HashMap behaves incorrectly in multithreaded Java application how to fix?

I am using a HashMap in a Java application where multiple threads add and read data concurrently. Sometimes values are missing or incorrect, and occasionally the program seems to hang. I know HashMap is not thread-safe, but I don’t fully understand what exactly can go wrong internally.

Here is a simplified example:

Map<String, Integer> map = new HashMap<>(); Runnable writer = () -> { for (int i = 0; i < 1000; i++) { map.put("key" + i, i); } }; Runnable reader = () -> { for (int i = 0; i < 1000; i++) { Integer value = map.get("key" + i); if (value != null) { System.out.println(value); } } }; Thread t1 = new Thread(writer); Thread t2 = new Thread(reader); t1.start(); t2.start();

I am seeing missing entries and sometimes the program freezes. I have tried Collections.synchronizedMap but it seems slower.

Why does HashMap fail in multithreaded scenarios?

What internal issues cause corrupted data or freezes?

What is the best thread-safe alternative for frequent reads and writes?

I want a solution that is thread-safe, high-performance, and doesn’t block threads unnecessarily.