How to rewrite a method for a multi-core processor?

Question

Java

The method works fine under two cores. How to multithread it so that it can load as many cores as you want? The number of cores is not known in advance.
in this form, he does not want to load ( Help please. very necessary.

private Map<String, String> makeAdreses() {
    
        Map<String, String> adresWhisW = new ConcurrentHashMap();
        Thread thread = new Thread(new IndexedRandomThread(adresWhisW));
        thread.start();
        for (int i = 0; i < 1000; i++) {
                t=//громоздкие вычисления которые нельзя разделить на части;
            adresWhisW( t, i);
        }
 
        try {
            thread.join();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        return adresWhisW;
    }
 
 
    public class IndexedRandomThread implements Runnable {
        private Map<String, String> outputMap;
 
        public IndexedRandomThread(Map<String, String> outputMap) {
            this.outputMap = outputMap;
        }
 
        @Override
        public void run() {
            for (int i = 2000; i < 3000; i++) {
                t=//громоздкие вычисления которые нельзя разделить на части
            outputMap.put( t, i);
 
            }
        }
    }