Unable to make field private final java.util.comparator java.util.treemap.comparator accessible: module java.base does not “opens java.util”

The error message “unable to make field private final java.util.Comparator accessible: module java.base does not ‘opens java.util'” is encountered when trying to access the private final field “java.util.Comparator” in the “java.util.TreeMap” class. This error typically occurs when using Java 9 or higher, as it has introduced a new module system that restricts access to certain packages and classes by default.

To understand this error, let’s break it down step by step:

1. Module System: Starting from Java 9, the Java platform introduced a modular system called the Java Platform Module System (JPMS). It divides the JDK (Java Development Kit) into separate modules, allowing for greater modularity and encapsulation of code.
2. Module java.base: The error message specifically mentions the “java.base” module. This module is the core module of the Java SE platform and includes fundamental classes and interfaces that are required by all modules.
3. “Opens” Directive: The error message further states that the module “java.base” does not “opens java.util”. The “opens” directive is used in module definitions to open a package to specific modules or all other modules for access.
4. Inaccessible Field: The error occurs because the private final field “java.util.Comparator” in the “java.util.TreeMap” class is not accessible due to the restrictions imposed by the module system. This field is critical for the internal functioning of the TreeMap class, and accessing it directly is not allowed.

To resolve this error, you should avoid trying to access private fields that are not intended for direct access. Instead, you should use the public methods provided by the TreeMap class to perform desired operations. For instance, if you need to specify a custom comparator for a TreeMap, you can use the constructor that accepts a Comparator as an argument:

        
        TreeMap<Integer, String> treeMap = new TreeMap<>(new Comparator<Integer>() {
            @Override
            public int compare(Integer o1, Integer o2) {
                // Custom comparison logic
                return o1.compareTo(o2);
            }
        });