4 child threads count, each thread counts 25 times

　　Requires that the main thread executes after the child thread executes.
This is easy to handle, the child threads are set to not be daemon threads

Then process 4 child threads

We can make each thread direct as follows

static class MyThread {
        public static void start(int begin, int step){
            Thread t = new Thread(()->{
                int localSum = 0;
                for (int i = begin;i<=100;i+=step){
                    localSum+=i;
                }
            });
            t.setDaemon(false);
            t.start();
        }
    }
}

Create a blocking queue to store the results of each task block

 private static final LinkedBlockingQueue<Integer> queue = 
 new LinkedBlockingQueue<>(10);

Put the execution result of the child thread into the blocking queue

queue.offer(localSum);

Accumulate the results using the atomic class of the concurrent package

 private static final AtomicInteger sum = new AtomicInteger(0);
 public static void test3()throws InterruptedException{
        int sum = 0;
        Lock lock = new ReentrantLock();
        AtomicInteger count = new AtomicInteger(0);
        for(int i = 0 ; ;i++){
            MyThread.start(i,4);
            count.getAndIncrement();
            if(i==4){
                break;
            }
        }
        while(count.decrementAndGet()>0){
            Integer integer = null;
            while(queue.size()<=0){
                Thread.sleep(0);
            }
            lock.lock();
            integer = queue.take();
            lock.unlock();
            sum+=Integer.valueOf(integer);
        }
        System.out.println(sum);
    }

This will solve the problem
full code