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THE MUTABLE STATE MONSTER AND HOW TO DEFEAT IT ANUP COWKUR

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PROLOGUE

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WHAT IS STATE?

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WHAT IS STATE? WIKIPEDIA A computer program stores data in variables, which represent storage locations in the computer's memory. The contents of these memory locations, at any given point in the program's execution, is called the program's state.

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STATE IS THE CURRENT VALUE OF DATA AT ANY POINT OF EXECUTION OF THE PROGRAM –Me TEXT

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CHAPTER 1: THE WIZARD

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A LONG TIME AGO, THERE WAS A GRAND WIZARD

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JOHN MCCARTHY ACTUAL PHOTO

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HE PIONEERED ARTIFICIAL INTELLIGENCE

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IN 1958, HE CAME UP WITH A LITTLE LANGUAGE CALLED LISP

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LISP ACTUAL SYNTAX

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LISP WAS THE FIRST LANGUAGE TO HAVE AUTOMATIC MEMORY MANAGEMENT USING A GARBAGE COLLECTOR

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IT WAS ALSO ONE OF THE FIRST LANGUAGE TO HAVE ATOMS - DATA THAT COULD NOT BE DIVIDED FURTHER

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BUT WHY?

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CHAPTER 2: THE MONSTER

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DO THE FOLLOWING PROGRAMS WORK CORRECTLY IN A MULTI- THREADED SYSTEM?

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PROGRAM 1

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public void checkAndPut(final String key,
 final String value) {
 if (!map.containsKey(key)) {
 map.put(key, value);
 }
 }

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public void checkAndPut(final String key,
 final String value) {
 if (!map.containsKey(key)) {
 map.put(key, value);
 }
 }

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public void checkAndPut(final String key,
 final String value) {
 if (!map.containsKey(key)) {
 map.put(key, value);
 }
 }

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public void checkAndPut(final String key,
 final String value) {
 if (!map.containsKey(key)) { < - Thread 1
 map.put(key, value);
 }
 }

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public void checkAndPut(final String key,
 final String value) {
 if (!map.containsKey(key)) {
 map.put(key, value);
 }
 } Thread 2 removes key from map

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public void checkAndPut(final String key,
 final String value) {
 if (!map.containsKey(key)) {
 map.put(key, value); <- Thread 1
 }
 }

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public void checkAndPut(final String key,
 final String value) {
 if (!map.containsKey(key)) {
 map.put(key, value);
 }
 }

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CHECK-THEN-ACT PROBLEM

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WE CAN FIX IT!

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public void checkAndPut(final String key,
 final String value) {
 synchronized (this) {
 if (!map.containsKey(key)) {
 map.put(key, value);
 }
 }
 }

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public void checkAndPut(final String key,
 final String value) {
 synchronized (this) {
 if (!map.containsKey(key)) {
 map.put(key, value);
 }
 }
 }

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PROGRAM 2

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public class EntCounter {
 private long count = 0;
 
 public long increment() {
 return ++count;
 }
 }

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public class EntCounter {
 private long count = 0;
 
 public long increment() {
 return ++count;
 }
 }

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public class EntCounter {
 private long count = 0;
 
 public long increment() {
 return ++count;
 }
 }

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public class EntCounter {
 private long count = 0;
 
 public long increment() {
 return ++count; <- Thread 1
 }
 }

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public class EntCounter {
 private long count = 0;
 
 public long increment() {
 1. Read
 return ++count; 2. Modify
 3. Write 
 }
 }

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public class EntCounter {
 private long count = 0;
 
 public long increment() {
 1. Read <- Thread 1
 return ++count; 2. Modify
 3. Write 
 }
 }

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public class EntCounter {
 private long count = 0;
 
 public long increment() {
 1. Read <- Thread 1
 return ++count; 2. Modify
 3. Write 
 }
 } Thread 2 modifies the value of count

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public class EntCounter {
 private long count = 0;
 
 public long increment() {
 1. Read
 return ++count; 2. Modify <- Thread 1
 3. Write 
 }
 }

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public class EntCounter {
 private long count = 0;
 
 public long increment() {
 1. Read
 return ++count; 2. Modify
 3. Write <- Thread 1 
 }
 }

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public class EntCounter {
 private long count = 0;
 
 public long increment() {
 1. Read
 return ++count; 2. Modify
 3. Write
 }
 }

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READ-MODIFY- WRITE PROBLEM

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BUT WE CAN FIX IT!

