How Does HashMap Handle Duplicate Keys and Values?

What occurs in a HashMap when duplicate keys are inserted?

Map<String, String> mymap = new HashMap();
mymap.put("1","one");
mymap.put("1","not one");
mymap.put("1","surely not one");
System.out.println(mymap.get("1")); // Output: surely not one

HashMap is a widely-used data structure in Java that allows for efficient key-value pair storage. One key characteristic of a HashMap is that it does not allow duplicate keys. When a duplicate key is inserted, the value associated with that key is overwritten with the new value provided. This behavior makes it essential to understand how HashMap manages memory and references to values.

Map<String, String> mymap = new HashMap();
mymap.put("1","one");
mymap.put("1","not one");
mymap.put("1","surely not one");
System.out.println(mymap.get("1")); // Output: surely not one

Causes

• HashMap uses a hashing algorithm to determine where to store key-value pairs. When a key is added, it computes a hash code to determine its position in the internal data structure.
• If a new key-value pair is inserted with a key that already exists, the HashMap will overwrite the existing value with the new one.

Solutions

• To handle the scenario of wanting to keep all values for a single key, you might consider using a Map structure that allows multiple values, such as a Map<String, List<String>>.
• Utilize a data structure like a Multimap that can store multiple values for a single key.

Mistake: Ignoring the overwrite behavior of HashMap when inserting duplicate keys.

Solution: Always check if the key you are inserting already exists if you need to retain previous values.

Mistake: Assuming HashMap can store duplicate keys like ArrayLists can store duplicate values.

Solution: Use appropriate data structures (e.g., Lists or Multimaps) if duplicate representations are required.