import java.util.HashMap;
import java.util.Map;
public class Example {
private final HashMap<String, String> map = new HashMap<String, String>();
public static void main(String[] args) {
new Example();
}
public Example() {
Thread thread1 = new Thread(new Runnable() {
@Override
public void run() {
int counter = 0;
while (true) {
Map<String, String> tmp = new HashMap<String, String>();
tmp.put("example" + counter, "example");
counter++;
map.putAll(tmp);
}
}
});
Thread thread2 = new Thread(new Runnable() {
@Override
public void run() {
while (true) {
map.values().remove("example");
}
}
});
thread1.start();
thread2.start();
}
}
Unfortunately I cannot copy/paste the running code directly from my current workstation so I retyped it here so there might be a typing error.
As you can see the first thread is continuously adding values while the second thread iterates over the values in the Map. When it starts iterating over the values it expects a number of values (this value is initialized at the construction of the iterator). However because thread1 is continuously adding items this value is not as expected when the Iterator checks the actual amount of values that are in the map when it actual executes the remove code. This causes the ConcurrentModificationException.