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public class EntCounter {
 private long count = 0;
 
 public long increment() {
 synchronized (this) {
 return ++count;
 }
 }
 }

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public class EntCounter {
 private long count = 0;
 
 public long increment() {
 synchronized (this) {
 return ++count;
 }
 }
 }

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PROGRAM 3

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public class RunFrodoRun {
 
 private boolean stopped = false;
 
 public void run() {
 while (!stopped) {
 // RUN!!!
 }
 }
 
 public void stop() {
 this.stopped = true;
 }
 }

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public class RunFrodoRun {
 
 private boolean stopped = false;
 
 public void run() {
 while (!stopped) {
 // RUN!!!
 }
 }
 
 public void stop() {
 this.stopped = true;
 }
 }

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public class RunFrodoRun {
 
 private boolean stopped = false;
 
 public void run() {
 while (!stopped) {
 // RUN!!!
 }
 }
 
 public void stop() {
 this.stopped = true;
 }
 }

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public class RunFrodoRun {
 
 private boolean stopped = false;
 
 public void run() {
 while (!stopped) {
 // RUN!!!
 }
 }
 
 public void stop() {
 this.stopped = true;
 }
 }

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public class RunFrodoRun {
 
 private boolean stopped = false;
 
 public void run() {
 while (!stopped) {
 // RUN!!!
 }
 }
 
 public void stop() {
 this.stopped = true;
 }
 }

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public class RunFrodoRun {
 
 private boolean stopped = false;
 
 public void run() {
 while (!stopped) {
 // RUN!!!
 }
 }
 
 public void stop() {
 this.stopped = true;
 }
 }

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public class RunFrodoRun {
 
 private boolean stopped = false;
 
 public void run() { <- Thread 1
 while (!stopped) {
 // RUN!!!
 }
 }
 
 public void stop() {
 this.stopped = true;
 }
 }

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public class RunFrodoRun {
 
 private boolean stopped = false;
 
 public void run() {
 while (!stopped) {
 // RUN!!!
 }
 }
 
 public void stop() { <- Thread 2
 this.stopped = true;
 }
 }

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public class RunFrodoRun {
 
 private boolean stopped = false;
 
 public void run() {
 while (!stopped) { <- Thread 1
 // RUN!!!
 }
 }
 
 public void stop() {
 this.stopped = true;
 }
 }

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public class RunFrodoRun {
 
 private boolean stopped = false;
 
 public void run() {
 while (!stopped) { <- Thread 1
 // RUN!!!
 }
 }
 
 public void stop() {
 this.stopped = true;
 }
 } MAIN MEMORY stopped

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public class RunFrodoRun {
 
 private boolean stopped = false;
 
 public void run() {
 while (!stopped) { <- Thread 1
 // RUN!!!
 }
 }
 
 public void stop() {
 this.stopped = true;
 }
 } MAIN MEMORY stopped THREAD LOCAL stopped

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public class RunFrodoRun {
 
 private boolean stopped = false;
 
 public void run() {
 while (!stopped) { <- Thread 1
 // RUN!!!
 }
 }
 
 public void stop() {
 this.stopped = true;
 }
 } MAIN MEMORY stopped THREAD LOCAL stopped L1/L2 PROCESSOR CACHE stopped

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public class RunFrodoRun {
 
 private boolean stopped = false;
 
 public void run() {
 while (!stopped) {
 // RUN!!!
 }
 }
 
 public void stop() {
 this.stopped = true;
 }
 }

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VISIBILITY PROBLEM

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WE CAN SOOOO FIX THIS!

