Add basic implmentation of FieldDeclarationsShouldBeAtStart for apex

pmd-apex/src/main/java/net/sourceforge/pmd/lang/apex/rule/codestyle/FieldDeclarationsShouldBeAtStartRule.java

@@ -0,0 +1,67 @@
+/**
+ * BSD-style license; for more info see http://pmd.sourceforge.net/license.html
+ */
+
+package net.sourceforge.pmd.lang.apex.rule.codestyle;
+
+import java.util.Comparator;
+import java.util.List;
+import java.util.Optional;
+
+import net.sourceforge.pmd.lang.apex.ast.ASTFieldDeclaration;
+import net.sourceforge.pmd.lang.apex.ast.ASTMethod;
+import net.sourceforge.pmd.lang.apex.ast.ASTUserClass;
+import net.sourceforge.pmd.lang.apex.ast.ApexNode;
+import net.sourceforge.pmd.lang.apex.rule.AbstractApexRule;
+
+public class FieldDeclarationsShouldBeAtStartRule extends AbstractApexRule {
+    @Override
+    public Object visit(ASTUserClass node, Object data) {
+        // Unfortunately the parser re-orders the AST to put field declarations before method declarations
+        // so we have to rely on line numbers / positions to work out where the first method starts so we
+        // can check if the fields are in acceptable places.
+        List<ASTFieldDeclaration> fields = node.findDescendantsOfType(ASTFieldDeclaration.class);
+        List<ASTMethod> methods = node.findDescendantsOfType(ASTMethod.class);
+
+        Optional<NodeAndLocation> firstMethod =
+            methods.stream().map(NodeAndLocation::new)
+                .min(Comparator.naturalOrder());
+
+        if (!firstMethod.isPresent()) {
+            // there are no methods so the field declaration has to come first
+            return data;
+        }
+
+        for (ASTFieldDeclaration field : fields) {
+            NodeAndLocation fieldPosition = new NodeAndLocation(field);
+            if (fieldPosition.compareTo(firstMethod.get()) > 0) {
+                addViolation(data, field);
+            }
+        }
+
+        return data;
+    }
+
+    private static class NodeAndLocation implements Comparable<NodeAndLocation> {
+        public int line;
+        public int column;
+        public ApexNode<?> node;
+
+        public NodeAndLocation(ApexNode<?> node) {
+            this.node = node;
+
+            line = node.getBeginLine();
+            column = node.getBeginColumn();
+        }
+
+        @Override
+        public int compareTo(NodeAndLocation other) {
+            int lineCompare = Integer.compare(line, other.line);
+            if (lineCompare != 0) {
+                return lineCompare;
+            }
+
+            return Integer.compare(column, other.column);
+        }
+    }
+}