I am trying to understand the SynchronizedMap and I ran the below code. I get the below Output with an exception. According to my understanding the exception is caused when the get() methods are trying to access the syncmap when a thread is still executing a write on the map or Thread is in the Sleep state. Is my understanding correct or am I missing something ?
class MapHelper1 implements Runnable {
Map<String, Integer> map;
public MapHelper1(Map<String, Integer> map) {
this.map = map;
new Thread(this, "MapHelper1").start();
}
public void run() {
map.put("One", 1);
try {
System.out.println("MapHelper1 sleeping");
Thread.sleep(100);
} catch (Exception e) {
System.out.println(e);
}
}
}
class MapHelper2 implements Runnable {
Map<String, Integer> map;
public MapHelper2(Map<String, Integer> map) {
this.map = map;
new Thread(this, "MapHelper3").start();
}
public void run() {
map.put("two", 1);
try {
System.out.println("MapHelper2 sleeping");
Thread.sleep(100);
} catch (Exception e) {
System.out.println(e);
}
}
}
class MapHelper3 implements Runnable {
Map<String, Integer> map;
public MapHelper3(Map<String, Integer> map) {
this.map = map;
new Thread(this, "MapHelper3").start();
}
public void run() {
map.put("three", 1);
try {
System.out.println("MapHelper3 sleeping");
Thread.sleep(100);
} catch (Exception e) {
System.out.println(e);
}
}
}
public class MainClass {
public static void main(String[] args) {
Map<String, Integer> hashMap = new HashMap<>();
Map<String, Integer> syncMap = Collections.synchronizedMap(hashMap);
MapHelper1 mapHelper1 = new MapHelper1(syncMap);
MapHelper2 mapHelper2 = new MapHelper2(syncMap);
MapHelper3 mapHelper3 = new MapHelper3(syncMap);
for (Map.Entry<String, Integer> e : syncMap.entrySet()) {
System.out.println(e.getKey() + "=" + e.getValue());
}
}
}OUTPUT:
MapHelper1 sleeping
MapHelper2 sleeping
MapHelper3 sleeping
Exception in thread "main" java.util.ConcurrentModificationException
at java.base/java.util.HashMap$HashIterator.nextNode(HashMap.java:1494)
at java.base/java.util.HashMap$EntryIterator.next(HashMap.java:1527)
at java.base/java.util.HashMap$EntryIterator.next(HashMap.java:1525)
at MainClass.main(MainClass.java:137)
Command exited with non-zero status 1