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public class RunFrodoRun {
 
 private volatile boolean stopped = false;
 
 public void run() {
 while (!stopped) {
 // RUN!!!
 }
 }
 
 public void stop() {
 this.stopped = true;
 }
 }

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public class RunFrodoRun {
 
 private volatile boolean stopped = false;
 
 public void run() {
 while (!stopped) {
 // RUN!!!
 }
 }
 
 public void stop() {
 this.stopped = true;
 }
 }

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PROGRAM 4

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public class OrcFunerals {
 int orcsKilledYesterday = 0;
 int orcsKilledToday = 0;
 
 public void init() {
 orcsKilledYesterday = 1;
 orcsKilledToday = 2;
 }
 
 public int getDeadOrcs() {
 return orcsKilledYesterday + orcsKilledToday;
 }
 }

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public class OrcFunerals {
 int orcsKilledYesterday = 0;
 int orcsKilledToday = 0;
 
 public void init() {
 orcsKilledYesterday = 1;
 orcsKilledToday = 2;
 }
 
 public int getDeadOrcs() {
 return orcsKilledYesterday + orcsKilledToday;
 }
 }

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public class OrcFunerals {
 int orcsKilledYesterday = 0;
 int orcsKilledToday = 0;
 
 public void init() {
 orcsKilledYesterday = 1;
 orcsKilledToday = 2;
 }
 
 public int getDeadOrcs() {
 return orcsKilledYesterday + orcsKilledToday;
 }
 }

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public class OrcFunerals {
 int orcsKilledYesterday = 0;
 int orcsKilledToday = 0;
 
 public void init() {
 orcsKilledYesterday = 1;
 orcsKilledToday = 2;
 }
 
 public int getDeadOrcs() {
 return orcsKilledYesterday + orcsKilledToday;
 }
 }

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public class OrcFunerals {
 int orcsKilledYesterday = 0;
 int orcsKilledToday = 0;
 
 public void init() {
 orcsKilledYesterday = 1;
 orcsKilledToday = 2;
 }
 
 public int getDeadOrcs() {
 return orcsKilledYesterday + orcsKilledToday;
 }
 }

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public class OrcFunerals {
 int orcsKilledYesterday = 0;
 int orcsKilledToday = 0;
 
 public void init() {
 orcsKilledYesterday = 1;
 orcsKilledToday = 2;
 }
 
 public int getDeadOrcs() {
 return orcsKilledYesterday + orcsKilledToday;
 }
 }

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public class OrcFunerals {
 int orcsKilledYesterday = 0;
 int orcsKilledToday = 0;
 
 public void init() {
 orcsKilledYesterday = 1; <- thread 1
 orcsKilledToday = 2;
 }
 
 public int getDeadOrcs() {
 return orcsKilledYesterday + orcsKilledToday;
 }
 }

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public class OrcFunerals {
 int orcsKilledYesterday = 0;
 int orcsKilledToday = 0;
 
 public void init() {
 orcsKilledYesterday = 1;
 orcsKilledToday = 2;
 }
 
 public int getDeadOrcs() { <- Thread 2
 return orcsKilledYesterday + orcsKilledToday;
 }
 }

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public class OrcFunerals {
 int orcsKilledYesterday = 0;
 int orcsKilledToday = 0;
 
 public void init() {
 orcsKilledYesterday = 1;
 orcsKilledToday = 2;
 }
 
 public int getDeadOrcs() {
 return orcsKilledYesterday + orcsKilledToday;
 }
 } Result should be 1

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public class OrcFunerals {
 int orcsKilledYesterday = 0;
 int orcsKilledToday = 0;
 
 public void init() {
 orcsKilledYesterday = 1;
 orcsKilledToday = 2;
 }
 
 public int getDeadOrcs() {
 return orcsKilledYesterday + orcsKilledToday;
 }
 } But the JVM can re-order incorrectly synchronized operations

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public class OrcFunerals {
 int orcsKilledYesterday = 0;
 int orcsKilledToday = 0;
 
 public void init() {
 orcsKilledYesterday = 1; <- Operation 1
 orcsKilledToday = 2; <- Operation 2
 }
 
 public int getDeadOrcs() {
 return orcsKilledYesterday + orcsKilledToday;
 }
 }

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public class OrcFunerals {
 int orcsKilledYesterday = 0;
 int orcsKilledToday = 0;
 
 public void init() { orcsKilledToday = 2; <- Operation 2
 orcsKilledYesterday = 1; <- Operation 1
 }
 
 public int getDeadOrcs() {
 return orcsKilledYesterday + orcsKilledToday;
 }
 }

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public class OrcFunerals {
 int orcsKilledYesterday = 0;
 int orcsKilledToday = 0;
 
