投射同步的LinkedList时出错

Question

I have a simple code:

private List<String> requests = Collections.synchronizedList(new LinkedList<String>());

and

synchronized (requests) {
    ((LinkedList<String>)requests).addLast(message);
}

at runtime I get this error:

FATAL EXCEPTION: main
java.lang.IllegalStateException: Could not execute method of the activity
    at android.view.View$1.onClick(View.java:2144)
    at android.view.View.performClick(View.java:2485)
    at android.view.View$PerformClick.run(View.java:9080)
    at android.os.Handler.handleCallback(Handler.java:587)
    at android.os.Handler.dispatchMessage(Handler.java:92)
    at android.os.Looper.loop(Looper.java:123)
    at android.app.ActivityThread.main(ActivityThread.java:3683)
    at java.lang.reflect.Method.invokeNative(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:507)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
    at dalvik.system.NativeStart.main(Native Method)
Caused by: java.lang.reflect.InvocationTargetException
    at java.lang.reflect.Method.invokeNative(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:507)
    at android.view.View$1.onClick(View.java:2139)
    ... 11 more
Caused by: java.lang.ClassCastException: java.util.Collections$SynchronizedList
    at com.countryst.nabard.turnbased.client.networking.ClientNetworking.login(ClientNetworking.java:72)
    at com.countryst.nabard.turnbased.client.MainActivity.loginToServer(MainActivity.java:87)
    at com.countryst.nabard.turnbased.client.MainActivity.onButtonClicked(MainActivity.java:217)
    ... 14 more

This is part of an android program, I have similar code in my plain java code and works fine.

Answer 1

synchronizedList doesn't document that it will return something that can be cast back to the underlying list, just that it returns something that implements List<E> that is synchronized.

If you need to use methods specific to LinkedList , keep a reference to the original list:

private LinkedList<String> requestsLinkedList = new LinkedList<String>();
private List<String> requests = Collections.synchronizedList(requestsLinkedList);

and then synchronize like this:

synchronized (requests) {
    requestsLinkedList.addLast(message);
}

But in this case, you don't need to, because LinkedList#addLast is equivalent to List#add .