Question

It may be a bad practice, but I haven’t been able to figure out any better solution for my problem. So I have this map

// Map<state, Map<transition, Map<property, value>>>
private Map<String, Map<String, Map<String, String>>> properties;

and I want to initialize it so I don’t get NullPointerException with this

properties.get("a").get("b").get("c");

I tried this one but I didn’t work (obviously)

properties = new HashMap<String, Map<String, Map<String,String>>>();

Other things I tried didn’t compile.

Also if you have any ideas how to avoid this nested maps, I would appreciate it.

Answer 1

You need to put maps in your maps in your map. Literally:

properties = new HashMap<String, Map<String, Map<String,String>>>();
properties.put("a", new HashMap<String, Map<String,String>>());
properites.get("a").put("b", new HashMap<String,String>());

If your target is lazy initialization without NPE you have to create your own map:

private static abstract class MyMap<K, V> extends HashMap<K, V> {
    @Override
    public V get(Object key) {
        V val = super.get(key);
        if (val == null && key instanceof K) {
            put((K)key, val = create());
        }
        return val;
    }

    protected abstract V create();
}


public void initialize() {
    properties = new MyMap<String, Map<String, Map<String, String>>>() {
        @Override
        protected Map<String, Map<String, String>> create() {
            return new MyMap<String, Map<String, String>>() {
                @Override
                protected Map<String, String> create() {
                    return new HashMap<String, String>();
                }
            };
        }
    };

}