diff --git a/tests/net/java/com/android/server/net/NetworkStatsSubscriptionsMonitorTest.java b/tests/net/java/com/android/server/net/NetworkStatsSubscriptionsMonitorTest.java
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+++ b/tests/net/java/com/android/server/net/NetworkStatsSubscriptionsMonitorTest.java
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+/*
+ * Copyright (C) 2020 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.android.server.net;
+
+import static org.junit.Assert.assertEquals;
+import static org.mockito.Mockito.any;
+import static org.mockito.Mockito.anyInt;
+import static org.mockito.Mockito.eq;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.never;
+import static org.mockito.Mockito.reset;
+import static org.mockito.Mockito.times;
+import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.when;
+
+import android.annotation.NonNull;
+import android.content.Context;
+import android.os.Looper;
+import android.telephony.PhoneStateListener;
+import android.telephony.ServiceState;
+import android.telephony.SubscriptionManager;
+import android.telephony.TelephonyManager;
+
+import com.android.internal.util.CollectionUtils;
+import com.android.server.net.NetworkStatsSubscriptionsMonitor.Delegate;
+import com.android.server.net.NetworkStatsSubscriptionsMonitor.RatTypeListener;
+
+import org.junit.Before;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+import org.mockito.ArgumentCaptor;
+import org.mockito.Mock;
+import org.mockito.MockitoAnnotations;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.concurrent.Executor;
+import java.util.concurrent.Executors;
+
+@RunWith(JUnit4.class)
+public final class NetworkStatsSubscriptionsMonitorTest {
+    private static final int TEST_SUBID1 = 3;
+    private static final int TEST_SUBID2 = 5;
+    private static final String TEST_IMSI1 = "466921234567890";
+    private static final String TEST_IMSI2 = "466920987654321";
+    private static final String TEST_IMSI3 = "466929999999999";
+
+    @Mock private Context mContext;
+    @Mock private PhoneStateListener mPhoneStateListener;
+    @Mock private SubscriptionManager mSubscriptionManager;
+    @Mock private TelephonyManager mTelephonyManager;
+    @Mock private Delegate mDelegate;
+    private final List<Integer> mTestSubList = new ArrayList<>();
+
+    private final Executor mExecutor = Executors.newSingleThreadExecutor();
+    private NetworkStatsSubscriptionsMonitor mMonitor;
+
+    @Before
+    public void setUp() {
+        MockitoAnnotations.initMocks(this);
+
+        if (Looper.myLooper() == null) {
+            Looper.prepare();
+        }
+
+        when(mTelephonyManager.createForSubscriptionId(anyInt())).thenReturn(mTelephonyManager);
+
+        when(mContext.getSystemService(eq(Context.TELEPHONY_SUBSCRIPTION_SERVICE)))
+                .thenReturn(mSubscriptionManager);
+        when(mContext.getSystemService(eq(Context.TELEPHONY_SERVICE)))
+                .thenReturn(mTelephonyManager);
+
+        mMonitor = new NetworkStatsSubscriptionsMonitor(mContext, mExecutor, mDelegate);
+    }
+
+    @Test
+    public void testStartStop() {
+        // Verify that addOnSubscriptionsChangedListener() is never called before start().
+        verify(mSubscriptionManager, never())
+                .addOnSubscriptionsChangedListener(mExecutor, mMonitor);
+        mMonitor.start();
+        verify(mSubscriptionManager).addOnSubscriptionsChangedListener(mExecutor, mMonitor);
+
+        // Verify that removeOnSubscriptionsChangedListener() is never called before stop()
+        verify(mSubscriptionManager, never()).removeOnSubscriptionsChangedListener(mMonitor);
+        mMonitor.stop();
+        verify(mSubscriptionManager).removeOnSubscriptionsChangedListener(mMonitor);
+    }
+
+    @NonNull
+    private static int[] convertArrayListToIntArray(@NonNull List<Integer> arrayList) {
+        final int[] list = new int[arrayList.size()];
+        for (int i = 0; i < arrayList.size(); i++) {
+            list[i] = arrayList.get(i);
+        }
+        return list;
+    }
+
+    private void setRatTypeForSub(List<RatTypeListener> listeners,
+            int subId, int type) {
+        final ServiceState serviceState = mock(ServiceState.class);
+        when(serviceState.getDataNetworkType()).thenReturn(type);
+        final RatTypeListener match = CollectionUtils
+                .find(listeners, it -> it.getSubId() == subId);
+        if (match != null) {
+            match.onServiceStateChanged(serviceState);
+        }
+    }
+
+    private void addTestSub(int subId, String subscriberId) {
+        // add SubId to TestSubList.
+        if (!mTestSubList.contains(subId)) {
+            mTestSubList.add(subId);
+        }
+        final int[] subList = convertArrayListToIntArray(mTestSubList);
+        when(mSubscriptionManager.getCompleteActiveSubscriptionIdList()).thenReturn(subList);
+        when(mTelephonyManager.getSubscriberId(subId)).thenReturn(subscriberId);
+        mMonitor.onSubscriptionsChanged();
+    }
+
+    private void removeTestSub(int subId) {
+        // Remove subId from TestSubList.
