Lambda表达式详解（一）

Lambda表达式是什么

• Lambda表达式是Java SE 8中一个重要的新特性
• lambda表达式允许你通过表达式来代替功能接口
• Lambda表达式还增强了集合库Function/Stream

使用Lambda表达式的优点

简化了匿名委托的使用，让你让代码更加简洁，优雅

Lambda表达式应用场景

使用ForEach遍历

自定义一个bean

package com.zyxr.vip.mall.goods.bean;

import lombok.AllArgsConstructor;
import lombok.Getter;
import lombok.NoArgsConstructor;
import lombok.Setter;
import lombok.ToString;


@Getter
@Setter
@ToString
@AllArgsConstructor
@NoArgsConstructor
public class GoodsRule {
    public Long ruleId;
    public Long goodsId;
    public Integer firstType;
    public Integer secondType;
    public String secondValue;
    public GoodsRule(Long goodsId, Integer firstType, Integer secondType, String secondValue) {
        this(firstType, secondType, secondValue);
        this.goodsId = goodsId;

    }
    public GoodsRule(Integer firstType, Integer secondType, String secondValue) {
        this.firstType = firstType;
        this.secondType = secondType;
        this.secondValue = secondValue;
    }
}

测试类

@Before
    public void init() {
        log.info("begin to init data！");
        ruleList = new ArrayList<>();
        ruleList.add(new GoodsRule(1L, 1, 1, "3"));
        ruleList.add(new GoodsRule(1L, 2, 1, "呵呵"));
        ruleList.add(new GoodsRule(1L, 2, 2, "3"));
        ruleList.add(new GoodsRule(1L, 3, 1, "1232"));
        ruleList.add(new GoodsRule(2L, 1, 1, "3"));
        ruleList.add(new GoodsRule(2L, 2, 1, "qqq"));
        log.info("end to init data！");
    }

    @Test
    public void testForEach() {
        log.info("传统的遍历！");
        for (GoodsRule rule : ruleList) {
            log.info(gs.toJson(rule));
        }
        log.info("使用Lambda表达式遍历！");
        ruleList.forEach(rule -> log.info(gs.toJson(rule)));
    }

执行结果

INFO  [VIPApp] 2018-08-11 16:51:50.125 [main] com.zyxr.vip.test.mall.AdvisorTest 252 - begin to init data！
INFO  [VIPApp] 2018-08-11 16:51:50.125 [main] com.zyxr.vip.test.mall.AdvisorTest 260 - end to init data！
INFO  [VIPApp] 2018-08-11 16:51:50.125 [main] com.zyxr.vip.test.mall.AdvisorTest 265 - 传统的遍历！
INFO  [VIPApp] 2018-08-11 16:51:50.125 [main] com.zyxr.vip.test.mall.AdvisorTest 267 - {"goodsId":1,"firstType":1,"secondType":1,"secondValue":"3"}
INFO  [VIPApp] 2018-08-11 16:51:50.126 [main] com.zyxr.vip.test.mall.AdvisorTest 267 - {"goodsId":1,"firstType":2,"secondType":1,"secondValue":"呵呵"}
INFO  [VIPApp] 2018-08-11 16:51:50.126 [main] com.zyxr.vip.test.mall.AdvisorTest 267 - {"goodsId":1,"firstType":2,"secondType":2,"secondValue":"3"}
INFO  [VIPApp] 2018-08-11 16:51:50.126 [main] com.zyxr.vip.test.mall.AdvisorTest 267 - {"goodsId":1,"firstType":3,"secondType":1,"secondValue":"1232"}
INFO  [VIPApp] 2018-08-11 16:51:50.126 [main] com.zyxr.vip.test.mall.AdvisorTest 267 - {"goodsId":2,"firstType":1,"secondType":1,"secondValue":"3"}
INFO  [VIPApp] 2018-08-11 16:51:50.126 [main] com.zyxr.vip.test.mall.AdvisorTest 267 - {"goodsId":2,"firstType":2,"secondType":1,"secondValue":"qqq"}
INFO  [VIPApp] 2018-08-11 16:51:50.126 [main] com.zyxr.vip.test.mall.AdvisorTest 269 - 使用Lambda表达式遍历！
INFO  [VIPApp] 2018-08-11 16:51:50.126 [main] com.zyxr.vip.test.mall.AdvisorTest 270 - {"goodsId":1,"firstType":1,"secondType":1,"secondValue":"3"}
INFO  [VIPApp] 2018-08-11 16:51:50.126 [main] com.zyxr.vip.test.mall.AdvisorTest 270 - {"goodsId":1,"firstType":2,"secondType":1,"secondValue":"呵呵"}
INFO  [VIPApp] 2018-08-11 16:51:50.127 [main] com.zyxr.vip.test.mall.AdvisorTest 270 - {"goodsId":1,"firstType":2,"secondType":2,"secondValue":"3"}
INFO  [VIPApp] 2018-08-11 16:51:50.127 [main] com.zyxr.vip.test.mall.AdvisorTest 270 - {"goodsId":1,"firstType":3,"secondType":1,"secondValue":"1232"}
INFO  [VIPApp] 2018-08-11 16:51:50.127 [main] com.zyxr.vip.test.mall.AdvisorTest 270 - {"goodsId":2,"firstType":1,"secondType":1,"secondValue":"3"}
INFO  [VIPApp] 2018-08-11 16:51:50.127 [main] com.zyxr.vip.test.mall.AdvisorTest 270 - {"goodsId":2,"firstType":2,"secondType":1,"secondValue":"qqq"}

