Java8Predicate花样用法详解

2024-04-02 19:55

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1. 简介

本文介绍Java 8 Predicate链.

2. 基本用法

怎么使用简单的Predicate来过滤list中的name

@Test
public void whenFilterList_thenSuccess(){
   List<String> names = Arrays.asList("Adam", "Alexander", "John", "Tom");
   List<String> result = names.stream()
     .filter(name -> name.startsWith("A"))
     .collect(Collectors.toList());
    
   assertEquals(2, result.size());
   assertThat(result, contains("Adam","Alexander"));
}

我们使用Predicate来筛选以大写字母A开头的姓名。

name -> name.startsWith("A")

那么如果多个条件这么办？

3. 多条件过滤

@Test
public void whenFilterListWithMultipleFilters_thenSuccess(){
    List<String> result = names.stream()
      .filter(name -> name.startsWith("A"))
      .filter(name -> name.length() < 5)
      .collect(Collectors.toList());
 
    assertEquals(1, result.size());
    assertThat(result, contains("Adam"));
}

用两个filter传入两个 Predicate分别过滤【以A开头的】和【姓名长度小于5】的。

4. 复杂条件

@Test
public void whenFilterListWithComplexPredicate_thenSuccess(){
    List<String> result = names.stream()
      .filter(name -> name.startsWith("A") && name.length() < 5)
      .collect(Collectors.toList());
 
    assertEquals(1, result.size());
    assertThat(result, contains("Adam"));
}

使用一个 filter 传入复杂的Predicate.

5. 组合使用Predicate

Predicates可以将 Predicate.and(), Predicate.or() 和 Predicate.negate()组合起来使用。

5.1. Predicate.and()

@Test
public void whenFilterListWithCombinedPredicatesUsingAnd_thenSuccess(){
    Predicate<String> predicate1 =  str -> str.startsWith("A");
    Predicate<String> predicate2 =  str -> str.length() < 5;
   
    List<String> result = names.stream()
      .filter(predicate1.and(predicate2))
      .collect(Collectors.toList());
         
    assertEquals(1, result.size());
    assertThat(result, contains("Adam"));
}

两个条件都要满足

5.2. Predicate.or()

满足其中一个即可

@Test
public void whenFilterListWithCombinedPredicatesUsingOr_thenSuccess(){
    Predicate<String> predicate1 =  str -> str.startsWith("J");
    Predicate<String> predicate2 =  str -> str.length() < 4;
     
    List<String> result = names.stream()
      .filter(predicate1.or(predicate2))
      .collect(Collectors.toList());
     
    assertEquals(2, result.size());
    assertThat(result, contains("John","Tom"));
}

5.3. Predicate.negate()

将此条件取反

Predicate<String> predicate2 =  str -> str.length() < 4;

相当于

Predicate<String> predicate2 =  str -> str.length() >= 4;

@Test
public void whenFilterListWithCombinedPredicatesUsingOrAndNegate_thenSuccess(){
    Predicate<String> predicate1 =  str -> str.startsWith("J");
    Predicate<String> predicate2 =  str -> str.length() < 4;
     
    List<String> result = names.stream()
      .filter(predicate1.or(predicate2.negate()))
      .collect(Collectors.toList());
     
    assertEquals(3, result.size());
    assertThat(result, contains("Adam","Alexander","John"));
}

5.4. 内联的方式组合使用Predicates

@Test
public void whenFilterListWithCombinedPredicatesInline_thenSuccess(){
    List<String> result = names.stream()
      .filter(((Predicate<String>)name -> name.startsWith("A"))
      .and(name -> name.length()<5))
      .collect(Collectors.toList());
 
    assertEquals(1, result.size());
    assertThat(result, contains("Adam"));
}

6. 组合Predicates集合

在开始介绍之前，简单介绍下 reduce 函数：

 `java.util.stream.Stream#reduce(T, java.util.function.BinaryOperator<T>)`

源码的注释中给出等价的写法：

     T result = identity;
     for (T element : this stream)
        result = accumulator.apply(result, element)
     return result;

即，第一个参数当做初始值，后续参数和第一个参数进行运算，最终得到结果。

接下来我们看下面 reduce 中 and 操作的例子：

@Test
public void whenFilterListWithCollectionOfPredicatesUsingAnd_thenSuccess(){
    List<Predicate<String>> allPredicates = new ArrayList<Predicate<String>>();
    allPredicates.add(str -> str.startsWith("A"));
    allPredicates.add(str -> str.contains("d"));        
    allPredicates.add(str -> str.length() > 4);
     
    List<String> result = names.stream()
      .filter(allPredicates.stream().reduce(x->true, Predicate::and))
      .collect(Collectors.toList());
     
    assertEquals(1, result.size());
    assertThat(result, contains("Alexander"));
}

注意这里初始条件是 true （如果初始条件为 false ，后续即使都满足，和初始值一起 and ，也没结果）

然后看 reduce 中使用 or 操作的例子：

@Test
public void whenFilterListWithCollectionOfPredicatesUsingOr_thenSuccess(){
    List<String> result = names.stream()
      .filter(allPredicates.stream().reduce(x->false, Predicate::or))
      .collect(Collectors.toList());
     
    assertEquals(2, result.size());
    assertThat(result, contains("Adam","Alexander"));
}