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  1. /* package whatever; // don't place package name! */
  2.  
  3. import java.util.*;
  4. import java.lang.*;
  5. import java.io.*;
  6. import java.util.concurrent.*;
  7.  
  8. /* Name of the class has to be "Main" only if the class is public. */
  9. class Ideone
  10. {
  11. 	static ConcurrentMap<String, List<Integer>> map = new ConcurrentHashMap<>();
  12.  
  13.     static void safeAdd(String key, Integer val) {
  14.         map.compute(key, (k, list) -> {
  15.             sleepSilently(2000);
  16.             if (list == null) {
  17.                 list = new ArrayList<>();
  18.             }
  19.             list.add(val);
  20.             return list;
  21.         });
  22.     }
  23.  
  24.     static List<Integer> safeGet(String key) {
  25.         List<Integer> list = map.compute(key, (k, l) -> l);
  26.         return list != null ? Collections.unmodifiableList(list) : null;
  27.     }
  28.  
  29.     public static void main(String[] args) {
  30.         map.put("A", new ArrayList<>(Arrays.asList(1, 2, 3)));
  31.         map.put("B", new ArrayList<>(Arrays.asList(1, 2, 3)));
  32.  
  33.         new Thread(() -> {
  34.             System.out.println("begin add to A");
  35.             safeAdd("A", 4);
  36.             System.out.println("finish add A");
  37.         }).start();
  38.         new Thread(() -> {
  39.             sleepSilently(1000);
  40.             System.out.println("begin get A");
  41.             System.out.println("A= " + safeGet("A"));
  42.         }).start();
  43.         new Thread(() -> {
  44.             sleepSilently(1300);
  45.             System.out.println("get A unsafely= " + map.get("A"));
  46.         }).start();
  47.         new Thread(() -> {
  48.             sleepSilently(1600);
  49.             System.out.println("begin get B");
  50.             System.out.println("B= " + safeGet("B"));
  51.         }).start();
  52.     }
  53.  
  54.     static void sleepSilently(long millis) {
  55.         try {
  56.             Thread.sleep(millis);
  57.         } catch (InterruptedException e) {
  58.             // ignore
  59.         }
  60.     }
  61. }
Success #stdin #stdout 0.12s 60532KB
comments (?)
stdin 
Standard input is empty
stdout 
begin add to A
begin get A
get A unsafely= [1, 2, 3]
begin get B
B= [1, 2, 3]
finish add A
A= [1, 2, 3, 4]
https://ideone.com/tCmTV2
language:
Java (HotSpot 12)
created:
3 years ago
visibility:
secret

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