+        mTestSubList.removeIf(it -> it == subId);
+        final int[] subList = convertArrayListToIntArray(mTestSubList);
+        when(mSubscriptionManager.getCompleteActiveSubscriptionIdList()).thenReturn(subList);
+        mMonitor.onSubscriptionsChanged();
+    }
+
+    private void assertRatTypeChangedForSub(String subscriberId, int ratType) {
+        assertEquals(mMonitor.getRatTypeForSubscriberId(subscriberId), ratType);
+        final ArgumentCaptor<Integer> typeCaptor = ArgumentCaptor.forClass(Integer.class);
+        // Verify callback with the subscriberId and the RAT type should be as expected.
+        // It will fail if get a callback with an unexpected RAT type.
+        verify(mDelegate).onCollapsedRatTypeChanged(eq(subscriberId), typeCaptor.capture());
+        final int type = typeCaptor.getValue();
+        assertEquals(ratType, type);
+    }
+
+    private void assertRatTypeNotChangedForSub(String subscriberId, int ratType) {
+        assertEquals(mMonitor.getRatTypeForSubscriberId(subscriberId), ratType);
+        // Should never get callback with any RAT type.
+        verify(mDelegate, never()).onCollapsedRatTypeChanged(eq(subscriberId), anyInt());
+    }
+
+    @Test
+    public void testSubChangedAndRatTypeChanged() {
+        final ArgumentCaptor<RatTypeListener> ratTypeListenerCaptor =
+                ArgumentCaptor.forClass(RatTypeListener.class);
+
+        mMonitor.start();
+        // Insert sim1, verify RAT type is NETWORK_TYPE_UNKNOWN, and never get any callback
+        // before changing RAT type.
+        addTestSub(TEST_SUBID1, TEST_IMSI1);
+        assertRatTypeNotChangedForSub(TEST_IMSI1, TelephonyManager.NETWORK_TYPE_UNKNOWN);
+
+        // Insert sim2.
+        addTestSub(TEST_SUBID2, TEST_IMSI2);
+        assertRatTypeNotChangedForSub(TEST_IMSI1, TelephonyManager.NETWORK_TYPE_UNKNOWN);
+        verify(mTelephonyManager, times(2)).listen(ratTypeListenerCaptor.capture(),
+                eq(PhoneStateListener.LISTEN_SERVICE_STATE));
+        reset(mDelegate);
+
+        // Set RAT type of sim1 to UMTS.
+        // Verify RAT type of sim1 after subscription gets onCollapsedRatTypeChanged() callback
+        // and others remain untouched.
+        setRatTypeForSub(ratTypeListenerCaptor.getAllValues(), TEST_SUBID1,
+                TelephonyManager.NETWORK_TYPE_UMTS);
+        assertRatTypeChangedForSub(TEST_IMSI1, TelephonyManager.NETWORK_TYPE_UMTS);
+        assertRatTypeNotChangedForSub(TEST_IMSI2, TelephonyManager.NETWORK_TYPE_UNKNOWN);
+        assertRatTypeNotChangedForSub(TEST_IMSI3, TelephonyManager.NETWORK_TYPE_UNKNOWN);
+        reset(mDelegate);
+
+        // Set RAT type of sim2 to LTE.
+        // Verify RAT type of sim2 after subscription gets onCollapsedRatTypeChanged() callback
+        // and others remain untouched.
+        setRatTypeForSub(ratTypeListenerCaptor.getAllValues(), TEST_SUBID2,
+                TelephonyManager.NETWORK_TYPE_LTE);
+        assertRatTypeNotChangedForSub(TEST_IMSI1, TelephonyManager.NETWORK_TYPE_UMTS);
+        assertRatTypeChangedForSub(TEST_IMSI2, TelephonyManager.NETWORK_TYPE_LTE);
+        assertRatTypeNotChangedForSub(TEST_IMSI3, TelephonyManager.NETWORK_TYPE_UNKNOWN);
+        reset(mDelegate);
+
+        // Remove sim2 and verify that callbacks are fired and RAT type is correct for sim2.
+        // while the other two remain untouched.
+        removeTestSub(TEST_SUBID2);
+        verify(mTelephonyManager).listen(any(), eq(PhoneStateListener.LISTEN_NONE));
+        assertRatTypeNotChangedForSub(TEST_IMSI1, TelephonyManager.NETWORK_TYPE_UMTS);
+        assertRatTypeChangedForSub(TEST_IMSI2, TelephonyManager.NETWORK_TYPE_UNKNOWN);
+        assertRatTypeNotChangedForSub(TEST_IMSI3, TelephonyManager.NETWORK_TYPE_UNKNOWN);
+        reset(mDelegate);
+
+        // Set RAT type of sim1 to UNKNOWN. Then stop monitoring subscription changes
+        // and verify that the listener for sim1 is removed.
+        setRatTypeForSub(ratTypeListenerCaptor.getAllValues(), TEST_SUBID1,
+                TelephonyManager.NETWORK_TYPE_UNKNOWN);
+        assertRatTypeChangedForSub(TEST_IMSI1, TelephonyManager.NETWORK_TYPE_UNKNOWN);
+        reset(mDelegate);
+
+        mMonitor.stop();
+        verify(mTelephonyManager, times(2)).listen(any(), eq(PhoneStateListener.LISTEN_NONE));
+        assertRatTypeChangedForSub(TEST_IMSI1, TelephonyManager.NETWORK_TYPE_UNKNOWN);
+    }
+}