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package bjc.data;
import java.util.*;
/**
* Utility methods for dealing with contexts.
*
* @author Ben Culkin
*
*/
public class Contexts {
/**
* The null context, which always throws an exception.
*/
public static final IContext NULL = new NullContextImpl();
private Contexts() {
throw new UnsupportedOperationException();
}
/**
* Create a new context with no parent.
*
* @return A context with no parent.
*/
public static IContext create() {
return new ContextImpl(NULL);
}
/**
* Create a context with the specified parent.
*
* @param parent The parent of this context.
*
* @return A context with the given context as its parent.
*/
public static IContext create(IContext parent) {
return new ContextImpl(parent);
}
private static class NullContextImpl implements IContext {
@Override
public IContext getParent() {
return this;
}
@Override
public void register(String name, Object o) {
throw new UnsupportedOperationException();
}
@Override
public Object get(String name) {
throw new NoSuchElementException();
}
@Override
public <T> T get(Class<T> contract) {
throw new NoSuchElementException();
}
}
private static class ContextImpl implements IContext {
private final IContext parent;
private final Map<String, Object> objects;
public ContextImpl(IContext parent) {
this.parent = parent;
this.objects = new HashMap<>();
}
@Override
public void register(String name, Object o) {
objects.put(name, o);
}
@Override
public Object get(String name) {
if (objects.containsKey(name)) {
return objects.get(name);
}
return parent.get(name);
}
@SuppressWarnings("unchecked")
@Override
public <T> T get(Class<T> contract) {
for (Object o : objects.values()) {
if (contract.isInstance(o)) {
return (T) o;
}
}
return parent.get(contract);
}
@Override
public IContext getParent() {
return parent;
}
}
}
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