Key existence check in HashMap

Do you mean that you've got code like

if(map.containsKey(key)) doSomethingWith(map.get(key))

all over the place ? Then you should simply check whether map.get(key) returned null and that's it. By the way, HashMap doesn't throw exceptions for missing keys, it returns null instead. The only case where containsKey is needed is when you're storing null values, to distinguish between a null value and a missing value, but this is usually considered bad practice.

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.

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

@Override
public boolean containsKey(Object key) {
    Entry<K, V> m = getEntry(key);
    return m != null;
}

@Override
public 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.

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Resources :

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

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
    }
}