Key existence check in HashMap

Do you ever store a null value? If not, you can just do:

Foo value = map.get(key);if (value != null) {    ...} else {    // No such key}

Otherwise, you could just check for existence if you get a null value returned:

Foo value = map.get(key);if (value != null) {    ...} else {    // Key might be present...    if (map.containsKey(key)) {       // Okay, there's a key but the value is null    } else {       // Definitely no such key    }}

You won't gain anything by checking that the key exists. This is the code of HashMap:

@Overridepublic boolean containsKey(Object key) {    Entry<K, V> m = getEntry(key);    return m != null;}@Overridepublic V get(Object key) {    Entry<K, V> m = getEntry(key);    if (m != null) {        return m.value;    }    return null;}

Just check if the return value for get() is different from null.

This is the HashMap source code.

Resources :

• HashMap source code Bad one
• HashMap source code Good one

Better way is to use containsKey method of HashMap. Tomorrow somebody will add null to the Map. You should differentiate between key presence and key has null value.