ReadThreadWithoutSout

// class version 52.0 (52)
// access flags 0x20
class Main$ReadThread extends java/lang/Thread {

  // compiled from: Main.java
  // access flags 0x8
  static INNERCLASS Main$ReadThread Main ReadThread

  // access flags 0x0
  <init>()V
   L0
    LINENUMBER 18 L0
    ALOAD 0
    INVOKESPECIAL java/lang/Thread.<init> ()V
    RETURN
   L1
    LOCALVARIABLE this LMain$ReadThread; L0 L1 0
    MAXSTACK = 1
    MAXLOCALS = 1

  // access flags 0x1
  public run()V
   L0
    LINENUMBER 21 L0
    GETSTATIC Main.i : I
    ISTORE 1
   L1
    LINENUMBER 22 L1
   FRAME APPEND [I]
    ILOAD 1
    ICONST_5
    IF_ICMPGE L2
   L3
    LINENUMBER 24 L3
    ILOAD 1
    GETSTATIC Main.i : I
    IF_ICMPEQ L1
   L4
    LINENUMBER 25 L4
    GETSTATIC java/lang/System.out : Ljava/io/PrintStream;
    NEW java/lang/StringBuilder
    DUP
    INVOKESPECIAL java/lang/StringBuilder.<init> ()V
    LDC "New value of i "
    INVOKEVIRTUAL java/lang/StringBuilder.append (Ljava/lang/String;)Ljava/lang/StringBuilder;
    ILOAD 1
    INVOKEVIRTUAL java/lang/StringBuilder.append (I)Ljava/lang/StringBuilder;
    INVOKEVIRTUAL java/lang/StringBuilder.toString ()Ljava/lang/String;
    INVOKEVIRTUAL java/io/PrintStream.println (Ljava/lang/String;)V
   L5
    LINENUMBER 26 L5
    GETSTATIC Main.i : I
    ISTORE 1
    GOTO L1
   L2
    LINENUMBER 29 L2
   FRAME SAME
    RETURN
   L6
    LOCALVARIABLE this LMain$ReadThread; L0 L6 0
    LOCALVARIABLE localI I L1 L6 1
    MAXSTACK = 3
    MAXLOCALS = 2
}

ReadThreadWithSout

// class version 52.0 (52)
// access flags 0x20
class Main$ReadThread extends java/lang/Thread {

  // compiled from: Main.java
  // access flags 0x8
  static INNERCLASS Main$ReadThread Main ReadThread

  // access flags 0x0
  <init>()V
   L0
    LINENUMBER 18 L0
    ALOAD 0
    INVOKESPECIAL java/lang/Thread.<init> ()V
    RETURN
   L1
    LOCALVARIABLE this LMain$ReadThread; L0 L1 0
    MAXSTACK = 1
    MAXLOCALS = 1

  // access flags 0x1
  public run()V
   L0
    LINENUMBER 21 L0
    GETSTATIC Main.i : I
    ISTORE 1
   L1
    LINENUMBER 22 L1
   FRAME APPEND [I]
    ILOAD 1
    ICONST_5
    IF_ICMPGE L2
   L3
    LINENUMBER 23 L3
    GETSTATIC java/lang/System.out : Ljava/io/PrintStream;
    LDC ""
    INVOKEVIRTUAL java/io/PrintStream.println (Ljava/lang/String;)V
   L4
    LINENUMBER 24 L4
    ILOAD 1
    GETSTATIC Main.i : I
    IF_ICMPEQ L1
   L5
    LINENUMBER 25 L5
    GETSTATIC java/lang/System.out : Ljava/io/PrintStream;
    NEW java/lang/StringBuilder
    DUP
    INVOKESPECIAL java/lang/StringBuilder.<init> ()V
    LDC "New value of i "
    INVOKEVIRTUAL java/lang/StringBuilder.append (Ljava/lang/String;)Ljava/lang/StringBuilder;
    ILOAD 1
    INVOKEVIRTUAL java/lang/StringBuilder.append (I)Ljava/lang/StringBuilder;
    INVOKEVIRTUAL java/lang/StringBuilder.toString ()Ljava/lang/String;
    INVOKEVIRTUAL java/io/PrintStream.println (Ljava/lang/String;)V
   L6
    LINENUMBER 26 L6
    GETSTATIC Main.i : I
    ISTORE 1
    GOTO L1
   L2
    LINENUMBER 29 L2
   FRAME SAME
    RETURN
   L7
    LOCALVARIABLE this LMain$ReadThread; L0 L7 0
    LOCALVARIABLE localI I L1 L7 1
    MAXSTACK = 3
    MAXLOCALS = 2
}

Test concurrency without volatile

public class Main {
    static int i = 0;

    static class WriteThread extends Thread {
        @Override
        public void run() {
            while (i < 5) {
                System.out.println("Increment i to " + (++i));
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }
    }

    static class ReadThread extends Thread {
        @Override
        public void run() {
            int localI = i;
            while (localI < 5) {
//                System.out.println("");
                if (localI != i) {
                    System.out.println("New value of i " + localI);
                    localI = i;
                }
            }
        }
    }

    public static void main(String[] args) throws InterruptedException {
        ReadThread readThread = new ReadThread();
        WriteThread writeThread = new WriteThread();
        writeThread.start();
        readThread.start();
    }
}