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Implementation of linear-chain Conditional Random Fields (CRF) in pure Java

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package com.asher_stern.crf.utilities;

/**
 * A "cache" which remembers one item only.
 * The cache remembers the value for a given key. When a new key-value pair is put, then the previous one is forgotten.
 * 
 * @author Asher Stern
 * Date: Nov 13, 2014
 *
 * @param  cannot be null
 * @param  cannot be null
 */
public class LastCache
{
	/**
	 * Remember the given value for the given key. Forget the older key-value pair.
	 * @param key A key. Cannot be null.
	 * @param value A value corresponds to this key. Cannot be null.
	 */
	public synchronized void put(K key, V value)
	{
		if (null==key) {throw new CrfException("null key");}
		if (null==value) {throw new CrfException("null value");}
		
		this.key = key;
		this.value = value;
	}
	
	/**
	 * Get the value that was put earlier for this key. Return null if the value is unknown.
	 * 
	 * @param key
	 * @return
	 */
	public synchronized V get(K key)
	{
		if (null==key) {throw new CrfException("null key");}
		else if (key.equals(this.key))
		{
			return value;
		}
		else
		{
			return null;
		}
	}

	private K key = null;
	private V value = null;
}