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copied!<p>The question is vague on what "long" means. Here are some possible interpretations:</p> <h3>Interpretation #1: The constructor has many parameters</h3> <p>Constructors with many parameters can lead to poor readability, and better alternatives exist.</p> <p>Here's a quote from <em>Effective Java 2nd Edition, Item 2: Consider a builder pattern when faced with many constructor parameters</em>:</p> <blockquote> <p>Traditionally, programmers have used the <em>telescoping constructor</em> pattern, in which you provide a constructor with only the required parameters, another with a single optional parameters, a third with two optional parameters, and so on...</p> </blockquote> <p>The telescoping constructor pattern is essentially something like this:</p> <pre><code>public class Telescope { final String name; final int levels; final boolean isAdjustable; public Telescope(String name) { this(name, 5); } public Telescope(String name, int levels) { this(name, levels, false); } public Telescope(String name, int levels, boolean isAdjustable) { this.name = name; this.levels = levels; this.isAdjustable = isAdjustable; } } </code></pre> <p>And now you can do any of the following:</p> <pre><code>new Telescope("X/1999"); new Telescope("X/1999", 13); new Telescope("X/1999", 13, true); </code></pre> <p>You can't, however, currently set only the <code>name</code> and <code>isAdjustable</code>, and leaving <code>levels</code> at default. You can provide more constructor overloads, but obviously the number would explode as the number of parameters grow, and you may even have multiple <code>boolean</code> and <code>int</code> arguments, which would really make a mess out of things.</p> <p>As you can see, this isn't a pleasant pattern to write, and even less pleasant to use (What does "true" mean here? What's 13?).</p> <p>Bloch recommends using a builder pattern, which would allow you to write something like this instead:</p> <pre><code>Telescope telly = new Telescope.Builder("X/1999").setAdjustable(true).build(); </code></pre> <p>Note that now the parameters are named, and you can set them in any order you want, and you can skip the ones that you want to keep at default values. This is certainly much better than telescoping constructors, especially when there's a huge number of parameters that belong to many of the same types.</p> <h3>See also</h3> <ul> <li><a href="http://en.wikipedia.org/wiki/Builder_pattern" rel="nofollow noreferrer">Wikipedia/Builder pattern</a></li> <li><em>Effective Java 2nd Edition, Item 2: Consider a builder pattern when faced with many constructor parameters</em> (<a href="http://www.codeproject.com/KB/books/EffectiveJava.aspx" rel="nofollow noreferrer">excerpt online</a>)</li> </ul> <h3>Related questions</h3> <ul> <li><a href="https://stackoverflow.com/questions/328496/when-would-you-use-the-builder-pattern">When would you use the Builder Pattern?</a></li> <li><a href="https://stackoverflow.com/questions/2637268/is-this-a-well-known-design-pattern-what-is-its-name/">Is this a well known design pattern? What is its name?</a></li> </ul> <hr> <h3>Interpretation #2: The constructor does a lot of work that costs time</h3> <p>If the work must be done at construction time, then doing it in the constructor or in a helper method doesn't really make too much of a difference. When a constructor delegates work to a helper method, however, make sure that it's not overridable, because that could lead to a lot of problems.</p> <p>Here's some quote from <em>Effective Java 2nd Edition, Item 17: Design and document for inheritance, or else prohibit it</em>:</p> <blockquote> <p>There are a few more restrictions that a class must obey to allow inheritance. <strong>Constructors must not invoke overridable methods</strong>, directly or indirectly. If you violate this rule, program failure will result. The superclass constructor runs before the subclass constructor, so the overriding method in the subclass will be invoked before the subclass constructor has run. If the overriding method depends on any initialization performed by the subclass constructor, the method will not behave as expected.</p> </blockquote> <p>Here's an example to illustrate:</p> <pre><code>public class ConstructorCallsOverride { public static void main(String[] args) { abstract class Base { Base() { overrideMe(); } abstract void overrideMe(); } class Child extends Base { final int x; Child(int x) { this.x = x; } @Override void overrideMe() { System.out.println(x); } } new Child(42); // prints "0" } } </code></pre> <p>Here, when <code>Base</code> constructor calls <code>overrideMe</code>, <code>Child</code> has not finished initializing the <code>final int x</code>, and the method gets the wrong value. This will almost certainly lead to bugs and errors.</p> <hr> <h3>Interpretation #3: The constructor does a lot of work that can be deferred</h3> <p>The construction of an object can be made faster when some work is deferred to when it's actually needed; this is called <em>lazy initialization</em>. As an example, when a <code>String</code> is constructed, it does not actually compute its hash code. It only does it when the hash code is first required, and then it will cache it (since strings are immutable, this value will not change).</p> <p>However, consider <em>Effective Java 2nd Edition, Item 71: Use lazy initialization judiciously</em>. Lazy initialization can lead to subtle bugs, and don't always yield improved performance that justifies the added complexity. Do not prematurely optimize.</p>

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