Java Snippets

Just some code snippets that I found useful. Not necessarily the best or fastest solutions.

Generics

Return Types

To get the compiler to understand a generic return type, simply add a dummy class parameter.

public static <A> A someMethod(Class<A> dummy) {
}

// calling the method
someMethod(Double.class);

Collections

Single Element List

Create a list of one single element.

Arrays.asList(element);

Iterable Iterator

Looping over an iteartor with a foreach is not possible. An Iterable is needed.

/**
 * Put this method in a Utility class. It can then be imported as class 
 * or statically.
 */
public static <T> Iterable<T> toIterable(final Iterator<T> the_iterator) {
   return new Iterable<T>() {
      public Iterator<T> iterator() { 
         return the_iterator;
      }
   };
}

...

for(T t : toIterable(an_iterator)) {
   // do something
}

Foreach on XML NodeList elements

Creating simple Iterable will allow us to iterate over NodeLists

Iterable<Element> toIterable(final NodeList list) {
   return new Iterable<Element>() {
      @Override public Iterator<Element> iterator() {
         return new Iterator<Element>() {
            int position = 0;

            @Override public void remove() {
               throw new UnsupportedOperationException();
            }

            @Override public Element next() {
               if (!hasNext()) throw new NoSuchElementException();
               return (Element) list.item(position++);
            }
     
            @Override public boolean hasNext() {
               // move to next
               while(position < list.getLength() && !(list.item(position) instanceof Element)) ++position;
               return position < list.getLength();
            }
         };
      }
   };
}

// used 
Element someElement = ...;
for (Element child : toIterable(someElement.getChildNodes()) {
   /// TODO something
}

Loops

Multiple nested loops break

// given a matrix of size n x k
// we want to know if it contains a value greater than val
double val = 1;
boolean found = false;

label: for(int i = 0; i < n; ++i) {
   for(int j = 0; j < k; ++j) {
      if (matrix[i][j] > val) {
         found = true;
         break label; // get out of both for loops
      }
   }
}

Note: this post might grow in time