 public void init() { orcsKilledToday = 2; <- Thread 1
 orcsKilledYesterday = 1;
 }
 
 public int getDeadOrcs() {
 return orcsKilledYesterday + orcsKilledToday;
 }
 }

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public class OrcFunerals {
 int orcsKilledYesterday = 0;
 int orcsKilledToday = 0;
 
 public void init() {
 orcsKilledYesterday = 1;
 orcsKilledToday = 2;
 }
 
 public int getDeadOrcs() { <- Thread 2
 return orcsKilledYesterday + orcsKilledToday;
 }
 }

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public class OrcFunerals {
 int orcsKilledYesterday = 0;
 int orcsKilledToday = 0;
 
 public void init() { orcsKilledToday = 2;
 orcsKilledYesterday = 1;
 }
 
 public int getDeadOrcs() {
 return orcsKilledYesterday + orcsKilledToday;
 }
 } Result in this case will be 2

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public class OrcFunerals {
 int orcsKilledYesterday = 0;
 int orcsKilledToday = 0;
 
 public void init() { orcsKilledToday = 2;
 orcsKilledYesterday = 1;
 }
 
 public int getDeadOrcs() {
 return orcsKilledYesterday + orcsKilledToday;
 }
 }

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ORDERING PROBLEM

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FIX!

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public class OrcFunerals {
 volatile int orcsKilledYesterday = 0;
 volatile int orcsKilledToday = 0;
 
 public void init() {
 orcsKilledYesterday = 1;
 orcsKilledToday = 2;
 }
 
 public int getDeadOrcs() {
 return orcsKilledYesterday + orcsKilledToday;
 }
 }

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public class OrcFunerals {
 volatile int orcsKilledYesterday = 0;
 volatile int orcsKilledToday = 0;
 
 public void init() {
 orcsKilledYesterday = 1;
 orcsKilledToday = 2;
 }
 
 public int getDeadOrcs() {
 return orcsKilledYesterday + orcsKilledToday;
 }
 }

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BUT WE FIXED IT, RIGHT?

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USER

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USER ORDERS

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USER ORDERS PAYMENT

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USER ORDERS PAYMENT ACTIVITY 1

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USER ORDERS PAYMENT ACTIVITY 1 ACTIVITY 2

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USER ORDERS PAYMENT ACTIVITY 1 ACTIVITY 2 SYNCHRONIZE

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USER ORDERS PAYMENT ACTIVITY 1 ACTIVITY 2 ACTIVITY 3 SYNCHRONIZE

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USER ORDERS PAYMENT ACTIVITY 1 ACTIVITY 2 ACTIVITY 3 SYNCHRONIZE CHAT

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USER ORDERS PAYMENT ACTIVITY 1 ACTIVITY 2 ACTIVITY 3 CHAT ACTIVITY 4 SYNCHRONIZE

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SYNCHRONIZE

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DEADLOCK

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SAY SYNCHRONIZE ONE MORE TIME!!!

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CONCURRENCY IS HARD

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OUR BRAINS ARE SMALL

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SO WHEN YOU MIX SHARED MUTABLE STATE WITH COMPLEXITY

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THE MUTABLE STATE MONSTER IS BORN ACTUAL PHOTO

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CHAPTER 3: THE PROPHET

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THERE ONCE WAS A PROPHET CALLED GORDON MOORE

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HE CO-FOUNDED A SEMICONDUCTOR COMPANY CALLED INTEL

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GORDON MOORE ACTUAL PHOTO

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IN 1965, HE MADE A PROPHECY

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THE NUMBER OF TRANSISTORS IN A DENSE INTEGRATED CIRCUIT DOUBLES APPROXIMATELY EVERY TWO YEARS GORDON MOORE MOORE’S LAW

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AND BEHOLD, THE PROPHECY WAS TRUE

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BUT IN THE PAST DECADE, THE PROPHECY HASN’T HELD

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OUR COMPUTERS WON’T BECOME SIGNIFICANTLY FASTER EVERY YEAR

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WE CANNOT STUFF MORE TRANSISTORS INTO OUR PROCESSORS ANYMORE

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WE HAVE TO MAKE OUR PROGRAMS USE MULTIPLE CORES EFFECTIVELY

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OUR TOOLS SUCK

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WE NEED TO DO BETTER

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WE’RE JUST GOING TO HAVE TO..

