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# Chapter 13: References
In this chapter, we add references and structs.
You can also read [this chapter](https://github.com/SeaOfNodes/Simple/tree/linear-chapter13) in a linear Git revision history on the [linear](https://github.com/SeaOfNodes/Simple/tree/linear) branch and [compare](https://github.com/SeaOfNodes/Simple/compare/linear-chapter12...linear-chapter13) it to the previous chapter.
## References
Structs can reference other structs; references can be mutually recursive.
Any reference field can allow null or not, same as a normal variable.
```java
struct Person {
String name; // No string type, yet
int age;
FamilyTree tree?; // A family tree, or not
}
struct FamilyTree {
Person father?; // FamilyTree can reference Person
Person mother?;
Person []kids?;
}
```
Normal field syntax works:
e.g. `person.tree.father = new Person;` or
`person.tree.father.name = "Dad";`.
Reference fields all start out null, and can be tested as such. However, a
non-null reference field can only be stored with a non-null ref.
```java
struct N { N next; int i; }
N n = new N;
return n.next; // Value reports as null, despite field typed as not-null
```
And:
```java
struct N { N next; int i; }
N n = new N;
n.next = null; // <<-- Compile error, cannot store null into not-null field
return n.next;
```
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