[MNG-6776] Inconsistent list of parameters for MojoDescriptor

It may lead to confusion as parameters (list) and
parametersMap (map) may "fall apart" easily.

Also, parametersMap did not honor parameter
ordering while parameters list makes it look
that order is important.
Simply, rebuild the map always and retain
ordering as well.

Added UT and also removed some cruft provided
by java8.

maven-plugin-api/src/main/java/org/apache/maven/plugin/descriptor/MojoDescriptor.java

@@ -19,10 +19,11 @@
  * under the License.
  */
 
-import java.util.HashMap;
-import java.util.LinkedList;
+import java.util.ArrayList;
+import java.util.LinkedHashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.Objects;
 
 import org.apache.maven.plugin.Mojo;
 import org.codehaus.plexus.component.repository.ComponentDescriptor;
@@ -55,9 +56,7 @@ public class MojoDescriptor
 
     private static final String DEFAULT_LANGUAGE = "java";
 
-    private List<Parameter> parameters;
-
-    private Map<String, Parameter> parameterMap;
+    private final ArrayList<Parameter> parameters;
 
     /** By default, the execution strategy is "once-per-session" */
     private String executionStrategy = SINGLE_PASS_EXEC_STRATEGY;
@@ -145,6 +144,7 @@ public class MojoDescriptor
      */
     public MojoDescriptor()
     {
+        this.parameters = new ArrayList<>();
         setInstantiationStrategy( DEFAULT_INSTANTIATION_STRATEGY );
         setComponentFactory( DEFAULT_LANGUAGE );
     }
@@ -186,11 +186,12 @@ public void setDeprecated( String deprecated )
     }
 
     /**
-     * @return the list of parameters
+     * @return the list of parameters copy. Any change to returned list is NOT reflected on this instance. To add
+     * parameters, use {@link #addParameter(Parameter)} method.
      */
     public List<Parameter> getParameters()
     {
-        return parameters;
+        return new ArrayList<>( parameters  );
     }
 
     /**
@@ -200,6 +201,7 @@ public List<Parameter> getParameters()
     public void setParameters( List<Parameter> parameters )
         throws DuplicateParameterException
     {
+        this.parameters.clear();
         for ( Parameter parameter : parameters )
         {
             addParameter( parameter );
@@ -213,37 +215,28 @@ public void setParameters( List<Parameter> parameters )
     public void addParameter( Parameter parameter )
         throws DuplicateParameterException
     {
-        if ( parameters != null && parameters.contains( parameter ) )
+        if ( parameters.contains( parameter ) )
         {
             throw new DuplicateParameterException( parameter.getName()
                 + " has been declared multiple times in mojo with goal: " + getGoal() + " (implementation: "
                 + getImplementation() + ")" );
         }
 
-        if ( parameters == null )
-        {
-            parameters = new LinkedList<>();
-        }
-
         parameters.add( parameter );
     }
 
     /**
-     * @return the list parameters as a Map
+     * @return the list parameters as a Map (keyed by {@link Parameter#getName()}) that is built from
+     * {@link #parameters} list on each call. In other words, the map returned is built on fly and is a copy.
+     * Any change to this map is NOT reflected on list and other way around!
      */
     public Map<String, Parameter> getParameterMap()
     {
-        if ( parameterMap == null )
-        {
-            parameterMap = new HashMap<>();
+        LinkedHashMap<String, Parameter> parameterMap = new LinkedHashMap<>();
 
-            if ( parameters != null )
-            {
-                for ( Parameter pd : parameters )
-                {
-                    parameterMap.put( pd.getName(), pd );
-                }
-            }
+        for ( Parameter pd : parameters )
+        {
+            parameterMap.put( pd.getName(), pd );
         }
 
         return parameterMap;
@@ -539,53 +532,21 @@ public boolean equals( Object object )
         {
             MojoDescriptor other = (MojoDescriptor) object;
 
-            if ( !compareObjects( getPluginDescriptor(), other.getPluginDescriptor() ) )
+            if ( !Objects.equals( getPluginDescriptor(), other.getPluginDescriptor() ) )
             {
                 return false;
             }
 
-            return compareObjects( getGoal(), other.getGoal() );
-
+            return Objects.equals( getGoal(), other.getGoal() );
         }
 
         return false;
     }
 
-    private boolean compareObjects( Object first, Object second )
-    {
-        if ( first == second )
-        {
-            return true;
-        }
-
-        if ( first == null || second == null )
-        {
-            return false;
-        }
-
-        return first.equals( second );
-    }
-
     /** {@inheritDoc} */
     public int hashCode()
     {
-        int result = 1;
-
-        String goal = getGoal();
-
-        if ( goal != null )
-        {
-            result += goal.hashCode();
-        }
-
-        PluginDescriptor pd = getPluginDescriptor();
-
-        if ( pd != null )
-        {
-            result -= pd.hashCode();
-        }
-
-        return result;
+        return Objects.hash( getGoal(), getPluginDescriptor() );
     }
 
     /**

maven-plugin-api/src/test/java/org/apache/maven/plugin/descriptor/MojoDescriptorTest.java

@@ -0,0 +1,53 @@
+package org.apache.maven.plugin.descriptor;
+
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *  http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+import org.junit.jupiter.api.Test;
+
+import java.util.List;
+import java.util.Map;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class MojoDescriptorTest
+{
+    @Test
+    public void getParameterMap() throws DuplicateParameterException
+    {
+        MojoDescriptor mojoDescriptor = new MojoDescriptor();
+        Parameter param1 = new Parameter();
+        param1.setName( "param1" );
+        param1.setDefaultValue( "value1" );
+        mojoDescriptor.addParameter( param1 );
+
+        assertEquals( 1, mojoDescriptor.getParameters().size() );
+
+        assertEquals( mojoDescriptor.getParameters().size(), mojoDescriptor.getParameterMap().size() );
+
+        Parameter param2 = new Parameter();
+        param2.setName( "param2" );
+        param2.setDefaultValue( "value2" );
+        mojoDescriptor.addParameter( param2 );
+
+        assertEquals( 2, mojoDescriptor.getParameters().size() );
+        assertEquals( mojoDescriptor.getParameters().size(), mojoDescriptor.getParameterMap().size() );
+    }
+
+}