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CHAPTER 4: THE FELLOWSHIP OF THE FUNCTIONAL PROGRAMMERS

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JOHN MCCARTHY, ALONZO CHURCH, HASKELL CURRY, RICH HICKEY… ACTUAL GROUP PHOTO

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THEY WERE THINKING OF AN ALTERNATIVE WAY OF REPRESENTING COMPUTATION

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CAN WE REPRESENT PROGRAMS AS A SERIES OF DATA TRANSFORMATIONS INSTEAD OF A SERIES OF SEQUENTIAL DATA MUTATIONS?

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IMPERATIVE PROGRAM

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int[] numbers = {5,10,15};
 
 int sum() {
 int total = 0;
 
 for(int i=0; i

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int[] numbers = {5,10,15};
 
 int sum() {
 int total = 0;
 
 for(int i=0; i

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int[] numbers = {5,10,15};
 
 int sum() {
 int total = 0;
 
 for(int i=0; i

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int[] numbers = {5,10,15};
 
 int sum() {
 int total = 0;
 
 for(int i=0; i

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int[] numbers = {5,10,15};
 
 int sum() {
 int total = 0;
 
 for(int i=0; i

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int[] numbers = {5,10,15};
 
 int sum() {
 int total = 0;
 
 for(int i=0; i

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int[] numbers = {5,10,15};
 
 int sum() {
 int total = 0;
 
 for(int i=0; i

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int[] numbers = {5,10,15};
 
 int sum() {
 int total = 0;
 
 for(int i=0; i

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FUNCTIONAL PROGRAM

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int compute() {
 return sum(sum(sum(0, 5), 10), 15);
 }
 
 int sum(int x, int y) {
 return x + y;
 }

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int compute() {
 return sum(sum(sum(0, 5), 10), 15);
 }
 
 int sum(int x, int y) {
 return x + y;
 }

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int compute() {
 return sum(sum(sum(0, 5), 10), 15);
 }
 
 int sum(int x, int y) {
 return x + y;
 }

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int compute() {
 return sum(sum(sum(0, 5), 10), 15);
 }
 
 int sum(int x, int y) {
 return x + y;
 }

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int compute() {
 return sum(sum(sum(0, 5), 10), 15);
 }
 
 int sum(int x, int y) {
 return x + y;
 }

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int compute() {
 return sum(sum(sum(0, 5), 10), 15);
 }
 
 int sum(int x, int y) {
 return x + y;
 } The result is still 30

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FUNCTIONAL PROGRAMMING IS ABOUT MODELLING OUR PROGRAMS IN TERMS OF EXPRESSIONS RATHER THAN STEP BY STEP INSTRUCTIONS

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THE IDEA IS TO BE AS DECLARATIVE AS POSSIBLE AND LET THE SYSTEM WORRY ABOUT LOW LEVEL DETAILS

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WE DON'T NEED TO KNOW EVERYTHING ABOUT FUNCTIONAL PROGRAMMING. WE JUST NEED TO STEAL SOME GOOD IDEAS

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IDEA 1: REFERENTIAL TRANSPARENCY

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REFERENTIAL TRANSPARENCY MEANS THERE IS NO DIFFERENCE BETWEEN THE VALUE AND THE REFERENCE TO A PIECE OF DATA

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Book book = new Book(“Vol 2”, “Green”);

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Book book = new Book(“Vol 2”, “Green”);

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Book book = new Book(“Vol 2”, “Green”); book.setTitle(“Vol 3");

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Book book = new Book(“Vol 2”, “Green”); book.setTitle(“Vol 3”); book.setColour(“Blue");

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Book book = new Book(“Vol 2”, “Green”); book.setTitle(“Vol 3”); book.setColour(“Blue");

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Book book = new Book(“Vol 2”, “Green”); book.setTitle(“Vol 3”); book.setColour(“Blue"); The same book object can have different values at different points in time

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Book book = new Book(“Vol 2”, “Green”); book.setTitle(“Vol 3”); book.setColour(“Blue"); Not referentially transparent X

