关于stream.foreach()和stream.peek()的区别解析

矫情吗；* 2021-12-12 15:17 575阅读 0赞

**改思考来源于日常工作中，特记此心得。**

**思考：如何快速将list中的每个item内部属性值改变并进行其他流体操作呢？**

**下面做个测试：****如何在list中根据某个属性的最小值取出该对象**

**1：随便新建一个测试bean:**

![ContractedBlock.gif][] ![ExpandedBlockStart.gif][]

1 package com.dev.model;
     2 
     3 import javax.persistence.*;
     4 
     5 public class Aopi {
     6     /**
     7      * id
     8      */
     9     @Id
    10     @GeneratedValue(strategy = GenerationType.IDENTITY)
    11     private Integer id;
    12 
    13     /**
    14      * 姓名
    15      */
    16     private String name;
    17 
    18     /**
    19      * 年龄
    20      */
    21     private Integer age;
    22 
    23     /**
    24      * 获取id
    25      *
    26      * @return id - id
    27      */
    28     public Integer getId() {
    29         return id;
    30     }
    31 
    32     /**
    33      * 设置id
    34      *
    35      * @param id id
    36      */
    37     public void setId(Integer id) {
    38         this.id = id;
    39     }
    40 
    41     /**
    42      * 获取姓名
    43      *
    44      * @return name - 姓名
    45      */
    46     public String getName() {
    47         return name;
    48     }
    49 
    50     /**
    51      * 设置姓名
    52      *
    53      * @param name 姓名
    54      */
    55     public void setName(String name) {
    56         this.name = name;
    57     }
    58 
    59     /**
    60      * 获取年龄
    61      *
    62      * @return age - 年龄
    63      */
    64     public Integer getAge() {
    65         return age;
    66     }
    67 
    68     /**
    69      * 设置年龄
    70      *
    71      * @param age 年龄
    72      */
    73     public void setAge(Integer age) {
    74         this.age = age;
    75     }
    76 
    77     public Aopi(String name, Integer age) {
    78         this.name = name;
    79         this.age = age;
    80     }
    81 
    82     public Aopi() {
    83     }
    84 
    85     @Override
    86     public String toString() {
    87         return "Aopi{" +
    88                 "id=" + id +
    89                 ", name='" + name + '\'' +
    90                 ", age=" + age +
    91                 '}';
    92     }
    93 }

**2：新建一个单元测试：**

@Test
        public void test01() {
            List<Aopi> aopiList = Lists.newArrayList();
    
            Aopi aopi = new Aopi("1", 1);
            Aopi aop2 = new Aopi("2", 2);
            Aopi aop3 = new Aopi("3", 3);
            Aopi aop4 = new Aopi("4", 4);
    
            aopiList.addAll(Arrays.asList(aopi, aop2, aop3, aop4));
    
            //第一种方式
            aopiList.forEach(item -> item.setName(item.getName() + "_test"));
            System.out.println(
                    aopiList.stream().min((o1, o2) -> {
                        if (Objects.equals(o1.getAge(), o2.getAge()))
                            return 0;
                        return o1.getAge() > o2.getAge() ? 1 : -1;
                    }).get().toString()
            );
    
            System.out.println("zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz");
    
            //第二种方式
            System.out.println(
                    aopiList.stream().peek(item -> item.setName(item.getName() + "_test")).min((o1, o2) -> {
                        if (Objects.equals(o1.getAge(), o2.getAge()))
                            return 0;
                        return o1.getAge() > o2.getAge() ? 1 : -1;
                    }).get().toString()
            );
    
        }

**notice1：测试第一种方式注释掉第二种，反之亦如此**

**notice2：list.stream().foreach  ->  list.foreach()**

**3：看测试结果：**

**第一种测试结果：**

![1537140-20190703110410726-1940400163.png][]

**第二种测试结果：**

**![1537140-20190703110303950-1923708775.png][]**

**结论：**

**（1）：使用stream.foreach也可以更改list中的每个item的内部属性值等等，但是要进行“二次流处理”，才能得到list中最小的item（根据age筛选）**

**（2）：stream.peek比stream.foreach()可以跟直接拿到最小的item（根据age筛选）**

**原因：**

**（1）：stream.foreach的操作是void的，除了更改属性值还可以进行其他操作等。因此要做“二次流处理”。**

default void forEach(Consumer<? super T> action) {
            Objects.requireNonNull(action);
            for (T t : this) {
                action.accept(t);
            }
        }

**（1）：stream.peek的操作是返回一个新的stream的，且设计的初衷是用来debug调试的，因此使用steam.peek()必须对流进行一次处理再产生一个新的stream。**

/**
         * Returns a stream consisting of the elements of this stream, additionally
         * performing the provided action on each element as elements are consumed
         * from the resulting stream.
         *
         * <p>This is an <a href="package-summary.html#StreamOps">intermediate
         * operation</a>.
         *
         * <p>For parallel stream pipelines, the action may be called at
         * whatever time and in whatever thread the element is made available by the
         * upstream operation.  If the action modifies shared state,
         * it is responsible for providing the required synchronization.
         *
         * @apiNote This method exists mainly to support debugging, where you want
         * to see the elements as they flow past a certain point in a pipeline:
         * <pre>{
         @code
         *     Stream.of("one", "two", "three", "four")
         *         .filter(e -> e.length() > 3)
         *         .peek(e -> System.out.println("Filtered value: " + e))
         *         .map(String::toUpperCase)
         *         .peek(e -> System.out.println("Mapped value: " + e))
         *         .collect(Collectors.toList());
         * }</pre>
         *
         * @param action a <a href="package-summary.html#NonInterference">
         *                 non-interfering</a> action to perform on the elements as
         *                 they are consumed from the stream
         * @return the new stream
         */
        Stream<T> peek(Consumer<? super T> action);

**bye~^\_^**

转载于:https://www.cnblogs.com/zgq7/p/11125419.html

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[1537140-20190703110303950-1923708775.png]: /images/20211210/21dfa24b66a441fe9e64711788c7b27f.png