可以简化匿名内部类的实现

测试类

@Before
    public void init() {
        log.info("begin to init data！");
        ruleList = new ArrayList<>();
        ruleList.add(new GoodsRule(1L, 1, 1, "3"));
        ruleList.add(new GoodsRule(1L, 2, 1, "呵呵"));
        ruleList.add(new GoodsRule(1L, 2, 2, "3"));
        ruleList.add(new GoodsRule(1L, 3, 1, "1232"));
        ruleList.add(new GoodsRule(2L, 1, 1, "3"));
        ruleList.add(new GoodsRule(2L, 2, 1, "qqq"));
        log.info("end to init data！");
    }
    @Test
    public void testTraditionSort() {
        log.info("排序前：");
        ruleList.forEach(rule -> log.info(gs.toJson(rule)));
        //排序规则：按照goodsId降序
        Collections.sort(ruleList, new Comparator<GoodsRule>() {
            @Override
            public int compare(GoodsRule o1, GoodsRule o2) {
                return (int) (o2.goodsId - o1.goodsId);
            }
        });
        log.info("排序后:");
        ruleList.forEach(rule -> log.info(gs.toJson(rule)));

    }

    @Test
    public void testLambdaSort() {
        log.info("排序前：");
        ruleList.forEach(rule -> log.info(gs.toJson(rule)));
        //排序规则：按照goodsId降序
        Collections.sort(ruleList, (GoodsRule o1, GoodsRule o2) -> (int) (o2.goodsId - o1.goodsId));
        log.info("排序后:");
        ruleList.forEach(rule -> log.info(gs.toJson(rule)));
    }

执行结果

两个测试案例的结果一样

INFO  [VIPApp] 2018-08-11 16:59:20.348 [main] com.zyxr.vip.test.mall.AdvisorTest 252 - begin to init data！
INFO  [VIPApp] 2018-08-11 16:59:20.349 [main] com.zyxr.vip.test.mall.AdvisorTest 260 - end to init data！
INFO  [VIPApp] 2018-08-11 16:59:20.349 [main] com.zyxr.vip.test.mall.AdvisorTest 280 - 排序前：
INFO  [VIPApp] 2018-08-11 16:59:20.350 [main] com.zyxr.vip.test.mall.AdvisorTest 281 - {"goodsId":1,"firstType":1,"secondType":1,"secondValue":"3"}
INFO  [VIPApp] 2018-08-11 16:59:20.350 [main] com.zyxr.vip.test.mall.AdvisorTest 281 - {"goodsId":1,"firstType":2,"secondType":1,"secondValue":"呵呵"}
INFO  [VIPApp] 2018-08-11 16:59:20.350 [main] com.zyxr.vip.test.mall.AdvisorTest 281 - {"goodsId":1,"firstType":2,"secondType":2,"secondValue":"3"}
INFO  [VIPApp] 2018-08-11 16:59:20.350 [main] com.zyxr.vip.test.mall.AdvisorTest 281 - {"goodsId":1,"firstType":3,"secondType":1,"secondValue":"1232"}
INFO  [VIPApp] 2018-08-11 16:59:20.350 [main] com.zyxr.vip.test.mall.AdvisorTest 281 - {"goodsId":2,"firstType":1,"secondType":1,"secondValue":"3"}
INFO  [VIPApp] 2018-08-11 16:59:20.350 [main] com.zyxr.vip.test.mall.AdvisorTest 281 - {"goodsId":2,"firstType":2,"secondType":1,"secondValue":"qqq"}
INFO  [VIPApp] 2018-08-11 16:59:20.351 [main] com.zyxr.vip.test.mall.AdvisorTest 284 - 排序后:
INFO  [VIPApp] 2018-08-11 16:59:20.352 [main] com.zyxr.vip.test.mall.AdvisorTest 285 - {"goodsId":2,"firstType":1,"secondType":1,"secondValue":"3"}
INFO  [VIPApp] 2018-08-11 16:59:20.352 [main] com.zyxr.vip.test.mall.AdvisorTest 285 - {"goodsId":2,"firstType":2,"secondType":1,"secondValue":"qqq"}
INFO  [VIPApp] 2018-08-11 16:59:20.352 [main] com.zyxr.vip.test.mall.AdvisorTest 285 - {"goodsId":1,"firstType":1,"secondType":1,"secondValue":"3"}
INFO  [VIPApp] 2018-08-11 16:59:20.352 [main] com.zyxr.vip.test.mall.AdvisorTest 285 - {"goodsId":1,"firstType":2,"secondType":1,"secondValue":"呵呵"}
INFO  [VIPApp] 2018-08-11 16:59:20.353 [main] com.zyxr.vip.test.mall.AdvisorTest 285 - {"goodsId":1,"firstType":2,"secondType":2,"secondValue":"3"}
INFO  [VIPApp] 2018-08-11 16:59:20.353 [main] com.zyxr.vip.test.mall.AdvisorTest 285 - {"goodsId":1,"firstType":3,"secondType":1,"secondValue":"1232"}

总结

Lambda表达式可以简化遍历和匿名内部类，能使代码更加的优雅。在ideal中使用匿名类会提示

Anonymous new Comparator<GoodsRule>() can be replaced with lambda less... (Ctrl+F1) 
This inspection reports all anonymous classes which can be replaced with lambda expressions
Lambda syntax is not supported under Java 1.7 or earlier JVMs.

由此看出，官方也推荐使用Lambda表达式。但更大的应用场景是和Stream联合使用，将在下一篇中详细介绍。