Why is null printed before the string, in my string array?

Question

Sorry if this question has been asked before I haven't seen it asked on stack overflow. The code prompts the user for their name and module mark and then prints the names and module marks to the user at the end.

public class Main
{
  public static void main( String args[] )
  {
      String name_array[] = new String[4];
      int mark_array[] = new int[4];
      System.out.println("Please enter in your name");
      String name = BIO.getString();
      System.out.println("Please enter in your module mark");
      int mark = BIO.getInt();

      name_array[0] += name;
      mark_array[0] += mark;

      int i = 1;
      int z = 1;
      while ( ! name.equals("END"))
      {
          while ( ! name.equals("END"))
          {
              System.out.println("Please enter in your name");
              name = BIO.getString();

              name_array[i] += name;
              ++i;
              break;
          }
          while ( ! name.equals("END"))
          {
              System.out.println("Please enter in your module mark");
              mark = BIO.getInt();

              mark_array[z] += mark;
              ++z;
              break;
          }
          if (i == name_array.length)
          {
              break;
          }
      }
      for (int y = 0; y < name_array.length; ++y)
      {
          System.out.println(name_array[y] + ' ' + mark_array[y]);
      }
  }
}

The code prints null before my string which is entered. If I enter Harry Houdini as the name, nullHarry Houdini will be printed.

How do I avoid this?

because if you have a string initialized as null and you are using += to append something to it, it will cast the null to be "null" and append the following value to "null", which finally results in something like "nullA" for example. — SomeJavaGuy
– SomeJavaGuy, Commented Nov 10, 2015 at 10:46

Mena · Accepted Answer · 2015-11-10 10:44:09Z

4

You are appending a value to the default value of a String[] element.

The default value for String is null, and your array is initialized with default values: String name_array[] = new String[4];.

Therefore, you get null[the name] as a value.

Here is a self-contained example:

String[] foo = new String[1];
foo[0] += "bar";
System.out.println(foo[0]);

Output

nullbar

Also worth of notice, the String literal for null is "null", e.g. System.out.println((String)null); will literally print null.

You may want to try assigning the value instead of appending it: name_array[0] = name;

answered Nov 10, 2015 at 10:44

Mena

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Comments

Joel Min · Accepted Answer · 2015-11-10 10:43:55Z

1

Change name_array[0] += name; to name_array[0] = name; in the beginning.

answered Nov 10, 2015 at 10:43

Joel Min

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