Null

Author: bowbahdoe

README.md

@@ -6,7 +6,7 @@
 
 Book teaching how to write modern and effective Java. It is maintained by the community, anyone can contribute.
 
-Feel free to join our [discord server](https://discord.com/invite/XXFUXzK)
+Feel free to join our [discord server](https://discord.gg/together-java-272761734820003841)
 if you have any questions, or require assistance with the project. :relaxed:
 
 ## Getting started

src/SUMMARY.md

@@ -95,15 +95,17 @@
 # Control Flow I
 
 - [Branching Paths](./branching_paths.md)
+
   - [If](./branching_logic/if.md)
-  - [Else](./branching_logic/else.md)
   - [Nested Ifs](./branching_logic/nested_ifs.md)
+  - [Else](./branching_logic/else.md)
   - [Else If](./branching_logic/else_if.md)
   - [Relation to Delayed Assignment](./branching_logic/relation_to_delayed_assignment.md)
   - [Scoped Variables](./branching_logic/scoped_variables.md)
   - [Conditional Operator](./branching_logic/conditional_operator.md)
   - [Boolean Expressions](./branching_logic/boolean_expressions.md)
   - [Challenges](./branching_logic/challenges.md)
+
 - [Loops](./loops.md)
   - [While](./loops/while.md)
   - [Endless Loops](./loops/endless_loops.md)
@@ -165,11 +167,10 @@
   - [Declaration](./methods/declaration.md)
   - [Invocation](./methods/invocation.md)
   - [Scope](./methods/scope.md)
-  - [Return](./methods/return.md)
-  - [Unreachable Statements](./methods/unreachable_statements.md)
   - [main](./methods/main.md)
   - [Challenges](./methods/challenges.md)
 - [Arguments](./arguments.md)
+
   - [Declaration](./arguments/declaration.md)
   - [Invocation with Arguments](./arguments/invocation_with_arguments.md)
   - [Reassignment](./arguments/reassignment.md)
@@ -179,19 +180,16 @@
   - [Inferred Types](./arguments/inferred_types.md)
   - [Challenges](./arguments/challenges.md)
 
-- [Return Values](./return_values.md)
-  - [void](./return_values/void.md)
+- [Return](./return_values.md)
+  - [Return Values](./return_values/return_values.md)
   - [Conversion](./return_values/conversion.md)
+  - [void](./return_values/void.md)
+  - [Unreachable Statements](./return_values/unreachable_statements.md)
   - [Pure Functions](./return_values/pure_functions.md)
   - [Impure Functions](./return_values/impure_functions.md)
 
-
-
 # Data Types III
 
-- [Identity Types](./identity_types.md)
-  - [Comparison with ==](./identity_types/comparison_with_equalsequals.md)
-- [Primitive Types](./primitive_types.md)
 - [null](./null.md)
   - [Checking for null](./null/checking_for_null.md)
   - [Field Access](./null/field_access.md)
@@ -205,7 +203,7 @@
   - [Unboxing Conversion](./boxed_primitives/unboxing_conversion.md)
   - [Boxing Conversion](./boxed_primitives/boxing_conversion.md)
   - [Arrays of Boxed Primitives](./boxed_primitives/arrays_of_boxed_primitives.md)
-  - [Challenges](./arguments/challenges.md)
+  - [Challenges](./boxed_primitives/challenges.md)
 - [Arrays II](./arrays_ii.md)
   - [Default Values](./arrays_ii/default_values.md)
   - [Populate Array]()

src/arguments/challenges.md

@@ -53,3 +53,4 @@ void main() {
 ```
 
 ## Challenge 3.
+

src/arguments/final_arguments.md

@@ -32,6 +32,5 @@ This has the same use as regular final variables. If there are lots of lines
 of code where a variable might be reassigned, it can be useful to not have
 to read all that code to know that it does happen.[^opinion]
 
-[^opinion]:
-    Adding `final` to all arguments can make it harder to read the code, simply
+[^opinion]: Adding `final` to all arguments can make it harder to read the code, simply
     because of visual noise.

src/boxed_primitives/challenges.md

@@ -0,0 +1 @@
+# Challenges

src/first_steps.md

@@ -109,4 +109,4 @@ public class Main {
 You should get in the habit of, whenever you see some bit of code, trying to physically type it out, run it,
 tweak it, and play with it.
 
-So try playing around with this program. If you are not actively engaging then the whole thing is a bit of a wash.
+So try playing around with this program. If you actively engage then you will retain information better.

src/methods/challenges.md

@@ -9,6 +9,13 @@ Remember the rules for this are
 
 Declare a method named `printSquare`. When invoked it should print a square.
 
+```
+*****
+*****
+*****
+*****
+```
+
 ```java
 // CODE HERE
 

src/methods/return_values.md

@@ -1,4 +1,40 @@
 # null
 
-There is a special value called `null` which is assignable to
-any `String`
+There is a special value called `null` which is assignable to most types.
+
+```java
+void main() {
+    String name = null;
+    int[] numbers = null;
+
+    System.out.println(name);
+    System.out.println(numbers);
+}
+```
+
+The only types which `null` cannot be assigned to are `int`, `double`, `char`, and `boolean`.[^aswellas]
+
+```java
+void main() {
+    // Will not work
+    int x = null;
+}
+```
+
+When something has the value of `null` it represents the absence of a "legitimate" value.
+
+A good example is when a puppy is born. It starts without a name and it is later given a name.
+`null` is a way to represent the state of the world in the time before it is named.
+
+```java
+void main() {
+    // Just born, it has no name
+    String puppyName = null;
+    System.out.println("At the start, the name is " + puppyName);
+
+    puppyName = "Sally";
+    System.out.println("Later it was given the name " + puppyName);
+}
+```
+
+[^aswellas]: As well as `long`, `short`, `byte`, and `float` but I haven't shown you those yet.

src/null.md

@@ -1,4 +1,4 @@
-# Return Values
+# Return
 
 In addition to running some code, a method can return a value to the code that is calling it.
 
@@ -10,28 +10,3 @@ int returnsEight() {
     return 8;
 }
 ```
-
-When the method is called, that expression can be used on the right hand side of an `=` to get the value returned by the method.
-
-```java
-int returnsEight() {
-    return 8;
-}
-
-void main() {
-    int value = returnsEight();
-    System.out.println(value);
-}
-```
-
-The method call can also be used directly in expressions without assigning its value to a variable first.
-
-```java
-String returnsName() {
-    return "Mariah";
-}
-
-void main() {
-    System.out.println(returnsName() + " is my name");
-}
-```