多线程|| 爪哇

Question

I've a program that's as follows below. I want three concurrent threads to add different Strings to infiList ("This", "is", "infinite") using thread synchronization.

I want to append certain threads also, for example;

• If the last word in infiList is currently “This”, the thread should append word “is” to infiList .

• If the last word in infiList is currently “is”, the thread should append word “infinite” to infiList .

• If the last word in infiList is currently “infinite”, or if infiList is still empty, the thread should append word “This” to infiList .

• At any time, infiList should contain “This” only at the beginning of the list or directly after an occurrence of “infinite”, “is” should occur in the list only directly after a “This”, and an “infinite” should occur only directly after an “is”.

Any help as to how to do this is appreciated.

import java.util.ArrayList;

public class Multithreading implements Runnable {

public static ArrayList<String> infiList = new ArrayList<>();

@Override
public void run() {
    for (int i=0; i<100; i++) {
        String s = null;
        synchronized (infiList) {
            if(infiList.isEmpty())
                infiList.add("This");
            else
            {
                s = infiList.get(infiList.size()-1);

            if(s.equals("This"))
                infiList.add("is");
            else if(s.equals("is"))
                infiList.add("infinite");
            else if(s.equals("infinite"))
                infiList.add("This");
            }
        }
    }
}

public static void main (String args[]) {
    // Create three concurrent threads
    new Thread(new Multithreading()).start();
    new Thread(new Multithreading()).start();
    new Thread(new Multithreading()).start();

}
}

Answer 1

Very naive quick fix for the problem. Synchronized will obtain a lock, on the arraylist, check the element and insert it based on your rules. But due to your logic being depending on the last element while adding a element to the array while maintaning the order of the elements this is actually not multithreaded but a sequential program.

This is simply because, when multithreaded programs run, you don't care abount the sequentioning, because you can never guarentee it. In most cases you will go into a Divide and Conqueer style algorithm, where the algorithm will be split up into pieces and calculated in pieces.

@Override
public void run() {
    for(int i = 0; i < 100; i++) {
        String s = null;
        synchronized (infiList) {
            if(infiList.isEmpty())
                infiList.add("This");
            else
            {
                s = infiList.get(infiList.size()-1);

            if(s.equals("This"))
                infiList.add("is");
            else if(s.equals("is"))
                infiList.add("infinite");
            else if(s.equals("infinite"))
                infiList.add("This");
            }
        }
    }
}

How to utilize multithreading

If we look at your example in another case, where you needed to calculate something before you put it into the array. This could lead to utilzing the multithreading performance better.

@Override
public void run() {
    while (true) {
        String s = null;

        CalculateSomethingBig();

        synchronized (infiList) {
            ...
        }
    }
}

If we play with the thought, that the primary runtime lies within CalculateSomethingBig() , this will now utilize more of the computers multitasking capabilities, because more of the threads will use time to calculate and utilize processing power then to wait on a lock being released.

How to get output

    public static void main(String args[]) {
    // Create three concurrent threads
    new Thread(new Multithreading()).start();
    new Thread(new Multithreading()).start();
    new Thread(new Multithreading()).start();

    for(String s : infiList)
        System.out.println(s);
}