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TO ACHIEVE REFERENTIAL TRANSPARENCY, WE NEED TO STEAL ANOTHER IDEA

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IDEA 2: IMMUTABILITY

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IMMUTABILITY MEANS DATA ONCE CREATED CANNOT BE MODIFIED

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Book book = new Book(“Vol 2”, “Green”);

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Book book = new Book(“Vol 2”, “Green”);

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Book book = new Book(“Vol 2”, “Green”); book.setTitle(“Vol 3”);

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Book book = new Book(“Vol 2”, “Green”); book.setTitle(“Vol 3”); Can’t change immutable data X

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INSTEAD OF MODIFYING, WE CAN MAKE A WHOLE NEW OBJECT WITH OUR NEW PROPERTIES

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MUTABLE BOOK STORE

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public class BookStore {
 private List books;
 
 public BookStore(final List books) {
 this.books = books;
 }
 
 public void addBooks(List additionalBooks) {
 this.books.addAll(additionalBooks);
 }
 
 public List getBooks() {
 return books;
 }
 }

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public class BookStore {
 private List books;
 
 public BookStore(final List books) {
 this.books = books;
 }
 
 public void addBooks(List additionalBooks) {
 this.books.addAll(additionalBooks);
 }
 
 public List getBooks() {
 return books;
 }
 }

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public class BookStore {
 private List books;
 
 public BookStore(final List books) {
 this.books = books;
 }
 
 public void addBooks(List additionalBooks) {
 this.books.addAll(additionalBooks);
 }
 
 public List getBooks() {
 return books;
 }
 }

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public class BookStore {
 private List books;
 
 public BookStore(final List books) {
 this.books = books;
 }
 
 public void addBooks(List additionalBooks) {
 this.books.addAll(additionalBooks);
 }
 
 public List getBooks() {
 return books;
 }
 }

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public class BookStore {
 private List books;
 
 public BookStore(final List books) {
 this.books = books;
 }
 
 public void addBooks(List additionalBooks) {
 this.books.addAll(additionalBooks);
 }
 
 public List getBooks() {
 return books;
 }
 }

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public class BookStore {
 private List books;
 
 public BookStore(final List books) {
 this.books = books;
 }
 
 public void addBooks(List additionalBooks) {
 this.books.addAll(additionalBooks); <- Mutation
 }
 
 public List getBooks() {
 return books;
 }
 }

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public class BookStore {
 private List books;
 
 public BookStore(final List books) {
 this.books = books;
 }
 
 public void addBooks(List additionalBooks) {
 this.books.addAll(additionalBooks);
 }
 
 public List getBooks() {
 return books;
 }
 } How do we make this immutable?

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IMMUTABLE BOOK STORE

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public class BookStore {
 private final List books;
 
 public BookStore(final List books) {
 this.books = books;
 }
 
 public BookStore addBooks(List additionalBooks) {
 List newBooks = getBooks();
 
 newBooks.addAll(additionalBooks);
 
 return new BookStore(newBooks);
 }
 
 public List getBooks() {
 List copy = new ArrayList<>();
 for (String book : books) {
 copy.add(book);
 }
 return copy;
 }
 }

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public class BookStore {
 private final List books;
 
 public BookStore(final List books) {
 this.books = books;
 }
 
 public BookStore addBooks(List additionalBooks) {
 List newBooks = getBooks();
 
 newBooks.addAll(additionalBooks);
 
 return new BookStore(newBooks);
 }
 
 public List getBooks() {
 List copy = new ArrayList<>();
 for (String book : books) {
 copy.add(book);
 }
 return copy;
 }
 }

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public class BookStore {
 private final List books;
 
 public BookStore(final List books) {
 this.books = books;
 }
 
 public BookStore addBooks(List additionalBooks) {
 List newBooks = getBooks();
 
 newBooks.addAll(additionalBooks);
 
 return new BookStore(newBooks);
 }
 
 public List getBooks() {
 List copy = new ArrayList<>();
 for (String book : books) {
 copy.add(book);
 }
 return copy;
 }
 }

