title: Error: The operator += is undefined for the argument type(s) double, int[]
body: <p>I am trying to use the operator += for one of the methods in my program which takes the total of the amounts of food for the gerbils, and divides it by the number of gerbils, in kind of like an average. The errors are the following:</p>
  
  <ol>
  <li><p>"The operator += is undefined for the argument type(s) double, int[]" at the following code:<br>
  average +=  g.getAmountFood();</p></li>
  <li><p>"The operator / is undefined for the argument type(s) Gerbil, int" at the following code: 
    average = gerbil[i] / gerbil.length</p></li>
  </ol>
  
  <p>Here is my code for my first main Class:
  import java.util.Scanner;</p>
  
  <pre><code>public class Gerbilfood {
  
  static int n8;
  static int n3;
  static String n55;
  static String n35;
  static String n2;
  public static Gerbil[] gerbil;
  
  public static void main(String[] args) {
  
  Scanner scanner = new Scanner(System.in);
  
  System.out.println("Please input how many types of food items the  gerbils  eat as an integer");
  String n0 = scanner.nextLine();
  int n1 = Integer.parseInt(n0);
  
  String[] food = new String[n1];
  
  for (int i = 0; i &lt; n1; i++) {
  
      System.out.println("Please enter a food name");
  
      String n2 = scanner.nextLine();
  
      food[i] = n2;
  
      int[] maximum = new int[n1];
  
      System.out.println("Please enter maximum amount of this food per day");
      String n33 = scanner.nextLine();
      int n3 = Integer.parseInt(n33);
      maximum[i] = n3;
  }
  
  System.out.println("Please enter in the number of gerbils in the lab");
  String n73 = scanner.nextLine();
  int n4 = Integer.parseInt(n73);
  //gerbil = new Gerbil[n4];
  
  gerbil = new Gerbil[n4];
  
  int[] combo = new int[n4];
  String[] ids = new String[n4];
  for (int i = 0; i &lt; n4; i++) {
  
      //Gerbil g = new Gerbil(n1);
      System.out.println("Please enter in the lab id for one of the gerbils");
      String n5 = scanner.nextLine();
      //g.setId(n5);
  
      //ids[i] = n5;
      //String[] names = new String[n4];
      System.out.println("Please enter in the name given to gerbil whose lab id you just entered");
      String n6 = scanner.nextLine(); // gerbil name
      //g.setName(n6);
  
      //String[] amountfood = new String[n1];
      int[] amountfood = new int[n1];
  
      for (int j = 0; j &lt; n1; j++) {
          System.out.println("how much of " + food[j]
                  + " did this gerbil eat");
          String n8 = scanner.nextLine();
          //amountfood[j = n8;
          amountfood[j] = Integer.parseInt(n8);
  
      }
      boolean[] bite = new boolean[n4];
      System.out
              .println("Does this Gerbil bite? Enter True or False");
      String n77 = scanner.nextLine();
      if (n77.equalsIgnoreCase("True")) {
          bite[i] = true;
      } else {
          bite[i] = false;
      }
  
      boolean[] escape = new boolean[n4];
      System.out
              .println("Does this Gerbil escape? Enter True or False");
      String n89 = scanner.nextLine();
  
      if (n89.equalsIgnoreCase("True")) {
          escape[i] = true;
      } else {
          escape[i] = false;
      }
  
      gerbil[i] = new Gerbil(n5, n6, amountfood, escape[i], bite[i], food);
  
  }
  
  System.out.println("What information would you like to know?");
  String n55 = scanner.nextLine();
  
  String n33 = "search";
  String n34 = "average";
  String n35 = "restart";
  String n36 = "quit";
  
  if (n55.equalsIgnoreCase(n34)) {
      System.out.println(averagefood());
  
  } else {
      if (n55.equalsIgnoreCase(n33)) {
  
          System.out.println("Please type the lab id of the gerbil you wish to search for");
  
          String n87 = scanner.nextLine();
  
          Gerbil g = searchForGerbil(n87);
  
          Gerbil gerbilattributes = searchForGerbil(n87);
  
          String gerbid = g.getId();
  
          String gerbname = g.getName();
  
          boolean gerbbite = g.getBite();
  
          boolean gerbescape = g.getEscape();
  
          for (int i = 0; i &lt; n1; i++) {
              food = g.getTypeFood();
          }
  
  
              int[] gerbfoods = g.getAmountFood();
  
           System.out.print("Lab :"+gerbid + " Name:"+ gerbname + " ("+ ((gerbbite==true)?"will bite":"will not bite")  +  "," + ((gerbescape==true)?"will escape":"will not escape") + ")");
          for (int i = 0; i &lt; n1; i++) {
              System.out.print( " " + food[i] + ":"+ gerbfoods[i]);
          }
  
      } else {
          if (n55.equalsIgnoreCase(n35)) {
  
              //GO BACK
          } else {
              if (n55.equalsIgnoreCase(n36)) {
  
                  System.exit(0);
              } else {
                  System.out.println("ERROR");
              }
          }
      }
  }
  }
  
         public static String averagefood() {
  
  
  // girbil[0] .. girbil[n] / n = average!!!
  average = gerbil[i] / gerbil.length
  double average = 0.0;
  for (int i = 0; i &lt;= gerbil.length; i++) {
      Gerbil g = gerbil[i];
  
  
      average +=  g.getAmountFood();
  }
  average /= gerbil.length;
  
  for (int i = 0; i &lt;= gerbil.length; i++) {
      Gerbil g = gerbil[i];
      String gid = g.getId();
      String gname = g.getName();
      String everything = gid + " " + gname + " " + average + "\n";
  
  }
  
  
  
  int i = 0;
  Gerbil g = gerbil[i];
  String gid = g.getId();
  
  String gname = g.getName();
  long percent = Math.round(n8 * 100.0 / n3);
  
  String everything = gid + " " + gname + " " + percent + "\n";
  for (i = 0; i &lt;= gerbil.length; i++) {
  
      //turn everything;
  }
  
  return everything;
  }
  
  public static Gerbil searchForGerbil(String n87) {
  
  for (int i = 0; i &lt; gerbil.length; i++) {
      Gerbil g = gerbil[i];
  
      if (n87.equals(g.getId())) {
          return gerbil[i];
  
      }
  
  // return (new Gerbil[i]);
  }
  return null;
  
  }
  }
  </code></pre>
  
  <p>The following is my second "gerbil" class</p>
  
  <pre><code>public class Gerbil {
  
  private String id;
  private String name;
  private int[] amountfood;
  private int numbergerbils;
  private String[] food;
  
  private boolean escape;
  private boolean bite;
  
  public Gerbil(String n5, String n6, int[] numOfFood, boolean newEscape, boolean newBite, String[] n2) {
  
  id = n5;
  name = n6;
  amountfood = numOfFood;
  escape = newEscape;
  bite = newBite;
  food = n2;
  }
  
  public Gerbil(String[] typefood) {
  
  food = typefood;
  }
  
  public Gerbil(int[] numOfFood) {
  amountfood = numOfFood;
  
  }
  
  public int[] getAmountFood() {
  
  return amountfood;
  }
  
  public boolean getBite() {
  
  return bite;
  }
  
  public boolean getEscape() {
  
  return escape;
  }
  
  public String getId() {
  
  return id;
  }
  
  public String getName() {
  return name;
  }
  
  public void setId(String newId) {
  id = newId;
  }
  
  public void setName(String newName) {
  name = newName;
  }
  
  public String[] getTypeFood() {
  
  return food;
  
  
  }
  
  }
  </code></pre>