refactor: Introduce AlterExpression subclass hierarchy (internal, API-compatible)

The grammar now instantiates operation-specific subclasses instead of
the monolithic AlterExpression for all ALTER TABLE operations. All
existing API contracts are preserved — getAlterExpressions() still
returns List<AlterExpression>, all getters/setters remain on the base
class, instanceof AlterExpression still works for every instance.

New public subclasses in net.sf.jsqlparser.statement.alter:
- AlterExpressionDrop:       DROP column/constraint/index/PK/UK/FK/PARTITION
- AlterExpressionPartition:  all 12 partition maintenance operations
- AlterExpressionRename:     RENAME column/table/index/key/constraint
- AlterExpressionCharset:    CONVERT/DEFAULT CHARACTER SET/COLLATE
- AlterExpressionTableOption: ENGINE/ALGORITHM/LOCK/COMMENT/ENCRYPTION/
                              AUTO_INCREMENT/KEY_BLOCK_SIZE/TABLESPACE/KEYS

Each subclass overrides appendBody() with focused rendering logic,
replacing the 330-line if/else dispatch chain in the base class.
ADD/ALTER/MODIFY/CHANGE remain on the base class for now.

AlterExpression changes:
- toString() is now final, delegates to appendBody() + appendCommonTail()
- appendBody() and all toString helpers promoted from private to protected
- Base class appendBody() retained as fallback for non-subclassed operations

Grammar changes:
- AlterExpressionDrop(), AlterExpressionPartitionOp(),
  AlterExpressionDiscardOrImport() changed from void to returning
  AlterExpression, instantiating the correct subclass internally
- Each branch in AlterExpression() creates the appropriate subclass

This lays the groundwork for a future major version where fields can
migrate to subclasses and a visitor pattern can be introduced, while
keeping the current release fully backward-compatible.

Signed-off-by: Andreas Reichel <andreas@manticore-projects.com>
Signed-off-by: manticore-projects <andreas@manticore-projects.com>

Author: manticore-projects

src/main/java/net/sf/jsqlparser/statement/alter/AlterExpression.java

@@ -0,0 +1,22 @@
+/*-
+ * #%L
+ * JSQLParser library
+ * %%
+ * Copyright (C) 2004 - 2019 JSQLParser
+ * %%
+ * Dual licensed under GNU LGPL 2.1 or Apache License 2.0
+ * #L%
+ */
+package net.sf.jsqlparser.statement.alter;
+
+/**
+ * Internal subclass for character set and collation operations within ALTER TABLE.
+ * Handles CONVERT TO CHARACTER SET, DEFAULT CHARACTER SET, CHARACTER SET, and COLLATE.
+ */
+public class AlterExpressionCharset extends AlterExpression {
+
+    @Override
+    protected void appendBody(StringBuilder b) {
+        toStringConvert(b);
+    }
+}

src/main/java/net/sf/jsqlparser/statement/alter/AlterExpressionCharset.java

@@ -0,0 +1,68 @@
+/*-
+ * #%L
+ * JSQLParser library
+ * %%
+ * Copyright (C) 2004 - 2019 JSQLParser
+ * %%
+ * Dual licensed under GNU LGPL 2.1 or Apache License 2.0
+ * #L%
+ */
+package net.sf.jsqlparser.statement.alter;
+
+import net.sf.jsqlparser.statement.select.PlainSelect;
+
+/**
+ * Internal subclass for DROP operations within ALTER TABLE.
+ * Handles DROP column, DROP CONSTRAINT, DROP INDEX/KEY, DROP PRIMARY KEY,
+ * DROP UNIQUE, DROP FOREIGN KEY, and DROP PARTITION.
+ */
+public class AlterExpressionDrop extends AlterExpression {
+
+    @Override
+    protected void appendBody(StringBuilder b) {
+        switch (getOperation()) {
+            case DROP_PRIMARY_KEY:
+                b.append("DROP PRIMARY KEY ");
+                break;
+            case DROP_UNIQUE:
+                b.append("DROP UNIQUE (")
+                        .append(PlainSelect.getStringList(getPkColumns())).append(')');
+                break;
+            case DROP_FOREIGN_KEY:
+                b.append("DROP FOREIGN KEY (")
+                        .append(PlainSelect.getStringList(getPkColumns())).append(')');
+                break;
+            case DROP_PARTITION:
+                b.append("DROP PARTITION ")
+                        .append(PlainSelect.getStringList(getPartitions()));
+                break;
+            default:
+                toStringDropDefault(b);
+                break;
+        }
+    }
+
+    private void toStringDropDefault(StringBuilder b) {
+        b.append("DROP ");
+        if (getColumnName() == null && getPkColumns() != null && !getPkColumns().isEmpty()) {
+            // Oracle Multi Column Drop
+            b.append("(").append(PlainSelect.getStringList(getPkColumns())).append(')');
+        } else if (getConstraintName() != null) {
+            b.append("CONSTRAINT ");
+            if (isUsingIfExists()) {
+                b.append("IF EXISTS ");
+            }
+            b.append(getConstraintName());
+        } else if (getColumnName() != null) {
+            if (hasColumn()) {
+                b.append("COLUMN ");
+            }
+            if (isUsingIfExists()) {
+                b.append("IF EXISTS ");
+            }
+            b.append(getColumnName());
+        } else if (getIndex() != null) {
+            b.append(getIndex());
+        }
+    }
+}