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public class BookStore {
 private final List books;
 
 public BookStore(final List books) {
 this.books = books;
 }
 
 public BookStore addBooks(List additionalBooks) {
 List newBooks = getBooks();
 
 newBooks.addAll(additionalBooks);
 
 return new BookStore(newBooks);
 }
 
 public List getBooks() {
 List copy = new ArrayList<>();
 for (String book : books) {
 copy.add(book);
 }
 return copy;
 }
 }

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public class BookStore {
 private final List books;
 
 public BookStore(final List books) {
 this.books = books;
 }
 
 public BookStore addBooks(List additionalBooks) {
 List newBooks = getBooks();
 
 newBooks.addAll(additionalBooks);
 
 return new BookStore(newBooks);
 }
 
 public List getBooks() {
 List copy = new ArrayList<>();
 for (String book : books) { <- Deep copy
 copy.add(book);
 }
 return copy;
 }
 }

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public class BookStore {
 private final List books;
 
 public BookStore(final List books) {
 this.books = books;
 }
 
 public BookStore addBooks(List additionalBooks) {
 List newBooks = getBooks();
 
 newBooks.addAll(additionalBooks);
 
 return new BookStore(newBooks);
 }
 
 public List getBooks() {
 List copy = new ArrayList<>();
 for (String book : books) {
 copy.add(book);
 }
 return copy; <- Return deep copy, not the original
 }
 }

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public class BookStore {
 private final List books;
 
 public BookStore(final List books) {
 this.books = books;
 }
 
 public BookStore addBooks(List additionalBooks) {
 List newBooks = getBooks();
 
 newBooks.addAll(additionalBooks);
 
 return new BookStore(newBooks);
 }
 
 public List getBooks() {
 List copy = new ArrayList<>();
 for (String book : books) {
 copy.add(book);
 }
 return copy;
 }
 }

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public class BookStore {
 private final List books;
 
 public BookStore(final List books) {
 this.books = books;
 }
 
 public BookStore addBooks(List additionalBooks) {
 List newBooks = getBooks(); <- Get deep copy
 
 newBooks.addAll(additionalBooks);
 
 return new BookStore(newBooks);
 }
 
 public List getBooks() {
 List copy = new ArrayList<>();
 for (String book : books) {
 copy.add(book);
 }
 return copy;
 }
 }

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public class BookStore {
 private final List books;
 
 public BookStore(final List books) {
 this.books = books;
 }
 
 public BookStore addBooks(List additionalBooks) {
 List newBooks = getBooks();
 
 newBooks.addAll(additionalBooks); <- Add to deep copy 
 
 return new BookStore(newBooks);
 }
 
 public List getBooks() {
 List copy = new ArrayList<>();
 for (String book : books) {
 copy.add(book);
 }
 return copy;
 }
 }

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public class BookStore {
 private final List books;
 
 public BookStore(final List books) {
 this.books = books;
 }
 
 public BookStore addBooks(List additionalBooks) {
 List newBooks = getBooks();
 
 newBooks.addAll(additionalBooks);
 
 return new BookStore(newBooks); <- Return a new BookStore
 }
 
 public List getBooks() {
 List copy = new ArrayList<>();
 for (String book : books) {
 copy.add(book);
 }
 return copy;
 }
 }

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STORE 1

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STORE 1 ADD SOME BOOKS

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STORE 1 ADD SOME BOOKS STORE 2

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The original book store is always the original book store

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Referential Transparency!

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Referential Transparency!

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HOW DOES THIS STUFF MAKE CONCURRENCY SAFER IN OUR APPS?

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BOOK STORE

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BOOK STORE

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BOOK STORE UI Thread

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BOOK STORE UI Thread Observable.just(bookStore)
 .map(addSomeBooks())
 .map(sortAllBooks())
 .subscribeOn(Schedulers.io())
 .observeOn(AndroidSchedulers.mainThread())
 .subscribe(
 // Update list with new BookStore
 );

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BOOK STORE UI Thread Observable.just(bookStore)
 .map(addSomeBooks())
 .map(sortAllBooks())
 .subscribeOn(Schedulers.io())
 .observeOn(AndroidSchedulers.mainThread())
 .subscribe(
 // Update list with new BookStore
 );

