In Java, pattern matching with switch statements or expressions can fail with a compiler error like:

'does not cover all possible input values'

This happens because the compiler requires exhaustive coverage of all possible cases for a given type. For InetAddress, which is a non-sealed abstract class, the compiler cannot ensure that all possible subclasses of InetAddress are covered.

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Key Concepts

1. Exhaustiveness Check:
   • For switch to work, all possible types must be covered explicitly, or a default case must be provided.
   • This is achievable for enum types, sealed classes, or when you use a default branch.
2. InetAddress Hierarchy:
   • InetAddress is an abstract class with two known direct subclasses in the standard library: Inet4Address and Inet6Address.
   • However, InetAddress is not sealed, meaning additional, user-defined subclasses can exist. The compiler cannot assume that Inet4Address and Inet6Address are the only subclasses.

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Example of the Problem

import java.net.InetAddress;

public class InetAddressSwitch {
    public static void main(String[] args) throws Exception {
        InetAddress address = InetAddress.getByName("127.0.0.1");

        switch (address) {
            case Inet4Address inet4Address -> System.out.println("IPv4 Address");
            case Inet6Address inet6Address -> System.out.println("IPv6 Address");
        }
    }
}

Compiler Error:

The switch statement does not cover all possible input values.

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Why Does This Happen?

The compiler cannot verify that all possible subclasses of InetAddress are handled because:

• InetAddress is not sealed (in Java 17 or later).
• New, user-defined subclasses of InetAddress could exist, making the hierarchy open-ended.

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Fixes

1. Add a default Case

The simplest fix is to provide a default case to handle any unmatched values.

import java.net.InetAddress;
import java.net.Inet4Address;
import java.net.Inet6Address;

public class InetAddressSwitch {
    public static void main(String[] args) throws Exception {
        InetAddress address = InetAddress.getByName("127.0.0.1");

        switch (address) {
            case Inet4Address inet4Address -> System.out.println("IPv4 Address");
            case Inet6Address inet6Address -> System.out.println("IPv6 Address");
            default -> System.out.println("Unknown Address Type");
        }
    }
}

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2. Use instanceof for Pattern Matching

If you’re using an earlier Java version or want to avoid switch, you can use instanceof.

import java.net.InetAddress;
import java.net.Inet4Address;
import java.net.Inet6Address;

public class InetAddressInstanceOf {
    public static void main(String[] args) throws Exception {
        InetAddress address = InetAddress.getByName("127.0.0.1");

        if (address instanceof Inet4Address) {
            System.out.println("IPv4 Address");
        } else if (address instanceof Inet6Address) {
            System.out.println("IPv6 Address");
        } else {
            System.out.println("Unknown Address Type");
        }
    }
}

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3. Manually Verify Exhaustiveness

If you are confident that no other subclasses of InetAddress will appear (e.g., in a controlled environment), you can suppress the compiler warning (not recommended in most cases).

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Key Takeaways

1. The error occurs because the switch cannot guarantee that all possible subclasses of InetAddress are handled.
2. To resolve the issue:
   • Add a default case to handle unmatched cases.
   • Use instanceof checks as an alternative to switch.
3. This limitation arises from InetAddress being non-sealed, allowing user-defined subclasses that the compiler cannot anticipate.