Using anonymous class in Java

The Java Language Specification, Third Edition
http://java.sun.com/docs/books/jls/third_edition/html/j3TOC.html

The Java Tutorials
http://java.sun.com/docs/books/tutorial/index.html

Java in a Nutshell
http://docstore.mik.ua/orelly/java-ent/jnut/ch03_12.htm

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;


public class TestAnonymous {

    @SuppressWarnings("serial")
    public static void main(String[] args) {
        
        Map<Object, Object> map = new HashMap<Object, Object>() {
            
            @SuppressWarnings("unchecked")
            @Override
            public Object put(Object key, Object value) {
                if (containsKey(key)) {
                    ((List<Object>) get(key)).add(value);
                } else  {
                    List<Object> list = new ArrayList<Object>();
                    list.add(value);
                    super.put(key, list);
                } 
                return value;
            }
        };
        
        map.put("k1", 1);        
        map.put("k1", 2);
        map.put("k1", "1");
        map.put("k1", "2");
        
        map.put("k2", 1);
        map.put("k2", "1");
        
        map.put("k3", 1);
        
        System.out.println(map.toString());
    

    }

}

{k3=[1], k1=[1, 2, 1, 2], k2=[1, 1]}

Anonymous class converted to nested class

private static final class HashMapConvertList extends
            HashMap<Object, Object> {
        @SuppressWarnings("unchecked")
        @Override
        public Object put(Object key, Object value) {
            if (containsKey(key)) {
                ((List<Object>) get(key)).add(value);
            } else  {
                List<Object> list = new ArrayList<Object>();
                list.add(value);
                super.put(key, list);
            } 
            return value;
        }
    }
...

Map<Object, Object> map = new HashMapConvertList();