Add an abstract function that can change they column data type

Implementation of #197
Thanks @Endinc!

Author: jeffgbutler

src/main/java/org/mybatis/dynamic/sql/select/function/AbstractFunction.java

@@ -1,5 +1,5 @@
 /**
- *    Copyright 2016-2018 the original author or authors.
+ *    Copyright 2016-2020 the original author or authors.
  *
  *    Licensed under the Apache License, Version 2.0 (the "License");
  *    you may not use this file except in compliance with the License.
@@ -16,31 +16,17 @@
 package org.mybatis.dynamic.sql.select.function;
 
 import java.sql.JDBCType;
-import java.util.Objects;
 import java.util.Optional;
 
 import org.mybatis.dynamic.sql.BindableColumn;
 
-public abstract class AbstractFunction<T, U extends AbstractFunction<T, U>> implements BindableColumn<T> {
-    protected BindableColumn<T> column;
-    protected String alias;
+public abstract class AbstractFunction<T, U extends AbstractFunction<T, U>>
+        extends AbstractTypeConvertingFunction<T, T, U> {
 
     protected AbstractFunction(BindableColumn<T> column) {
-        this.column = Objects.requireNonNull(column);
+        super(column);
     }
 
-    @Override
-    public Optional<String> alias() {
-        return Optional.ofNullable(alias);
-    }
-
-    @Override
-    public U as(String alias) {
-        U newThing = copy();
-        newThing.alias = alias;
-        return newThing;
-    }
-
     @Override
     public Optional<JDBCType> jdbcType() {
         return column.jdbcType();
@@ -50,6 +36,4 @@ public Optional<JDBCType> jdbcType() {
     public Optional<String> typeHandler() {
         return column.typeHandler();
     }
-    
-    protected abstract U copy();
 }

src/main/java/org/mybatis/dynamic/sql/select/function/AbstractTypeConvertingFunction.java

@@ -0,0 +1,59 @@
+/**
+ *    Copyright 2016-2020 the original author or authors.
+ *
+ *    Licensed 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.
+ */
+package org.mybatis.dynamic.sql.select.function;
+
+import java.util.Objects;
+import java.util.Optional;
+
+import org.mybatis.dynamic.sql.BindableColumn;
+
+/**
+ * Represents a function that can change the underlying type. For example, converting a binary field for a base64
+ * string, or an integer to a string, etc.
+ * 
+ * <p>Thanks to @endink for the idea.
+ * 
+ * @author Jeff Butler
+ *
+ * @param <T> The type of the underlying column. For example, if a function converts a VARCHR to an INT, then the
+ *     underlying type will be a String 
+ * @param <R> The type of the column after the conversion. For example, if a function converts a VARCHR to an INT, then
+ *     the converted type will be Integer
+ * @param <U> the specific subtype that implements the function
+ */
+public abstract class AbstractTypeConvertingFunction<T, R, U extends AbstractTypeConvertingFunction<T, R, U>>
+        implements BindableColumn<R> {
+    protected BindableColumn<T> column;
+    protected String alias;
+
+    protected AbstractTypeConvertingFunction(BindableColumn<T> column) {
+        this.column = Objects.requireNonNull(column);
+    }
+    
+    @Override
+    public Optional<String> alias() {
+        return Optional.ofNullable(alias);
+    }
+
+    @Override
+    public U as(String alias) {
+        U newThing = copy();
+        newThing.alias = alias;
+        return newThing;
+    }
+
+    protected abstract U copy();
+}