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

Unable to make field private final java.util.Comparator java.util.TreeMap.comparator accessible

This error occurs when you are trying to access or modify a private final field in the java.util.TreeMap class called comparator. However, the module java.base does not open the java.util package to the unnamed module, which results in this error.

Explanation

In Java 9 and above, a module system was introduced where access to internal Java packages can be restricted. The java.util package is part of the Java base module, and it is exported by default to all other modules, but it is not opened by default. The java.util.TreeMap class uses the java.util.Comparator interface internally, and the comparator field is declared as private final.

When you try to access or modify the comparator field, the module system prevents it because the java.util package is not opened to the unnamed module. An unnamed module means that the code is running outside of any specific module, such as in the classpath.

Example

Here is an example that reproduces the error:

// Java code
    
    import java.util.TreeMap;
    
    public class Main {
        public static void main(String[] args) {
            TreeMap<String, Integer> treeMap = new TreeMap<>();
            treeMap.comparator(); // Error occurs here
        }
    }

In the above code, the comparator() method is called on the treeMap object. This method internally tries to access the comparator field, which results in the error.

Solution

To solve this issue, you can use reflection to access the private field:

// Java code
    
    import java.lang.reflect.Field;
    import java.util.TreeMap;
    
    public class Main {
        public static void main(String[] args) throws NoSuchFieldException, IllegalAccessException {
            TreeMap<String, Integer> treeMap = new TreeMap<>();
            
            Field comparatorField = TreeMap.class.getDeclaredField("comparator");
            comparatorField.setAccessible(true);
            
            System.out.println(comparatorField.get(treeMap));
        }
    }

In the above code, we use the getDeclaredField() method from the java.lang.reflect.Field class to access the private field called comparator. Then we call setAccessible(true) to make the field accessible, and finally, we use get() to retrieve the value of the field.

Keep in mind that accessing private fields using reflection should be used with caution as it can break encapsulation and lead to unexpected behavior. It is generally recommended to use the public API provided by classes whenever possible.