IocKeySample.java

import java.util.HashMap;
import java.util.Map;

public class IocKeySample {

    public static class Key<T> {
        
        private final String key;

        private Key(String key) {
            this.key = key;
        }

        private Key(Class<T> clazz, String key) {
            this(key);
        }

        public String getKey() {
            return this.key;
        }
        
    }

    public static class MakeKey {

        public static <T> Key<T> from(String key) {
            return new Key<T>(key);
        }

        public static <T> Key<T> from(Class<T> clazz, String key) {
            return new Key<T>(clazz, key);
        }
        
    }

    public static class IOC {

        static final Map<String, Object> container = new HashMap<String, Object>();
        static {
            container.put("integer", 42);
            container.put("string", "42");
        }

        public static <T> T resolve(Key<T> key, Object... params) {
            return (T) container.get(key.getKey());     // compilation warning: Unchecked cast: 'java.lang.Object' to 'T'
        }

    }


    public static void main(String[] args) {
        // requires explicit casting
        Integer i1 = (Integer) IOC.resolve(MakeKey.from("integer"));
        String s1 = (String) IOC.resolve(MakeKey.from("string"));

        // implicit casting
        Integer i2 = IOC.resolve(MakeKey.from(Integer.class, "integer"));
        String s2 = IOC.resolve(MakeKey.from(String.class, "string"));

        // wrong casting
        Integer i3 = IOC.resolve(MakeKey.from(String.class, "integer"));
            // compilation error: incompatible types: java.lang.String cannot be converted to java.lang.Integer
        String s3 = IOC.resolve(MakeKey.from(Integer.class, "string"));
            // compilation error: incompatible types: java.lang.Integer cannot be converted to java.lang.String

        // wrong key type
        Integer i4 = IOC.resolve(MakeKey.from(Integer.class, "string"));
            // runtime exception: java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Integer
        String s4 = IOC.resolve(MakeKey.from(String.class, "integer"));
            // runtime exception: java.lang.ClassCastException: java.lang.Integer cannot be cast to java.lang.String

        // However, this implementation doesn't stores and checks the T of the key,
        // addition of such check may avoid ClassCastException on caller side.
    }


}