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<div class="theme-default-content content__default"><h1 id="iterator-and-iterable"><a href="#iterator-and-iterable" class="header-anchor">#</a> Iterator and Iterable</h1> <h2 id="creating-your-own-iterable"><a href="#creating-your-own-iterable" class="header-anchor">#</a> Creating your own Iterable.</h2> <p>To create your own Iterable as with any interface you just implement the abstract methods in the interface. For <code>Iterable</code> there is only one which is called <code>iterator()</code>. But its return type <code>Iterator</code> is itself an interface with three abstract methods. You can return an iterator associated with some collection or create your own custom implementation:</p> <div class="language-java extra-class"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">class</span> <span class="token class-name">Alphabet</span> <span class="token keyword">implements</span> <span class="token class-name">Iterable</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">Character</span><span class="token punctuation">></span></span> <span class="token punctuation">{</span>
<span class="token annotation punctuation">@Override</span>
<span class="token keyword">public</span> <span class="token class-name">Iterator</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">Character</span><span class="token punctuation">></span></span> <span class="token function">iterator</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">return</span> <span class="token keyword">new</span> <span class="token class-name">Iterator</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">Character</span><span class="token punctuation">></span></span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">char</span> letter <span class="token operator">=</span> <span class="token string">'a'</span><span class="token punctuation">;</span>
<span class="token annotation punctuation">@Override</span>
<span class="token keyword">public</span> <span class="token keyword">boolean</span> <span class="token function">hasNext</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">return</span> letter <span class="token operator"><=</span> <span class="token string">'z'</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token annotation punctuation">@Override</span>
<span class="token keyword">public</span> <span class="token class-name">Character</span> <span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">return</span> letter<span class="token operator">++</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token annotation punctuation">@Override</span>
<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">remove</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">throw</span> <span class="token keyword">new</span> <span class="token class-name">UnsupportedOperationException</span><span class="token punctuation">(</span><span class="token string">"Doesn't make sense to remove a letter"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre></div><p>To use:</p> <div class="language-java extra-class"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">for</span><span class="token punctuation">(</span><span class="token keyword">char</span> c <span class="token operator">:</span> <span class="token keyword">new</span> <span class="token class-name">Alphabet</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"c = "</span> <span class="token operator">+</span> c<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre></div><p>The new <code>Iterator</code> should come with a state pointing to the first item, each call to next updates its state to point to the next one. The <code>hasNext()</code> checks to see if the iterator is at the end. If the iterator were connected to a modifiable collection then the iterator's optional <code>remove()</code> method might be implemented to remove the item currently pointed to from the underlying collection.</p> <h2 id="removing-elements-using-an-iterator"><a href="#removing-elements-using-an-iterator" class="header-anchor">#</a> Removing elements using an iterator</h2> <p>The <code>Iterator.remove()</code> method is an optional method that removes the element returned by the previous call to <code>Iterator.next()</code>. For example, the following code populates a list of strings and then removes all of the empty strings.</p> <div class="language-java extra-class"><pre class="language-java"><code><span class="token class-name">List</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">></span></span> names <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ArrayList</span><span class="token generics"><span class="token punctuation"><</span><span class="token punctuation">></span></span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
names<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token string">"name 1"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
names<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token string">"name 2"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
names<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token string">""</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
names<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token string">"name 3"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
names<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token string">""</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"Old Size : "</span> <span class="token operator">+</span> names<span class="token punctuation">.</span><span class="token function">size</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">Iterator</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">></span></span> it <span class="token operator">=</span> names<span class="token punctuation">.</span><span class="token function">iterator</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">while</span> <span class="token punctuation">(</span>it<span class="token punctuation">.</span><span class="token function">hasNext</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token class-name">String</span> el <span class="token operator">=</span> it<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>el<span class="token punctuation">.</span><span class="token function">equals</span><span class="token punctuation">(</span><span class="token string">""</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
it<span class="token punctuation">.</span><span class="token function">remove</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"New Size : "</span> <span class="token operator">+</span> names<span class="token punctuation">.</span><span class="token function">size</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre></div><p>Output :</p> <div class="language-java extra-class"><pre class="language-java"><code><span class="token class-name">Old</span> <span class="token class-name">Size</span> <span class="token operator">:</span> <span class="token number">5</span>
<span class="token class-name">New</span> <span class="token class-name">Size</span> <span class="token operator">:</span> <span class="token number">3</span>
