Why Doesn't My Java Method Compile with Multi-Catch Exception Handling?

Why doesn't method m2() compile when using multi-catch Exception handling in Java?

public int m2(boolean b) {
    try {
        throw b ? new Excep1() : new Excep2();
    } catch (Excep1 | Excep2 e) {
        return 0;
    }
}

The m2() method does not compile due to the way Java handles type resolution in the throw statement combined with multi-catch blocks. Let’s break down why this occurs and how to resolve it.

public int m2(boolean b) {
    try {
        if (b) {
            throw new Excep1();
        } else {
            throw new Excep2();
        }
    } catch (Excep1 | Excep2 e) {
        return 0;
    }
}

Causes

• Java requires that the type being thrown is known at compile time.
• The ternary operator in m2() introduces ambiguity because it evaluates to either Excep1 or Excep2, but not a common supertype that can be caught in a single catch block.

Solutions

• Refactor the m2() method to separate the catch blocks for the different exception types.
• Modify the throw statement to eliminate the ternary operator and explicitly throw each exception in an if-else structure.

Mistake: Using a ternary operator to throw different exception types.

Solution: Use an if-else statement to ensure that the compiler can clearly determine the exception type.

Mistake: Assuming all exception types can be caught in a single catch block without a common superclass in some syntaxes.

Solution: Make sure exceptions thrown in a single context can be caught by a multi-catch if they share a common superclass or refactor the statement.