title: Get value from one class to use it in another class (java)
body: <p>I have two classes</p>
  
  <p>in class ABC</p>
  
  <pre><code>double a = 0.5 
  public double lala()
  {
       return a;
  }
  </code></pre>
  
  <p>I want use it in another class, let us say class DEF</p>
  
  <pre><code>ABC abc;
  double baba = abc.lala();
  </code></pre>
  
  <p>But it says that java.lang.NullPointerException, any idea? Thx</p>
  
  <pre><code>public class AdapterDB 
  { 
          double cal;
      double rcarbohydrate;
      double rfat;
      double rprotein;
  
      public AdapterDB(double cal, double rcarbohydrate, double rfat, double rprotein, Context ctx)
      {
          this.cal = cal;
          this.rcarbohydrate = rcarbohydrate;
          this.rfat = rfat;
          this.rprotein = rprotein;
          this.context = ctx;
          DBHelper = new DatabaseHelper (context);        
      }
  
          public double Calorie()
      {
          return cal;
      }
  
      public double Carbohydrate()
      {
          return rcarbohydrate;
      }
  
      public double Protein()
      {
          return rprotein;
      }
  
      public double Fat()
      {
          return rfat;
      }
  
  }
  </code></pre>
  
  <p>Here is my code for class ABC, and I want to use some of them at another class >_&lt;</p>