1. A Primer for Understanding Recursion
The Function Stack
A Primer for Understanding Recursion

2. Let’s trace this code. Always start with main! class Main {
  static void method2() {
    PRINT("Method 2 was called");
  }
  static void method1() {
    PRINT("Method 1 was called");
    method2();
    PRINT("Method 1 still alive");
  public static void M/main (S/string[] args){
   method2();
   method1();
   PRINT("Finished!"); }
Let’s trace this code.
Always start with main!

3. Main is pushed onto the activation stack and has 3 things to do
class Main {
  static void method2() {
    PRINT("Method 2 was called");
  }
  static void method1() {
    PRINT("Method 1 was called");
    method2();
    PRINT("Method 1 still alive");
  public static void M/main (S/string[] args){
   method2();
   method1();
   PRINT("Finished!"); }
Main is pushed onto the activation stack and has 3 things to do
1 method2();
2 method1();
3 PRINT(…);
main

4. Time to call method2() main class Main { static void method2() {
    PRINT("Method 2 was called");
  }
  static void method1() {
    PRINT("Method 1 was called");
    method2();
    PRINT("Method 1 still alive");
  public static void M/main (S/string[] args){
   method2();
   method1();
   PRINT("Finished!"); }
Time to call method2()
1 method2();
2 method1();
3 PRINT(…);
main

5. method2() is pushed onto the stack
class Main {
  static void method2() {
    PRINT("Method 2 was called");
  }
  static void method1() {
    PRINT("Method 1 was called");
    method2();
    PRINT("Method 1 still alive");
  public static void M/main (S/string[] args){
   method2();
   method1();
   PRINT("Finished!"); }
method2
method2() is pushed onto the stack
1 method2();
2 method1();
3 PRINT(…);
main

6. method2() has 1 thing to do
class Main {
  static void method2() {
    PRINT("Method 2 was called");
  }
  static void method1() {
    PRINT("Method 1 was called");
    method2();
    PRINT("Method 1 still alive");
  public static void M/main (S/string[] args){
   method2();
   method1();
   PRINT("Finished!"); }
method2
1 PRINT(…);
method2() has 1 thing to do
1 method2();
2 method1();
3 PRINT(…);
main

7. method2 main class Main { static void method2() {
    PRINT("Method 2 was called");
  }
  static void method1() {
    PRINT("Method 1 was called");
    method2();
    PRINT("Method 1 still alive");
  public static void M/main (S/string[] args){
   method2();
   method1();
   PRINT("Finished!"); }
Method 2 was called
method2
1 PRINT(…);
1 method2();
2 method1();
3 PRINT(…);
main

8. method2 finishes and dies
class Main {
  static void method2() {
    PRINT("Method 2 was called");
  }
  static void method1() {
    PRINT("Method 1 was called");
    method2();
    PRINT("Method 1 still alive");
  public static void M/main (S/string[] args){
   method2();
   method1();
   PRINT("Finished!"); }
method2 finishes and dies
Method 2 was called
method2
1 PRINT(…);
1 method2();
2 method1();
3 PRINT(…);
main

9. Return to main for next item main class Main { static void method2() {
    PRINT("Method 2 was called");
  }
  static void method1() {
    PRINT("Method 1 was called");
    method2();
    PRINT("Method 1 still alive");
  public static void M/main (S/string[] args){
   method2();
   method1();
   PRINT("Finished!"); }
Return to main
for next item
Method 2 was called
1 method2();
2 method1();
3 PRINT(…);
main

10. Call method1 and put on the stack
class Main {
  static void method2() {
    PRINT("Method 2 was called");
  }
  static void method1() {
    PRINT("Method 1 was called");
    method2();
    PRINT("Method 1 still alive");
  public static void M/main (S/string[] args){
   method2();
   method1();
   PRINT("Finished!"); }
Call method1 and put on the stack
1 PRINT(…);
2 method2();
3 PRINT(…);
method1
Method 2 was called
1 method2();
2 method1();
3 PRINT(…);
main

11. method1 main class Main { static void method2() {
    PRINT("Method 2 was called");
  }
  static void method1() {
    PRINT("Method 1 was called");
    method2();
    PRINT("Method 1 still alive");
  public static void M/main (S/string[] args){
   method2();
   method1();
   PRINT("Finished!"); }
1 PRINT(…);
2 method2();
3 PRINT(…);
method1
Method 2 was called
Method 1 was called
1 method2();
2 method1();
3 PRINT(…);
main

12. method1 calls method2 method1 main class Main {
  static void method2() {
    PRINT("Method 2 was called");
  }
  static void method1() {
    PRINT("Method 1 was called");
    method2();
    PRINT("Method 1 still alive");
  public static void M/main (S/string[] args){
   method2();
   method1();
   PRINT("Finished!"); }
method1 calls
method2
1 PRINT(…);
2 method2();
3 PRINT(…);
method1
Method 2 was called
Method 1 was called
1 method2();
2 method1();
3 PRINT(…);
main

