corenlp sentiment Java program via Py4j in Python, raises errors
<p>I made a Java sentiment analysis program to be used in Python via Py4j. I can create the Java object in Python, but try to access the method, it gives java.lang.NullPointerException. Could you help please? Thanks.</p>

<p><strong>Java code</strong>: it compiles correctly, runs without errors.</p>

<pre><code>import java.util.List;
import java.util.Properties;
import edu.stanford.nlp.ling.CoreAnnotations;
import edu.stanford.nlp.neural.rnn.RNNCoreAnnotations;
import edu.stanford.nlp.pipeline.Annotation;
import edu.stanford.nlp.pipeline.StanfordCoreNLP;
import edu.stanford.nlp.sentiment.SentimentCoreAnnotations.SentimentAnnotatedTree;
import edu.stanford.nlp.trees.Tree;
import edu.stanford.nlp.util.ArrayCoreMap;
import edu.stanford.nlp.util.CoreMap;
import py4j.GatewayServer;
import org.ejml.simple.SimpleMatrix;


public class CoreNLPSentiScore {
static StanfordCoreNLP pipeline;

    public static void init() {
        Properties props = new Properties();
        props.setProperty("annotators", "tokenize, ssplit, parse, sentiment");
        pipeline = new StanfordCoreNLP(props);

    }

    public static void main(String[] args) {
        CoreNLPSentiScore app = new CoreNLPSentiScore();
        // app is now the gateway.entry_point
        GatewayServer server = new GatewayServer(app);
        server.start();
    }

    //public static void main(String tweet) {
    //public static String findSentiment(String tweet) {
    public String findSentiment(String tweet) {
        //String SentiReturn = "2";
        //String[] SentiClass ={"very negative", "negative", "neutral", "positive", "very positive"};

        //Sentiment is an integer, ranging from 0 to 4. 
        //0 is very negative, 1 negative, 2 neutral, 3 positive and 4 very positive.
        //int sentiment = 2;
        SimpleMatrix senti_score = new SimpleMatrix();
        if (tweet != null &amp;&amp; tweet.length() &gt; 0) {
            Annotation annotation = pipeline.process(tweet);

            List&lt;CoreMap&gt; sentences = annotation.get(CoreAnnotations.SentencesAnnotation.class);
            if (sentences != null &amp;&amp; sentences.size() &gt; 0) {

                ArrayCoreMap sentence = (ArrayCoreMap) sentences.get(0);                
                //Tree tree = sentence.get(SentimentAnnotatedTree.class);  
                Tree tree = sentence.get(SentimentAnnotatedTree.class);  
                senti_score = RNNCoreAnnotations.getPredictions(tree);             

                //SentiReturn = SentiClass[sentiment];
            }
        }
        //System.out.println(senti_score);
        return senti_score.toString();
        //System.out.println(senti_score);
    }

}
</code></pre>

<p><strong>Python code</strong>:</p>

<pre><code>from py4j.java_gateway import JavaGateway
gateway = JavaGateway()
app = gateway.entry_point
test = 'i like this product'
app.findSentiment(test)
</code></pre>

<p><strong>NullPointerException</strong></p>

<pre><code>Traceback (most recent call last):

  File "&lt;ipython-input-1-cefdf18ada67&gt;", line 5, in &lt;module&gt;
    app.findSentiment(test)

  File "C:\Anaconda3\envs\sandbox\lib\site-packages\py4j\java_gateway.py", line 1026, in __call__
    answer, self.gateway_client, self.target_id, self.name)

  File "C:\Anaconda3\envs\sandbox\lib\site-packages\py4j\protocol.py", line 316, in get_return_value
    format(target_id, ".", name), value)

Py4JJavaError: An error occurred while calling t.findSentiment.: java.lang.NullPointerException
at CoreNLPSentiScore.findSentiment(CoreNLPSentiScore.java:43)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at py4j.reflection.MethodInvoker.invoke(MethodInvoker.java:237)
at py4j.reflection.ReflectionEngine.invoke(ReflectionEngine.java:357)
at py4j.Gateway.invoke(Gateway.java:280)
at py4j.commands.AbstractCommand.invokeMethod(AbstractCommand.java:132)
at py4j.commands.CallCommand.execute(CallCommand.java:79)
at py4j.GatewayConnection.run(GatewayConnection.java:214)
at java.lang.Thread.run(Unknown Source)
</code></pre>

<p>I believe you forgot to call init().</p>

<p>This seems to be line #43 (see your stack trace):</p>

<pre><code>Annotation annotation = pipeline.process(tweet);
</code></pre>

<p>pipeline is likely to be null because it is instantiated in init() and init() is not called in the main() method or from Python.</p>

java python corenlp py4j