With pattern matching for switch (Java 21+), do we still need polymorphism?

Yes, polymorphism is still essential — pattern matching for switch and virtual dispatch solve different problems along the expression-problem axis. Polymorphism is for open sets of types where you keep adding implementations; pattern matching with sealed types is for closed sets where you keep adding operations from outside the data.

The expression problem

You have two axes of extension:

• Add a new type (e.g., a new Shape).
• Add a new operation (e.g., serialize, area, render).

A given language feature usually makes one easy and the other painful:

The two are duals. Neither replaces the other.

When polymorphism wins

interface Shape { double area(); }
record Circle(double r) implements Shape  { public double area() { return Math.PI*r*r; } }
record Square(double s) implements Shape  { public double area() { return s*s; } }
// Tomorrow: drop in Triangle. Nobody else changes.

This is the Open/Closed Principle in action. The set of Shapes is open — third parties, plugins, future you all add new ones. The operations on Shape are few and stable. Virtual dispatch is the right tool.

When pattern matching wins

sealed interface Json permits JsonNull, JsonBool, JsonNum, JsonStr, JsonArr, JsonObj {}
// ...
String render(Json j) {
    return switch (j) {
        case JsonNull n      -> "null";
        case JsonBool(var b) -> Boolean.toString(b);
        case JsonNum(var n)  -> n.toString();
        case JsonStr(var s)  -> "\"" + s + "\"";
        case JsonArr(var es) -> es.stream().map(this::render).collect(joining(",", "[", "]"));
        case JsonObj(var fs) -> ...;
    };   // exhaustive — compiler proves it
}

The set of Json node types is closed (the spec defines exactly six). The operations are many — render, validate, query, transform, diff. Putting all those operations as virtual methods on Json would bloat the interface and force every JSON library to provide all of them. Pattern matching keeps each operation as a standalone function and gives you exhaustiveness checking for free.

The senior synthesis