</code></pre></div><p>Note that is the code above is the safe way to remove elements while iterating a typical collection. If instead, you attempt to do remove elements from a collection like this:</p> <div class="language-java extra-class"><pre class="language-java"><code><span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token class-name">String</span> el<span class="token operator">:</span> names<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>el<span class="token punctuation">.</span><span class="token function">equals</span><span class="token punctuation">(</span><span class="token string">""</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
names<span class="token punctuation">.</span><span class="token function">remove</span><span class="token punctuation">(</span>el<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// WRONG!</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre></div><p>a typical collection (such as <code>ArrayList</code>) which provides iterators with <strong>fail fast</strong> iterator semantics will throw a <code>ConcurrentModificationException</code>.</p> <p>The <code>remove()</code> method can only called (once) following a <code>next()</code> call. If it is called before calling <code>next()</code> or if it is called twice following a <code>next()</code> call, then the <code>remove()</code> call will throw an <code>IllegalStateException</code>.</p> <p>The <code>remove</code> operation is described as an <strong>optional</strong> operation; i.e. not all iterators will allow it. Examples where it is not supported include iterators for immutable collections, read-only views of collections, or fixed sized collections. If <code>remove()</code> is called when the iterator does not support removal, it will throw an <code>UnsupportedOperationException</code>.</p> <h2 id="using-iterable-in-for-loop"><a href="#using-iterable-in-for-loop" class="header-anchor">#</a> Using Iterable in for loop</h2> <p>Classes implementing <code>Iterable<></code> interface can be used in <code>for</code> loops. This is actually only <a href="https://en.wikipedia.org/wiki/Syntactic_sugar" target="_blank" rel="noopener noreferrer">syntactic sugar<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a> for getting an iterator from the object and using it to get all elements sequentially; it makes code clearer, faster to write end less error-prone.</p> <div class="language-java extra-class"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">UsingIterable</span> <span class="token punctuation">{</span>
<span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token class-name">List</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">Integer</span><span class="token punctuation">></span></span> intList <span class="token operator">=</span> <span class="token class-name">Arrays</span><span class="token punctuation">.</span><span class="token function">asList</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">,</span><span class="token number">2</span><span class="token punctuation">,</span><span class="token number">3</span><span class="token punctuation">,</span><span class="token number">4</span><span class="token punctuation">,</span><span class="token number">5</span><span class="token punctuation">,</span><span class="token number">6</span><span class="token punctuation">,</span><span class="token number">7</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// List extends Collection, Collection extends Iterable</span>
<span class="token class-name">Iterable</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">Integer</span><span class="token punctuation">></span></span> iterable <span class="token operator">=</span> intList<span class="token punctuation">;</span>
<span class="token comment">// foreach-like loop</span>
<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token class-name">Integer</span> i<span class="token operator">:</span> iterable<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>i<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token comment">// pre java 5 way of iterating loops</span>
<span class="token keyword">for</span><span class="token punctuation">(</span><span class="token class-name">Iterator</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">Integer</span><span class="token punctuation">></span></span> i <span class="token operator">=</span> iterable<span class="token punctuation">.</span><span class="token function">iterator</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> i<span class="token punctuation">.</span><span class="token function">hasNext</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token class-name">Integer</span> item <span class="token operator">=</span> i<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>item<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre></div><h2 id="using-the-raw-iterator"><a href="#using-the-raw-iterator" class="header-anchor">#</a> Using the raw iterator</h2> <p>While using the foreach loop (or "extended for loop") is simple, it's sometimes beneficial to use the iterator directly. For example, if you want to output a bunch of comma-separated values, but don't want the last item to have a comma:</p> <div class="language-java extra-class"><pre class="language-java"><code><span class="token class-name">List</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">></span></span> yourData <span class="token operator">=</span> <span class="token comment">//...</span>
<span class="token class-name">Iterator</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">String</span><span class="token punctuation">></span></span> iterator <span class="token operator">=</span> yourData<span class="token punctuation">.</span><span class="token function">iterator</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">while</span> <span class="token punctuation">(</span>iterator<span class="token punctuation">.</span><span class="token function">hasNext</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
<span class="token comment">// next() "moves" the iterator to the next entry and returns it's value.</span>
<span class="token class-name">String</span> entry <span class="token operator">=</span> iterator<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">print</span><span class="token punctuation">(</span>entry<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>iterator<span class="token punctuation">.</span><span class="token function">hasNext</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
<span class="token comment">// If the iterator has another element after the current one:</span>
<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">print</span><span class="token punctuation">(</span><span class="token string">","</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre></div><p>This is much easier and clearer than having a <code>isLastEntry</code> variable or doing calculations with the loop index.</p> <h4 id="remarks"><a href="#remarks" class="header-anchor">#</a> Remarks</h4> <p>It is possible to iterate over an array using the <code>for</code>-each loop, though java arrays do not implement Iterable; iterating is done by JVM using a non-accessible index in the background.</p></div> <footer class="page-edit"><div class="edit-link"><a href="https://github.com/devtut/generate/edit/master/docs/java/iterator-and-iterable.md" target="_blank" rel="noopener noreferrer">Edit this page on GitHub</a> <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></div> <!----></footer> <div class="page-nav"><p class="inner"><span class="prev">
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