Prefer Collections over Arrays

Table of Contents

• Arrays are covariant; Generics are invariant
• Arrays and Generics do not mix well
• Collections provide correct toString, hashCode, equals implementations; Arrays don’t
• More Pitfalls of Object Array

Arrays are covariant; Generics are invariant

// Fails at runtime!
Object[] objectArray = new Long[1];
objectArray[0] = "I don't fit in"; // Throws ArrayStoreException

// Won't compile!
List<Object> ol = new ArrayList<Long>(); // Incompatible types
ol.add("I don't fit in");

Arrays and Generics do not mix well

• We can’t create an array of a generic type, a parameterized type, or a type parameter

Collections provide correct toString, hashCode, equals implementations; Arrays don’t

• Calling Array toString() directly or indirectly prints identity hash code(memory address), we need always rememberer to use Arrays.toString() or Arrays.toDeepString()
  • Arrays.toString()/toDeepString() doesn’t work for primitive array.
• Array hashCode method is inherited from Object: based on the reference, have to use Arrays.hashCode(shallow implementation) or Arrays.deepHashCode
• Array equals method is inherited from Object: based on the reference, have to use Arrays.equals(shallow implementation) or Arrays.deepEquals

More Pitfalls of Object Array

• Arrays are fixed in size but Collections are dynamic in size.
• Object Array is always mutable, a collection can be mutable or immutable: ImmutableList/Set/Map, ImmutableInt/Long/DoubleArray
• Object Array is not thread-safe, a collection can be thread safe: concurrent collections like ConcurrentHashMap,
• A collection can provide different views: unmodifiable, subList
• A collection provides many useful methods, an array doesn’t, have to use Arrays or some util classes.