Checked versus unchecked exceptions

Unchecked exceptions: Checked exceptions: It's somewhat confusing, but note as well that RuntimeException (unchecked) is itself a subclass of Exception (checked).

Example 1

Model Objects are the data-centric classes used to represent items in a particular domain. Model Object constructors need to handle both arbitrary user input, and input from underlying database ResultSets.

Model Object constructors should throw checked exceptions:

Here is an example Model Object, taken from the WEB4J example application. Its constructor throws ModelCtorException (a checked exception) :


import hirondelle.web4j.model.ModelCtorException;
import hirondelle.web4j.model.ModelUtil;
import hirondelle.web4j.model.Id;
import hirondelle.web4j.model.Decimal;
import static hirondelle.web4j.model.Decimal.ZERO;
import hirondelle.web4j.model.Check;
import hirondelle.web4j.model.Validator;
import static hirondelle.web4j.util.Consts.FAILS;

/** Model Object for a Restaurant. */
public final class Resto {

   Full constructor.
   @param aId underlying database internal identifier (optional) 1..50 characters
   @param aName of the restaurant (required), 2..50 characters
   @param aLocation street address of the restaurant (optional), 2..50 characters
   @param aPrice of the fish and chips meal (optional) $0.00..$100.00
   @param aComment on the restaurant in general (optional) 2..50 characters
  public Resto(
    Id aId, SafeText aName, SafeText aLocation, Decimal aPrice, SafeText aComment
  ) throws ModelCtorException {
    fId = aId;
    fName = aName;
    fLocation = aLocation;
    fPrice = aPrice;
    fComment = aComment;
  public Id getId() { return fId; }
  public SafeText getName() {  return fName; }
  public SafeText getLocation() {  return fLocation;  }
  public Decimal getPrice() { return fPrice; }
  public SafeText getComment() {  return fComment; }

  @Override public String toString(){
    return ModelUtil.toStringFor(this);
  @Override public  boolean equals(Object aThat){
    Boolean result = ModelUtil.quickEquals(this, aThat);
    if (result ==  null) {
      Resto that = (Resto) aThat;
      result = ModelUtil.equalsFor(
        this.getSignificantFields(), that.getSignificantFields()
    return result;
  @Override public int hashCode(){
    if (fHashCode == 0){
      fHashCode = ModelUtil.hashCodeFor(getSignificantFields());
    return fHashCode;
  private final Id fId;
  private final SafeText fName;
  private final SafeText fLocation;
  private final Decimal fPrice;
  private final SafeText fComment;
  private int fHashCode;
  private static final Decimal HUNDRED = Decimal.from("100");

  private void validateState() throws ModelCtorException {
    ModelCtorException ex = new ModelCtorException();
    if (FAILS == Check.optional(fId, Check.range(1,50))) {
      ex.add("Id is optional, 1..50 chars.");
    if (FAILS == Check.required(fName, Check.range(2,50))) {
      ex.add("Restaurant Name is required, 2..50 chars.");
    if (FAILS == Check.optional(fLocation, Check.range(2,50))) {
      ex.add("Location is optional, 2..50 chars.");
    Validator[] priceChecks = {Check.range(ZERO, HUNDRED), Check.numDecimalsAlways(2)};
    if (FAILS == Check.optional(fPrice, priceChecks)) {
      ex.add("Price is optional, 0.00 to 100.00.");
    if (FAILS == Check.optional(fComment, Check.range(2,50))) {
      ex.add("Comment is optional, 2..50 chars.");
    if ( ! ex.isEmpty() ) throw ex;
  private Object[] getSignificantFields(){
    return new Object[] {fName, fLocation, fPrice, fComment};

Example 2

Args is a convenient utility class. It performs common validations on method arguments. If a validation fails, then it throws an unchecked exception. It is suitable for checking the internal consistency of program, but not for checking arbitrary user input.

package hirondelle.web4j.util;

import java.util.regex.*;

 Utility methods for common argument validations.

<P>Replaces <tt>if</tt> statements at the start of a method with 
 more compact method calls.
 <P>Example use case.
 <P>Instead of :
 public void doThis(String aText){
   if (!Util.textHasContent(aText)){
     throw new IllegalArgumentException();
   //..main body elided
 <P>One may instead write :
 public void doThis(String aText){
   //..main body elided
public final class Args {
   If <code>aText</code> does not satisfy {@link Util#textHasContent}, then 
   throw an <code>IllegalArgumentException</code>.
   <P>Most text used in an application is meaningful only if it has visible content.
  public static void checkForContent(String aText){
    if( ! Util.textHasContent(aText) ){
      throw new IllegalArgumentException("Text has no visible content");

   If {@link Util#isInRange} returns <code>false</code>, then 
   throw an <code>IllegalArgumentException</code>. 
   @param aLow is less than or equal to <code>aHigh</code>.
  public static void checkForRange(int aNumber, int aLow, int aHigh) {
    if ( ! Util.isInRange(aNumber, aLow, aHigh) ) {
      throw new IllegalArgumentException(aNumber + " not in range " + aLow + ".." + aHigh);

   If <tt>aNumber</tt> is less than <tt>1</tt>, then throw an 
  public static void checkForPositive(int aNumber) {
    if (aNumber < 1) {
      throw new IllegalArgumentException(aNumber + " is less than 1");

   If {@link Util#matches} returns <tt>false</tt>, then 
   throw an <code>IllegalArgumentException</code>. 
  public static void checkForMatch(Pattern aPattern, String aText){
    if (! Util.matches(aPattern, aText)){
      throw new IllegalArgumentException(
        "Text " + Util.quote(aText) + " does not match '" +aPattern.pattern()+ "'"
   If <code>aObject</code> is null, then throw a <code>NullPointerException</code>.
   <P>Use cases :
   doSomething( Football aBall ){
     //1. call some method on the argument : 
     //if aBall is null, then exception is automatically thrown, so 
     //there is no need for an explicit check for null.
     //2. assign to a corresponding field (common in constructors): 
     //if aBall is null, no exception is immediately thrown, so 
     //an explicit check for null may be useful here
     Args.checkForNull( aBall );
     fBall = aBall;
     //3. pass on to some other method as parameter : 
     //it may or may not be appropriate to have an explicit check 
     //for null here, according the needs of the problem
     Args.checkForNull( aBall ); //??
     fReferee.verify( aBall );
  public static void checkForNull(Object aObject) {
    if (aObject == null) {
      throw new NullPointerException();
  private Args(){
    //empty - prevent construction

See Also :
Model Objects
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