title: get all table values from firebase null object reference firebase database
body: <pre><code>public class ShowBPGraphActivity extends AppCompatActivity {
  public GraphView graphView;
  public LineGraphSeries &lt;DataPoint&gt;lineGraphSeries; 
  public String systolicbp;
  public String diastolicbp;
  public String timebp;
  public String datebp;
  
  public DatabaseReference db;
  public BPFragmentTable bpFragmentTable = new BPFragmentTable(  );
  
   @Override
  protected void onCreate(Bundle savedInstanceState) {
      super.onCreate( savedInstanceState );
      setContentView( R.layout.activity_show_bp_graph );
      ButterKnife.bind( this );
  
       db = FirebaseDatabase.getInstance().getReference("bpfragmentTable");
  
      final GraphView graph = (GraphView) findViewById( R.id.graphView );
  
      final List&lt;BPFragmentTable&gt; bpFragmentTableList = new ArrayList&lt;&gt;();
  
      //Get datasnapshot at your "users" root node
      DatabaseReference ref = FirebaseDatabase.getInstance().getReference().child("bpfragmentTable");
      ref.addValueEventListener(new ValueEventListener() {
                  @Override
                  public void onDataChange(DataSnapshot dataSnapshot) {
                      List&lt;BPFragmentTable&gt; bpfragmentTableList = new ArrayList&lt;&gt;();
  
                      for(DataSnapshot dataSnapshot1 :dataSnapshot.getChildren()){
  
                          BPFragmentTable bpfragmentTable = dataSnapshot1.getValue(BPFragmentTable.class);
  
                          systolic = bpfragmentTable.getSystolicbp();
                          diastolic = bpfragmentTable.getDiastolicbp();
                          date = bpfragmentTable.getDatebp();
                          time = bpfragmentTable.getTimebp();
                          //bpfragmentTable.setSystolicbp(systolic);
                          //bpfragmentTable.setDiastolicbp(diastolic);
                          //bpfragmentTable.setDatebp(date);
                          //bpfragmentTable.setTimebp(time);
                          bpfragmentTableList.add(bpfragmentTable);
                          // Toast.makeText(MainActivity.this,""+name,Toast.LENGTH_LONG).show();
                      }
  
                  @Override
                  public void onCancelled(DatabaseError databaseError) {
                      //handle databaseError
                  }
              });
  // please help me why i get null value in systolic diastolic date and time
  
              System.out.println( "systolicbp" + systolicbp );
              System.out.println( "diastolicbp" + diastolicbp );
              System.out.println( "datebp" + datebp );
              System.out.println( "timebp" + timebp );
  
      try {
          @SuppressLint("SimpleDateFormat") SimpleDateFormat formatter = new SimpleDateFormat("hh:mm a\nMMM dd yyyy");
          dateValued = formatter.parse(datebp);
          dateInMills = dateValued.getTime();
      } catch (ParseException e) {
          e.printStackTrace();
      }
  
      final long xlong = dateInMills;
      final int ysystolicInt  = Integer.parseInt( systolicbp );
      final int ydiastolicInt = Integer.parseInt( diastolicbp );
  </code></pre>
  
  <p>this is the error in the picture </p>
  
  <p><img src="https://i.stack.imgur.com/Ft1i6.png" alt="getting null object reference from firebase database"></p>
  
  <p>I want to get <code>bpfragmentTable</code> all values and store all values in the string variable and use that variable to draw a graph in the graph view. I want to get all values from the table-BPFragment table  in the picture:</p>
  
  <p><img src="https://i.stack.imgur.com/AkFTp.png" alt="FireBase DataBase bpfragmentTable"></p>
  
  <p>and this is the class of getter and setter of BPFragmentTable.class</p>
  
  <pre><code>public class BPFragmentTable {
  
  public String bpid;
  public String systolicbp;
  public String diastolicbp;
  public String datebp;
  public String timebp;
  
  public BPFragmentTable(){}
  public BPFragmentTable(String bpid,String systolicbp,String diastolicbp,String datebp,String timebp) {
  
       this.bpid = bpid;
       this.systolicbp = systolicbp;
       this.diastolicbp = diastolicbp;
       this.datebp = datebp;
       this.timebp = timebp;
  
  }
  
  public String getBpid() {
      return bpid;
  }
  
  public void setBpid(String  bpid) {
      this.bpid = bpid;
  }
  
  public String getSystolicbp() {
      return systolicbp;
  }
  
  public void setSystolicbp(String systolicbp) {
      this.systolicbp = systolicbp;
  }
  
  public String getDiastolicbp() {
      return diastolicbp;
  }
  
  public void setDiastolicbp(String diastolicbp) {
      this.diastolicbp= diastolicbp;
  }
  
  public String getDatebp() {
      return datebp;
  }
  
  public void setDatebp(String datebp) {
      this.datebp= datebp;
  }
  
  public String getTimebp() {
      return timebp;
  }
  
  public void setTimebp(String timebp) {
      this.timebp= timebp;
  } 
  }
  </code></pre>
  
  <p>Why do I get null object reference?</p>