Java8 Predicate花樣用法詳解

更新時(shí)間：2022年03月01日 15:30:49 作者：明明如月學(xué)長(zhǎng)

本文主要介紹了Java 8 Predicate花樣用法詳解，文中通過示例代碼介紹的非常詳細(xì)，具有一定的參考價(jià)值，感興趣的小伙伴們可以參考一下

1. 簡(jiǎn)介

本文介紹Java 8 Predicate鏈.

2. 基本用法

怎么使用簡(jiǎn)單的Predicate來過濾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"));
}

我們使用Predicate來篩選以大寫字母A開頭的姓名。

name -> name.startsWith("A")

那么如果多個(gè)條件這么辦？

3. 多條件過濾

@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"));
}

用兩個(gè)filter傳入兩個(gè) Predicate分別過濾【以A開頭的】和【姓名長(zhǎng)度小于5】的。

4. 復(fù)雜條件

@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"));
}

使用一個(gè) filter 傳入復(fù)雜的Predicate.

5. 組合使用Predicate

Predicates可以將 Predicate.and(), Predicate.or() 和 Predicate.negate()組合起來使用。

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"));
}

兩個(gè)條件都要滿足

5.2. Predicate.or()

滿足其中一個(gè)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()

將此條件取反

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

相當(dāng)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. 內(nèi)聯(lián)的方式組合使用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. 組合Predicates集合

在開始介紹之前，簡(jiǎn)單介紹下 reduce 函數(shù)：

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

源碼的注釋中給出等價(jià)的寫法：

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

即，第一個(gè)參數(shù)當(dāng)做初始值，后續(xù)參數(shù)和第一個(gè)參數(shù)進(jìn)行運(yùn)算，最終得到結(jié)果。

接下來我們看下面 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"));
}

注意這里初始條件是 true （如果初始條件為 false ，后續(xù)即使都滿足，和初始值一起 and ，也沒結(jié)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"));
}