commented and finished the ASTPrinter.java

src/main/ASTPrinter.java

 package main;
 
+/**
+ * <h1>The ASTPrinter</h1>
+ * The ASTPrinter class creates a String out of an AST(represented through an Expression object)
+ * It prints it in a form the Evaluator class can work with it, it isn't primarily designed for
+ * returning a String that is perfect for human readability (e.g. Pow(x,3) would be printed instead of x^3)
+ *
+ * @version 1.0
+ * @since 09.01.2023
+ */
 public class ASTPrinter implements  Visitor<String>{
+
+    /**
+     * Builds a String representation of a given BinaryOperation
+     * It gets a String representation of its left and right Expressions (which may result in a
+     * recursive call) and combines them into one String together with the operator
+     * @param binaryOP The BinaryOperation that should be printed into a String
+     * @return String Returns the String that was build
+     */
     @Override
     public String visit(final Parser.BinaryOperation binaryOP) {
         StringBuffer sb = new StringBuffer();
+        //adds left bracket if the BinaryOperation is capsuled
         if(binaryOP.capsuled) {
             sb.append("(");
         }
         if(binaryOP.operator.contains("^")) {
+            //Exponents will be printed in the form of Pow(basis, exponent) so the Evaluator can work with it
             sb.append("Pow(");
             sb.append(binaryOP.leftExpression.accept(this));
             sb.append(",");
@@ -15,36 +34,53 @@ public class ASTPrinter implements  Visitor<String>{
             sb.append(")");
         }
         else {
+            //Adds the String representations of the left expression, operator and right expression
             sb.append(binaryOP.leftExpression.accept(this));
             sb.append(binaryOP.operator);
             sb.append(binaryOP.rightExpression.accept(this));
         }
-
+        //adds right bracket if the BinaryOperation is capsuled
         if(binaryOP.capsuled) {
             sb.append(")");
         }
         return sb.toString();
     }
 
+
+    /**
+     * Returns the result of ast.accept(this), which will be the core values of a Value(digits),
+     * Variable(variableName) or BinaryOperation(will lead to a recursive call)
+     * @param ast The Expression
+     * @return ast.accept(this)
+     */
+    public String visit(final Parser.Expression ast) {
+        if(ast==null) {
+            throw new RuntimeException("AST is null");
+        }
+        return ast.accept(this);
+    }
+
+    /**
+     * @see #visit(Parser.Expression)
+     */
     @Override
     public String visit(Parser.Variable variable) {
         return variable.variableName;
     }
 
+    /**
+     * @see #visit(Parser.Expression)
+     */
     @Override
     public String visit(Parser.Number number) {
         return number.digits;
     }
 
+    /**
+     * @see #visit(Parser.Expression)
+     */
     @Override
     public String visit(Parser.Decimal decimal) {
         return decimal.beforeDot.digits + "." + decimal.afterDot.digits;
     }
-
-    public String visit(final Parser.Expression ast) {
-        if(ast==null) {
-            throw new RuntimeException("AST is null");
-        }
-        return ast.accept(this);
-    }
 }