Guava----Function

1. Function接口，提供两个方法：

apply方法：　

　　可以自定义自己想实现的功能

 @Nullable T apply(@Nullable F input); 

１.　实例：

import com.google.common.base.Function;
import com.google.common.base.Joiner;
import org.junit.Test;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

/**
 * author: 龚细军
 * class-aim:
 */
public class Demo<F, T> {

    private Logger logger = LoggerFactory.getLogger(Demo.class);

    public void demoFunction(F input, Function<F, T> function) throws Exception {
        
        logger.info(String.valueOf(function.apply(input)));
    }

    @Test
    public void show() {

        Function<F, T> functionTest = new Function<F, T>() {
            @Override
            public T apply(F input) {
                return (T) Integer.valueOf(String.valueOf(input));
            }
        };

        try {
            demoFunction((F) "123", functionTest);
        } catch (Exception e) {
            logger.error(Joiner.on("").join("{}show发生异常", e));
        }
    }
}

一般而言，使用这种方法，可以让我们避免做重复的事情，比如，我们在使用jdbc时，会出现大量重复的代码，我们可以采用Function<F,T>来进行函数编程，简化代码：

import com.google.common.base.Function;
import com.google.common.collect.Lists;
import com.qunar.fresh.bean.PageVistor;
import com.qunar.fresh.bean.dbBean;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.beans.factory.BeanFactory;
import org.springframework.context.support.FileSystemXmlApplicationContext;

import java.sql.*;
import java.util.List;


/**
 * author: 龚细军
 * class-aim: dao-dbDemo
 */
public class DbUtil<F, T> {

    private static final String CONFIG_PROPERTIES = "classpath:Beans.xml";


    static Logger logger = LoggerFactory.getLogger(DbUtil.class);

    private Connection connection = null;
    private static dbBean dataBean;
    private PreparedStatement preparedStatement = null;
    private ResultSet resultSet = null;
    private List<PageVistor> PageVisitors;

    static {
        try {
            BeanFactory beanFactory = new FileSystemXmlApplicationContext(CONFIG_PROPERTIES);

            dataBean = (dbBean) beanFactory.getBean("dbBean");
            Class.forName(dataBean.getDriver());
        } catch (ClassNotFoundException e) {
            logger.info(String.format("sql exception0 : %s", e));
        }
    }


    private T execute(Function<F, T> function, String sql) {

        T execResult = null;

        try {
            connection = DriverManager.getConnection(dataBean.getUrl(),
                    dataBean.getUsername(), dataBean.getPassword());
            preparedStatement = connection.prepareStatement(sql);
            execResult = function.apply(null);
        } catch (SQLException e) {
            logger.info(String.format("sql exception１ : %s", e));
        } finally {
            try {
                if (!preparedStatement.isClosed())
                    preparedStatement.close();
                if (!connection.isClosed())
                    connection.close();
            } catch (SQLException e) {
                logger.info(String.format("sql exception２ : %s", e));
            }
        }

        return execResult;
    }


    public List<PageVistor> Query(String sql) {


        PageVisitors = Lists.newArrayList();

        Function<F, T> function = new Function<F, T>() {

            @Override
            public T apply(F input) {


                try {
                    resultSet = preparedStatement.executeQuery();

                    while (resultSet.next()) {
                        PageVisitors.add(new PageVistor(0, "", resultSet.getString(1)
                                , resultSet.getInt(2)));
                    }
                } catch (SQLException e) {
                    logger.info(String.format("sql exception3 : %s", e));
                } finally {
                    try {

                        if (resultSet != null) resultSet.close();

                    } catch (SQLException e) {
                        logger.info(String.format("sql exception４ : %s", e));
                    }
                }

                return null;
            }
        };
        this.execute(function, sql);

        return PageVisitors;
    }


    public void Update(String sql) {

        Function<F, T> function = new Function<F, T>() {

            @Override
            public T apply(F input) {

                try {
                    int var = preparedStatement.executeUpdate();
                    if (var == 0)
                        logger.error("{} 更新数据失败");
                } catch (SQLException e) {
                    logger.info(String.format("sql exception3 : %s", e));
                }
                return null;
            }
        };

        this.execute(function, sql);
    }
}