compositequerycreator.java

一个用于搜索本地文件内容的小型搜索引擎

package queries;

import java.util.Deque;

public class CompositeQueryCreator extends QueryCreator {
	private Deque<Query> operandStack;
	private volatile static CompositeQueryCreator uniqueInstance;
	
	private CompositeQueryCreator(Deque<Query> operandStack){
		this.operandStack = operandStack;
	}
	
	@Override
	public Query buildQuery(String expression) {
		Query result = null;
		if(expression.equals("and")){
			Query right = operandStack.pop();
			Query left = operandStack.pop();
			result = new AndQuery(left,right);
		}
		else if(expression.equals("or")){
			Query right = operandStack.pop();
			Query left = operandStack.pop();
			
			result = new OrQuery(left,right);
		}
		else if(expression.equals("not")){
			Query query = operandStack.pop();
			result = new NotQuery(query);
		}
		
		return result;
	}
	
	public static CompositeQueryCreator getInstance(Deque<Query> operandStack){
		if(uniqueInstance == null){
			synchronized(CompositeQueryCreator.class){
				if(uniqueInstance == null){
					uniqueInstance = new CompositeQueryCreator(operandStack);
				}
			}
		}
		return uniqueInstance;
	}
}