Abstract Tree Walker using Type-safe Heterogeneous Container pattern.

AstWalker.java

import com.google.common.collect.Multimap;
import com.google.common.collect.MultimapBuilder;

import java.util.Collection;

import java.util.function.Consumer;

public class AstWalker {
    private final Multimap<Class<?>, Consumer<?>> enterNodeListeners = MultimapBuilder.linkedHashKeys().arrayListValues().build();
    private final Multimap<Class<?>, Consumer<?>> exitNodeListeners = MultimapBuilder.linkedHashKeys().arrayListValues().build();

    public <T extends BaseNode> void registerEnterNodeListener(Class<T> key, Consumer<T> value) {
        enterNodeListeners.put( key, value );
    }
    public <T extends BaseNode> void registerExitNodeListener(Class<T> key, Consumer<T> value) {
        exitNodeListeners.put( key, value );
    }

    public <T extends BaseNode> Collection<Consumer<BaseNode>> getEnterNodeListeners(Class<T> key ) {
        return Collection.class.cast(enterNodeListeners.get( key ));
    }
    public <T extends BaseNode> Collection<Consumer<BaseNode>> getExitNodeListeners(Class<T> key ) {
        return Collection.class.cast(exitNodeListeners.get( key ));
    }

    public void visitNode(BaseNode node) {
        if (node == null) {
            return;
        }
        for (Consumer<BaseNode> consumer : getEnterNodeListeners(node.getClass())) {
            consumer.accept(node);
        }
        node.getChildren().forEach(child -> visitNode(child));
        for (Consumer<BaseNode> consumer : getExitNodeListeners(node.getClass())) {
            consumer.accept(node);
        }
    }
}