src/main/java/net/sf/jsqlparser/statement/alter/AlterExpressionDrop.java

@@ -0,0 +1,28 @@
+/*-
+ * #%L
+ * JSQLParser library
+ * %%
+ * Copyright (C) 2004 - 2019 JSQLParser
+ * %%
+ * Dual licensed under GNU LGPL 2.1 or Apache License 2.0
+ * #L%
+ */
+package net.sf.jsqlparser.statement.alter;
+
+import java.util.stream.Collectors;
+
+import net.sf.jsqlparser.statement.create.table.PartitionDefinition;
+import net.sf.jsqlparser.statement.select.PlainSelect;
+
+/**
+ * Internal subclass for partition maintenance operations within ALTER TABLE.
+ * Handles TRUNCATE, COALESCE, REORGANIZE, EXCHANGE, ANALYZE, CHECK, OPTIMIZE,
+ * REBUILD, REPAIR PARTITION, PARTITION BY, and REMOVE PARTITIONING.
+ */
+public class AlterExpressionPartition extends AlterExpression {
+
+    @Override
+    protected void appendBody(StringBuilder b) {
+        toStringPartition(b);
+    }
+}

src/main/java/net/sf/jsqlparser/statement/alter/AlterExpressionPartition.java

@@ -0,0 +1,38 @@
+/*-
+ * #%L
+ * JSQLParser library
+ * %%
+ * Copyright (C) 2004 - 2019 JSQLParser
+ * %%
+ * Dual licensed under GNU LGPL 2.1 or Apache License 2.0
+ * #L%
+ */
+package net.sf.jsqlparser.statement.alter;
+
+/**
+ * Internal subclass for RENAME operations within ALTER TABLE.
+ * Handles RENAME COLUMN, RENAME TO (table), RENAME INDEX/KEY/CONSTRAINT.
+ */
+public class AlterExpressionRename extends AlterExpression {
+
+    @Override
+    protected void appendBody(StringBuilder b) {
+        switch (getOperation()) {
+            case RENAME:
+                b.append("RENAME ");
+                if (hasColumn()) {
+                    b.append("COLUMN ");
+                }
+                b.append(getColumnOldName()).append(" TO ").append(getColumnName());
+                break;
+            case RENAME_TABLE:
+                b.append("RENAME TO ").append(getNewTableName());
+                break;
+            case RENAME_INDEX:
+            case RENAME_KEY:
+            case RENAME_CONSTRAINT:
+                toStringRename(b);
+                break;
+        }
+    }
+}

src/main/java/net/sf/jsqlparser/statement/alter/AlterExpressionRename.java

@@ -0,0 +1,35 @@
+/*-
+ * #%L
+ * JSQLParser library
+ * %%
+ * Copyright (C) 2004 - 2019 JSQLParser
+ * %%
+ * Dual licensed under GNU LGPL 2.1 or Apache License 2.0
+ * #L%
+ */
+package net.sf.jsqlparser.statement.alter;
+
+/**
+ * Internal subclass for table-level option operations within ALTER TABLE.
+ * Handles ENGINE, ALGORITHM, LOCK, KEY_BLOCK_SIZE, COMMENT, ENCRYPTION,
+ * AUTO_INCREMENT (SET_TABLE_OPTION), DISCARD/IMPORT TABLESPACE, DISABLE/ENABLE KEYS.
+ */
+public class AlterExpressionTableOption extends AlterExpression {
+
+    @Override
+    protected void appendBody(StringBuilder b) {
+        switch (getOperation()) {
+            case COMMENT:
+                b.append("COMMENT ");
+                b.append(getCommentText());
+                break;
+            case COMMENT_WITH_EQUAL_SIGN:
+                b.append("COMMENT = ");
+                b.append(getCommentText());
+                break;
+            default:
+                toStringSimpleKeyword(b);
+                break;
+        }
+    }
+}