13. Put method2 on the stack method2 method1 main class Main {
  static void method2() {
    PRINT("Method 2 was called");
  }
  static void method1() {
    PRINT("Method 1 was called");
    method2();
    PRINT("Method 1 still alive");
  public static void M/main (S/string[] args){
   method2();
   method1();
   PRINT("Finished!"); }
Put method2
on the stack
method2
1 PRINT(…);
1 PRINT(…);
2 method2();
3 PRINT(…);
method1
Method 2 was called
Method 1 was called
1 method2();
2 method1();
3 PRINT(…);
main

14. method2 method1 main class Main { static void method2() {
    PRINT("Method 2 was called");
  }
  static void method1() {
    PRINT("Method 1 was called");
    method2();
    PRINT("Method 1 still alive");
  public static void M/main (S/string[] args){
   method2();
   method1();
   PRINT("Finished!"); }
method2
1 PRINT(…);
1 PRINT(…);
2 method2();
3 PRINT(…);
method1
Method 2 was called
Method 1 was called
1 method2();
2 method1();
3 PRINT(…);
main

15. method2 dies and is removed
class Main {
  static void method2() {
    PRINT("Method 2 was called");
  }
  static void method1() {
    PRINT("Method 1 was called");
    method2();
    PRINT("Method 1 still alive");
  public static void M/main (S/string[] args){
   method2();
   method1();
   PRINT("Finished!"); }
method2 dies and is removed
method2
1 PRINT(…);
1 PRINT(…);
2 method2();
3 PRINT(…);
method1
Method 2 was called
Method 1 was called
1 method2();
2 method1();
3 PRINT(…);
main

16. Return to method1 method1 main class Main { static void method2() {
    PRINT("Method 2 was called");
  }
  static void method1() {
    PRINT("Method 1 was called");
    method2();
    PRINT("Method 1 still alive");
  public static void M/main (S/string[] args){
   method2();
   method1();
   PRINT("Finished!"); }
Return to method1
1 PRINT(…);
2 method2();
3 PRINT(…);
method1
Method 2 was called
Method 1 was called
1 method2();
2 method1();
3 PRINT(…);
main

17. method1 main class Main { static void method2() {
    PRINT("Method 2 was called");
  }
  static void method1() {
    PRINT("Method 1 was called");
    method2();
    PRINT("Method 1 still alive");
  public static void M/main (S/string[] args){
   method2();
   method1();
   PRINT("Finished!"); }
1 PRINT(…);
2 method2();
3 PRINT(…);
method1
Method 2 was called
Method 1 was called
Method 1 still alive
1 method2();
2 method1();
3 PRINT(…);
main

18. method1 removed from stack
class Main {
  static void method2() {
    PRINT("Method 2 was called");
  }
  static void method1() {
    PRINT("Method 1 was called");
    method2();
    PRINT("Method 1 still alive");
  public static void M/main (S/string[] args){
   method2();
   method1();
   PRINT("Finished!"); }
method1 removed from stack
1 PRINT(…);
2 method2();
3 PRINT(…);
method1
Method 2 was called
Method 1 was called
Method 1 still alive
1 method2();
2 method1();
3 PRINT(…);
main

19. Go to third thing in main
class Main {
  static void method2() {
    PRINT("Method 2 was called");
  }
  static void method1() {
    PRINT("Method 1 was called");
    method2();
    PRINT("Method 1 still alive");
  public static void M/main (S/string[] args){
   method2();
   method1();
   PRINT("Finished!"); }
Go to third thing in main
Method 2 was called
Method 1 was called
Method 1 still alive
1 method2();
2 method1();
3 PRINT(…);
main

20. main class Main { static void method2() {
    PRINT("Method 2 was called");
  }
  static void method1() {
    PRINT("Method 1 was called");
    method2();
    PRINT("Method 1 still alive");
  public static void M/main (S/string[] args){
   method2();
   method1();
   PRINT("Finished!"); }
Method 2 was called
Method 1 was called
Method 1 still alive
Finished!
1 method2();
2 method1();
3 PRINT(…);
main

21. Remove main from the stack
class Main {
  static void method2() {
    PRINT("Method 2 was called");
  }
  static void method1() {
    PRINT("Method 1 was called");
    method2();
    PRINT("Method 1 still alive");
  public static void M/main (S/string[] args){
   method2();
   method1();
   PRINT("Finished!"); }
Remove main from the stack
Method 2 was called
Method 1 was called
Method 1 still alive
Finished!
1 method2();
2 method1();
3 PRINT(…);
main

22. End of program class Main { static void method2() {
    PRINT("Method 2 was called");
  }
  static void method1() {
    PRINT("Method 1 was called");
    method2();
    PRINT("Method 1 still alive");
  public static void M/main (S/string[] args){
   method2();
   method1();
   PRINT("Finished!"); }
End of program
Method 2 was called
Method 1 was called
Method 1 still alive
Finished!
Follow the white rabbit...