Throw IllegalStateException if native objects are used after dispose.

This makes it easier to debug issues related to double dispose /
use after dispose.

Bug: webrtc:7566, webrtc:8297
Change-Id: I07429b2b794deabb62b5f3ea1cf92eea6f66a149
Reviewed-on: https://webrtc-review.googlesource.com/102540
Commit-Queue: Sami Kalliomäki <sakal@webrtc.org>
Reviewed-by: Paulina Hensman <phensman@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24894}

sdk/android/api/org/webrtc/DtmfSender.java

@@ -12,7 +12,7 @@ package org.webrtc;
 
 /** Java wrapper for a C++ DtmfSenderInterface. */
 public class DtmfSender {
-  final long nativeDtmfSender;
+  private long nativeDtmfSender;
 
   public DtmfSender(long nativeDtmfSender) {
     this.nativeDtmfSender = nativeDtmfSender;
@@ -22,6 +22,7 @@ public class DtmfSender {
    * @return true if this DtmfSender is capable of sending DTMF. Otherwise false.
    */
   public boolean canInsertDtmf() {
+    checkDtmfSenderExists();
     return nativeCanInsertDtmf(nativeDtmfSender);
   }
 
@@ -43,6 +44,7 @@ public class DtmfSender {
    * @return             true on success and false on failure.
    */
   public boolean insertDtmf(String tones, int duration, int interToneGap) {
+    checkDtmfSenderExists();
     return nativeInsertDtmf(nativeDtmfSender, tones, duration, interToneGap);
   }
 
@@ -50,6 +52,7 @@ public class DtmfSender {
    * @return The tones remaining to be played out
    */
   public String tones() {
+    checkDtmfSenderExists();
     return nativeTones(nativeDtmfSender);
   }
 
@@ -58,6 +61,7 @@ public class DtmfSender {
    *         insertDtmf() method, or the default value of 100 ms if insertDtmf() was never called.
    */
   public int duration() {
+    checkDtmfSenderExists();
     return nativeDuration(nativeDtmfSender);
   }
 
@@ -67,11 +71,20 @@ public class DtmfSender {
    *         called.
    */
   public int interToneGap() {
+    checkDtmfSenderExists();
     return nativeInterToneGap(nativeDtmfSender);
   }
 
   public void dispose() {
+    checkDtmfSenderExists();
     JniCommon.nativeReleaseRef(nativeDtmfSender);
+    nativeDtmfSender = 0;
+  }
+
+  private void checkDtmfSenderExists() {
+    if (nativeDtmfSender == 0) {
+      throw new IllegalStateException("DtmfSender has been disposed.");
+    }
   }
 
   private static native boolean nativeCanInsertDtmf(long dtmfSender);