Update and fix Java instructions

notes/java.md

@@ -1,9 +1,6 @@
 # Recommendations for solving lab 2 with Java
 
-The stub for lab 2 uses Java 21,
-which admits a more readable alternative to the visitor pattern suggested in the book.
-The following example code shows how you can process an object ```exp``` of the type type ```Exp```
-generated by the parser defined in the stub.
+The stub for lab 2 uses Java 21, which admits a more readable alternative to the visitor pattern suggested in the book. The following example code shows how you can process an object ```exp``` of the type ```Exp``` generated by the parser defined in the stub.
 
 ```java
 switch(exp) {
@@ -16,37 +13,60 @@ switch(exp) {
 }
 ```
 
-On the right hand side of the ```->``` you can extract further information from ```e```,
-for example, in the case ```EAnd e```, you can use the two operands ```e.exp_1``` and ```e.exp_2```.
+On the right hand side of the ```->``` you can extract further information from ```e```, for example, in the case ```EAnd e```, you can use the two operands ```e.exp_1``` and ```e.exp_2```.
 
 ```switch```, if used like above produces a Java-expression, which means that you can use it in assignments:
 
 ```java
 var typedExpression = switch(exp) { ... }
 ```
 
-To define the neccessary datatypes, ```record```s are very useful.
-You can define a record like this:
+To define the neccessary datatypes, ```record```s are very useful. You can define a record like this:
 
 ```java
-public record TypedAnd(TypedExpr e1, TypedExpr e2) {
+record TypedAnd(TypedExpr e1, TypedExpr e2) {
 }
 ```
 
 Objects of this type can be constructed with ```new TypedAnd(e1,e2)``` where ```e1``` and ```e2``` are ```TypedExpr```s. For solving the lab, it is reasonable to define an ```interface``` ```TypedExpr``` with a method which returns the type of an expression:
 
 ```java
-public interface TypedExpr {
+interface TypedExpr {
   Type type();
 }
 ```
 
 Where ```Type``` is a datatype for types that needs to be defined. With this interface, the ```TypedAnd``` from above can be turned into an implementation like this:
 
 ```java
-public record TypedAnd(TypedExpr e1, TypedExpr e2) {
+record TypedAnd(TypedExpr e1, TypedExpr e2) {
   Type type() {
     return new Type.Bool; // this is what it could look like if you implement basic types as an enum
   }
 }
 ```
+
+To keep a good overview, you can write implentations of an interface directly into the interface as "inner classes":
+
+```java
+interface TypedExpr {
+  Type type();
+
+  record And(TypedExpr e1, TypedExpr e2) implements TypedExpr {
+    Type type() {
+      return new Type.Bool;
+  }
+
+  record Var(String name, Type type) implements TypedExpr { }
+}
+```
+
+The second record `Var` shows a handy shortcut to overriding the method `type`: if the record has an attribute of the right Type and name ("`type`" in this case), it will automotically become the implementation.
+
+You can work with a variable of type `TypedExpr` like in the example above, by using `switch` expressions. If you want to avoid the `default` case, you can use a `sealed interface` instead. To make that work, you have to declare all implementations using `permits` like this:
+
+```java
+sealed interface TypedExpr permits And, Var {
+...
+}
+```