(Java)搭配synchronized達到StringBuilder的Thread-Safe

//https://coderanch.com/t/666742/java/understand-StringBuffer-thread-safe-StringBuilder

/*
 * Demonstrate StringBuilder is not thread safe
 */
public class StringBuilderWithMultThreadDemo implements Runnable {
 
    StringBuilder strBuilder;
 
    public StringBuilderWithMultThreadDemo() {
        strBuilder = new StringBuilder();
    }
 
    @Override
    public void run() {
 
        for (int i = 0; i < 50000; i++) {
            //addChar(); ///non Thread-Safe
        	addCharTS(); //Thread-Safe
        }
    }
 
    public void addChar() {
        /*
         * Here appended 6 A and removed 5 A at each call to this method. Total
         * 1 A at each call Expected Accurate Total StringBuilder length = loops
         * in run method * 1 i.e. 50000 * 1 for one thread Here we have 2
         * threads so = 100000
         */
 
        try {
            strBuilder.append("A");
            strBuilder.append("A");
            strBuilder.append("A");
            strBuilder.deleteCharAt(0);
            strBuilder.append("A");
            strBuilder.append("A");
            strBuilder.append("A");
            for (int i = 0; i < 4; i++) {
                strBuilder.deleteCharAt(0);
            }
        } catch (ArrayIndexOutOfBoundsException e) {
            System.out.println("A wasn't at index 0 " + e.getMessage());
        }
    }
    
    public synchronized void addCharTS() {
        /*
         * Here appended 6 A and removed 5 A at each call to this method. Total
         * 1 A at each call Expected Accurate Total StringBuilder length = loops
         * in run method * 1 i.e. 50000 * 1 for one thread Here we have 2
         * threads so = 100000
         */
 
        try {
            strBuilder.append("A");
            strBuilder.append("A");
            strBuilder.append("A");
            strBuilder.deleteCharAt(0);
            strBuilder.append("A");
            strBuilder.append("A");
            strBuilder.append("A");
            for (int i = 0; i < 4; i++) {
                strBuilder.deleteCharAt(0);
            }
        } catch (ArrayIndexOutOfBoundsException e) {
            System.out.println("A wasn't at index 0 " + e.getMessage());
        }
    }
     
 
    public static void main(String[] args) {
 
        StringBuilderWithMultThreadDemo strBldrWthThrdDmobj1 = new StringBuilderWithMultThreadDemo();
 
        Thread threadOne = new Thread(strBldrWthThrdDmobj1, "Thread One");
        Thread threadTwo = new Thread(strBldrWthThrdDmobj1, "Thread Two");
        threadOne.start();
        threadTwo.start();
 
        try {
            threadOne.join();
            threadTwo.join();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("Final StringBuilder Length: " + strBldrWthThrdDmobj1.strBuilder.length());
 
    }
}