Provide a New Java Rule checking fields by disallowing public fields

Closes gh-18

docs/USERGUIDE.md

@@ -84,6 +84,17 @@ Taikai.builder()
     .check();
 ```
 
+- **Fields Should Not Be Public**: Ensure that no fields in your Java classes are declared as `public`, except constants.
+
+```java
+Taikai.builder()
+    .namespace("com.enofex.taikai")
+    .java(java -> java
+        .fieldsShouldNotBePublic())
+    .build()
+    .check();
+```
+
 - **Imports Configuration**: Ensure that there are no cyclic dependencies in imports and disallow specific imports.
 
 ```java

src/main/java/com/enofex/taikai/java/FieldsShouldNotBePublic.java

@@ -0,0 +1,28 @@
+package com.enofex.taikai.java;
+
+import com.tngtech.archunit.core.domain.JavaField;
+import com.tngtech.archunit.core.domain.JavaModifier;
+import com.tngtech.archunit.lang.ArchCondition;
+import com.tngtech.archunit.lang.ConditionEvents;
+import com.tngtech.archunit.lang.SimpleConditionEvent;
+
+final class FieldsShouldNotBePublic {
+
+  private FieldsShouldNotBePublic() {
+  }
+
+  static ArchCondition<JavaField> notBePublic() {
+    return new ArchCondition<>("not be public") {
+      @Override
+      public void check(JavaField field, ConditionEvents events) {
+        if (!field.getModifiers().contains(JavaModifier.STATIC)
+            && field.getModifiers().contains(JavaModifier.PUBLIC)) {
+          String message = String.format("Field %s in class %s is public",
+              field.getName(),
+              field.getOwner().getFullName());
+          events.add(SimpleConditionEvent.violated(field, message));
+        }
+      }
+    };
+  }
+}

src/main/java/com/enofex/taikai/java/JavaConfigurer.java

@@ -1,11 +1,13 @@
 package com.enofex.taikai.java;
 
 import static com.enofex.taikai.java.Deprecations.notUseDeprecatedAPIs;
+import static com.enofex.taikai.java.FieldsShouldNotBePublic.notBePublic;
 import static com.enofex.taikai.java.HashCodeAndEquals.implementHashCodeAndEquals;
 import static com.enofex.taikai.java.UtilityClasses.beFinal;
 import static com.enofex.taikai.java.UtilityClasses.havePrivateConstructor;
 import static com.enofex.taikai.java.UtilityClasses.utilityClasses;
 import static com.tngtech.archunit.lang.syntax.ArchRuleDefinition.classes;
+import static com.tngtech.archunit.lang.syntax.ArchRuleDefinition.fields;
 import static com.tngtech.archunit.lang.syntax.ArchRuleDefinition.methods;
 
 import com.enofex.taikai.TaikaiRule;
@@ -66,6 +68,14 @@ public JavaConfigurer classesShouldImplementHashCodeAndEquals(Configuration conf
     return addRule(TaikaiRule.of(classes().should(implementHashCodeAndEquals()), configuration));
   }
 
+  public JavaConfigurer fieldsShouldNotBePublic() {
+    return fieldsShouldNotBePublic(null);
+  }
+
+  public JavaConfigurer fieldsShouldNotBePublic(Configuration configuration) {
+    return addRule(TaikaiRule.of(fields().should(notBePublic()), configuration));
+  }
+
   @Override
   public void disable() {
     disable(ImportsConfigurer.class);

src/test/java/com/enofex/taikai/ArchitectureTest.java

@@ -17,6 +17,7 @@ void shouldFulfilConstrains() {
             .classesShouldImplementHashCodeAndEquals()
             .methodsShouldNotThrowGenericException()
             .utilityClassesShouldBeFinalAndHavePrivateConstructor()
+            .fieldsShouldNotBePublic()
             .imports(imports -> imports
                 .shouldHaveNoCycles()
                 .shouldNotImport("..shaded..")