Initialize a Java List in one line with List.of() for an unmodifiable list, or new ArrayList<>(List.of(…)) when the list changes later. Compare Arrays.asList(), Stream.toList(), Collections and Guava by mutability and null support.
Assert two lists equal ignoring order with AssertJ, Hamcrest, Commons Collections or plain JUnit 6, and see which assertions count duplicates.
Convert List to Map in Java with Collectors.toMap() and groupingBy(), fix the duplicate key IllegalStateException with a merge function, keep order.
The Java List interface explained with examples of List.of(), positional methods, Java 21 sequenced methods and how to choose an implementation.
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