Demonstrates exception chaining : Exceptions « Language Basics « Java
Demonstrates exception chaining
// : c09:DynamicFields.java
// A Class that dynamically adds fields to itself.
// Demonstrates exception chaining.
// {ThrowsException}
// From 'Thinking in Java, 3rd ed.' (c) Bruce Eckel 2002
// www.BruceEckel.com. 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(fields[i][0]);
result.append(": ");
result.append(fields[i][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;
// 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);
System.out.println(df);
try {
df.setField("d", "A value for d");
df.setField("number", new Integer(47));
df.setField("number2", new Integer(48));
System.out.println(df);
df.setField("d", "A new value for d");
df.setField("number3", new Integer(11));
System.out.println(df);
System.out.println(df.getField("d"));
Object field = df.getField("a3"); // Exception
} catch (NoSuchFieldException e) {
throw new RuntimeException(e);
} catch (DynamicFieldsException e) {
throw new RuntimeException(e);
}
}
} ///:~
