Runtime Error

What This Error Means

This error occurs when a method or constructor may throw an exception, but you're trying to call it in a context where exceptions are not allowed, such as in a static initializer block.

Why It Happens

In Java, static initialization blocks are executed before the main method is called. If you try to put objects into a ConcurrentHashMap in a static initialization block, you may get this error because the ConcurrentHashMap's put method may throw a NullPointerException or a ClassCastException.

How to Fix It

1. 11. Move the code that uses ConcurrentHashMap to a non-static context, such as a method or a constructor.
2. 2 2. Use a try-catch block to catch any exceptions that may be thrown by the ConcurrentHashMap's put method.
3. 3 3. Initialize the ConcurrentHashMap with a default value to avoid NullPointerExceptions.

Example Code Solution

public class MyClass {
  public static ConcurrentHashMap<String, Integer> map = new ConcurrentHashMap<>();
  static {
    map.put("key", 5);
  }

  public static void main(String[] args) {
    System.out.println(map.get("key"));
  }
}

public class MyClass {
  public static ConcurrentHashMap<String, Integer> map = new ConcurrentHashMap<>();

  public static void main(String[] args) {
    try {
      map.put("key", 5);
    } catch (NullPointerException e) {
      System.out.println("NullPointerException: " + e.getMessage());
    }

    System.out.println(map.get("key"));
  }
}