meaning of RuntimeException("Stub!") in Android

This is because source code is not found in SDK.To see the source code, you need to download source for Android SDK, so Android studio can display the respective code.

I don't know where you looked, but the code for finish() is this

/** * Call this when your activity is done and should be closed.  The * ActivityResult is propagated back to whoever launched you via * onActivityResult(). */public void finish() {    finish(DONT_FINISH_TASK_WITH_ACTIVITY);}

which calls the private implementation

/** * Finishes the current activity and specifies whether to remove the task associated with this * activity. */private void finish(int finishTask) {    if (mParent == null) {        int resultCode;        Intent resultData;        synchronized (this) {            resultCode = mResultCode;            resultData = mResultData;        }        if (false) Log.v(TAG, "Finishing self: token=" + mToken);        try {            if (resultData != null) {                resultData.prepareToLeaveProcess(this);            }            if (ActivityManagerNative.getDefault()                    .finishActivity(mToken, resultCode, resultData, finishTask)) {                mFinished = true;            }        } catch (RemoteException e) {            // Empty        }    } else {        mParent.finishFromChild(this);    }}

Important here is ActivityManagerNative.getDefault().finishActivity which you can find at line 3359 in this file https://android.googlesource.com/platform/frameworks/base/+/master/core/java/android/app/ActivityManagerNative.java

If you want to dive deeper, you can just follow the trail.

You are checking in .class not .java file.