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BOOK STORE UI Thread Observable.just(bookStore)
 .map(addSomeBooks())
 .map(sortAllBooks())
 .subscribeOn(Schedulers.io())
 .observeOn(AndroidSchedulers.mainThread())
 .subscribe(
 // Update list with new BookStore
 );

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BOOK STORE UI Thread Observable.just(bookStore)
 .map(addSomeBooks())
 .map(sortAllBooks())
 .subscribeOn(Schedulers.io())
 .observeOn(AndroidSchedulers.mainThread())
 .subscribe(
 // Update list with new BookStore
 );

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BOOK STORE UI Thread Observable.just(bookStore)
 .map(addSomeBooks())
 .map(sortAllBooks())
 .subscribeOn(Schedulers.io())
 .observeOn(AndroidSchedulers.mainThread())
 .subscribe(
 // Update list with new BookStore
 );

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BOOK STORE UI Thread Observable.just(bookStore)
 .map(addSomeBooks())
 .map(sortAllBooks())
 .subscribeOn(Schedulers.io())
 .observeOn(AndroidSchedulers.mainThread())
 .subscribe(
 // Update list with new BookStore
 );

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BOOK STORE UI Thread Observable.just(bookStore)
 .map(addSomeBooks())
 .map(sortAllBooks())
 .subscribeOn(Schedulers.io())
 .observeOn(AndroidSchedulers.mainThread())
 .subscribe(
 // Update list with new BookStore
 );

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BOOK STORE UI Thread Observable.just(bookStore)
 .map(addSomeBooks())
 .map(sortAllBooks())
 .subscribeOn(Schedulers.io())
 .observeOn(AndroidSchedulers.mainThread())
 .subscribe(
 // Update list with new BookStore
 ); BOOK STORE 1 Background Thread

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BOOK STORE UI Thread Observable.just(bookStore)
 .map(addSomeBooks())
 .map(sortAllBooks())
 .subscribeOn(Schedulers.io())
 .observeOn(AndroidSchedulers.mainThread())
 .subscribe(
 // Update list with new BookStore
 ); BOOK STORE 1 Background Thread No incompletely modified data

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BOOK STORE UI Thread BOOK STORE 1 Background Thread Meanwhile, Another background thread writes to our store

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BOOK STORE UI Thread BOOK STORE 1 Background Thread It gets it’s own copy do whatever it wants BOOK STORE FOR THREAD 2 Background Thread 2

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BOOK STORE UI Thread BOOK STORE 1 Background Thread No lock based synchronization necessary BOOK STORE FOR THREAD 2 Background Thread 2

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BOOK STORE UI Thread Observable.just(bookStore)
 .map(addSomeBooks())
 .map(sortAllBooks())
 .subscribeOn(Schedulers.io())
 .observeOn(AndroidSchedulers.mainThread())
 .subscribe(
 // Update list with new BookStore
 ); BOOK STORE 1 Background Thread BOOK STORE 2

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UI Thread Observable.just(bookStore)
 .map(addSomeBooks())
 .map(sortAllBooks())
 .subscribeOn(Schedulers.io())
 .observeOn(AndroidSchedulers.mainThread())
 .subscribe(
 // Update list with new BookStore
 ); BOOK STORE 2

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UI Thread BOOK STORE 2

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BUT MY POOR GARBAGE COLLECTOR!

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CORRECTNESS IS OFTEN MORE IMPORTANT THAT RAW PERFORMANCE

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YOU CAN USE IMMUTABILITY ONLY WHERE YOU EXPECT CONCURRENCY

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CHAPTER 5: A NEW AGE

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FUNCTIONAL IS GETTING POPULAR

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FRONTEND

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BACKEND

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WHAT ABOUT GOOD OLD ANDROID?

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JAVA 8 ON ANDROID INTRODUCED STREAMS, LAMBDAS AND METHOD REFERENCES

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WE HAVE RX TO TRANSFORM DATA

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WE HAVE KOTLIN WHICH HAS FIRST CLASS SUPPORT FOR IMMUTABLE DATA

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YOU DON’T HAVE TO DO IT ALL AT ONCE

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AND YOU DON’T HAVE TO DO IT ALONE

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THE FUTURE IS BRIGHT

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THANK YOU! @anupcowkur