Demonstrates exception chaining : Exceptions « Language Basics « Java

Demonstrates exception chaining

Demonstrates exception chaining

// :
// A Class that dynamically adds fields to itself.
// Demonstrates exception chaining.
// {ThrowsException}
// From 'Thinking in Java, 3rd ed.' (c) Bruce Eckel 2002
// See copyright notice in CopyRight.txt.

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 };

  public String toString() {
    StringBuffer result = new StringBuffer();
    for (int i = 0; i < fields.length; i++) {
      result.append(": ");
    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;
    // Reursive 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);
    try {
      df.setField("d", "A value for d");
      df.setField("number", new Integer(47));
      df.setField("number2", new Integer(48));
      df.setField("d", "A new value for d");
      df.setField("number3", new Integer(11));
      Object field = df.getField("a3"); // Exception
    } catch (NoSuchFieldException e) {
      throw new RuntimeException(e);
    } catch (DynamicFieldsException e) {
      throw new RuntimeException(e);
} ///:~


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