How can I access the atoms in a structure?

BioJava provides a flexible data structure for managing protein structural data. The Structure class is the main container.

A Structure has a hierarchy of sub-objects:

Structure
   |
   Model(s)
        |
        Chain(s)
            |
             Group(s)
                 |
                 Atom(s)

Different ways are provided how to access the data contained in a Structure. If you want to directly access an array of Atoms you can use the StructureTools

// get all Calpha atoms in the structure Atom[] caAtoms =
StructureTools.getAtomArray(structure,new String[] {"CA"});

Another possibility is to use one of the iterators to iterate over Atoms or Groups.

```java public static int getNrAtoms(Structure s){

       int nrAtoms = 0;
       
       Iterator iter = new GroupIterator(s);
       
       while ( iter.hasNext()){
           Group g = (Group) iter.next();
           nrAtoms += g.size();
       }
       
       return nrAtoms;
   }

```java

`       AtomIterator iter = new AtomIterator(structure) ;`  
`       while (iter.hasNext()) {`  
`           Atom atom = (Atom) iter.next() ;`  
`           Calc.rotate(atom,rotationmatrix);`  
`       }`

Next: <BioJava:CookBook:PDB:atomsCalc> - How to do calculations on atoms.