catch block can handle multiple exception types with |:

java
try {
    process();
} catch (IOException | SQLException e) {
    log("Data error: " + e.getMessage());
}

• The variable e is implicitly final inside a multi-catch block — it cannot be reassigned.
• The types must not be in an inheritance relationship (would be a compile error):

java
catch (IOException | FileNotFoundException e) { }  // COMPILE ERROR — FileNotFoundException extends IOException

Rethrowing with Precise Type Inference

When a caught exception is rethrown and the compiler can determine its precise type, the throws declaration can be more specific:

java
void process() throws IOException, SQLException {
    try {
        riskyOperation();
    } catch (Exception e) {
        log(e);
        throw e;   // compiler infers IOException | SQLException, not Exception
    }
}

Custom Checked Exception

Extend Exception (or a checked subclass):

java
public class InsufficientFundsException extends Exception {

    private final double amount;

    public InsufficientFundsException(double amount) {
        super("Insufficient funds: need " + amount + " more");
        this.amount = amount;
    }

    public double getAmount() { return amount; }
}

Usage:

java
void withdraw(double amount) throws InsufficientFundsException {
    if (amount > balance) throw new InsufficientFundsException(amount - balance);
    balance -= amount;
}

Custom Unchecked Exception

Extend RuntimeException:

java
public class ConfigurationException extends RuntimeException {

    public ConfigurationException(String message) {
        super(message);
    }

    public ConfigurationException(String message, Throwable cause) {
        super(message, cause);
    }
}

The two-argument form preserves the original cause (useful for wrapping lower-level exceptions).

Exception Chaining

Wrapping one exception inside another preserves the root cause:

java
try {
    Files.readString(path);
} catch (IOException e) {
    throw new ConfigurationException("Cannot read config file", e);  // e is the cause
}

java
catch (ConfigurationException e) {
    System.out.println("Cause: " + e.getCause());
}

Best Practices

Common Standard Exceptions to Know