// exceptions/DynamicFields.java
// (c)2017 MindView LLC: see Copyright.txt
// We make no guarantees that this code is fit for any purpose.
// Visit http://OnJava8.com for more book information.
// A Class that dynamically adds fields to itself to
// demonstrate exception chaining

class DynamicFieldsException extends Exception {}

public class DynamicFields {
 private Object[][] fields;
 public DynamicFields(int initialSize) {
 fields = new Object[initialSize][2];
 for(int i = 0; i < initialSize; i++)
 fields[i] = new Object[] { null, null };
 }
 @Override
 public String toString() {
 StringBuilder result = new StringBuilder();
 for(Object[] obj : fields) {
 result.append(obj[0]);
 result.append(": ");
 result.append(obj[1]);
 result.append("\n");
 }
 return result.toString();
 }
 private int hasField(String id) {
 for(int i = 0; i < fields.length; i++)
 if(id.equals(fields[i][0]))
 return i;
 return -1;
 }
 private int getFieldNumber(String id)
 throws NoSuchFieldException {
 int fieldNum = hasField(id);
 if(fieldNum == -1)
 throw new NoSuchFieldException();
 return fieldNum;
 }
 private int makeField(String id) {
 for(int i = 0; i < fields.length; i++)
 if(fields[i][0] == null) {
 fields[i][0] = id;
 return i;
 }
 // No empty fields. Add one:
 Object[][] tmp = new Object[fields.length + 1][2];
 for(int i = 0; i < fields.length; i++)
 tmp[i] = fields[i];
 for(int i = fields.length; i < tmp.length; i++)
 tmp[i] = new Object[] { null, null };
 fields = tmp;
 // Recursive call with expanded fields:
 return makeField(id);
 }
 public Object
 getField(String id) throws NoSuchFieldException {
 return fields[getFieldNumber(id)][1];
 }
 public Object setField(String id, Object value)
 throws DynamicFieldsException {
 if(value == null) {
 // Most exceptions don't have a "cause"
 // constructor. In these cases you must use
 // initCause(), available in all
 // Throwable subclasses.
 DynamicFieldsException dfe =
 new DynamicFieldsException();
 dfe.initCause(new NullPointerException());
 throw dfe;
 }
 int fieldNumber = hasField(id);
 if(fieldNumber == -1)
 fieldNumber = makeField(id);
 Object result = null;
 try {
 result = getField(id); // Get old value
 } catch(NoSuchFieldException e) {
 // Use constructor that takes "cause":
 throw new RuntimeException(e);
 }
 fields[fieldNumber][1] = value;
 return result;
 }
 public static void main(String[] args) {
 DynamicFields df = new DynamicFields(3);
 System.out.println(df);
 try {
 df.setField("d", "A value for d");
 df.setField("number", 47);
 df.setField("number2", 48);
 System.out.println(df);
 df.setField("d", "A new value for d");
 df.setField("number3", 11);
 System.out.println("df: " + df);
 System.out.println("df.getField(\"d\") : "
 + df.getField("d"));
 Object field =
 df.setField("d", null); // Exception
 } catch(NoSuchFieldException |
 DynamicFieldsException e) {
 e.printStackTrace(System.out);
 }
 }
}
/* Output:
null: null
null: null
null: null

d: A value for d
number: 47
number2: 48

df: d: A new value for d
number: 47
number2: 48
number3: 11

df.getField("d") : A new value for d
DynamicFieldsException
 at
DynamicFields.setField(DynamicFields.java:65)
 at DynamicFields.main(DynamicFields.java:97)
Caused by: java.lang.NullPointerException
 at
DynamicFields.setField(DynamicFields.java:67)
 ... 1 more
*/