Java 8 Predicate Chain 介绍

1. Overview

In this quick tutorial, we’ll discuss different ways to chain Predicates in Java 8.

2. Basic Example

First, let’s see how to use a simple Predicate to filter a List of names:

@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"));
}

In this example, we filtered our List of names to only leave names that start with “A” using the Predicate:

name -> name.startsWith("A")

But what if we wanted to apply multiple Predicates?

3. Multiple Filters

If we wanted to apply multiple Predicates, one option is to simply chain multiple filters:

@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"));
}

We’ve now updated our example to filter our list by extracting names that start with “A” and have a length that is less than 5.

We used two filters — one for each Predicate.

4. Complex Predicate

Now, instead of using multiple filters, we can use one filter with a complex Predicate:

@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"));
}

This option is more flexible than the first one, as we can use bitwise operations to build the Predicate as complex as we want.

5. Combining Predicates

Next, if we don’t want to build a complex Predicate using bitwise operations, Java 8 Predicate has useful methods that we can use to combine Predicates.

*We’ll combine Predicates using the methods Predicate.and(), Predicate.or(), and Predicate.negate().*

5.1. Predicate.and()

In this example, we’ll define our Predicates explicitly, and then we’ll combine them using 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"));
}

As we can see, the syntax is fairly intuitive, and the method names suggest the type of operation. Using and(), we’ve filtered our List by extracting only names that fulfill both conditions.

5.2. Predicate.or()

We can also use Predicate.or() to combine Predicates.

Let’s extract names start with “J”, as well as names with a length that’s less than 4:

@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()

We can use Predicate.negate() when combining our Predicates as well:

@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"));
}

Here, we’ve used a combination of or() and negate() to filter the List by names that start with “J” or have a length that isn’t less than 4.

5.4. Combine Predicates Inline

We don’t need to explicitly define our Predicates to use and(),  or(), and negate().

We can also use them inline by casting the Predicate:

@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. Combining a Collection of Predicates

Finally, let’s see how to chain a collection of Predicates by reducing them.

In the following example, we have a List of Predicates that we combined using Predicate.and():

@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"));
}

Note that we use our base identity as:

x->true

But that will be different if we want to combine them using Predicate.or():

@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"));
}

7. Conclusion

In this article, we explored different ways to chain Predicates in Java 8, by using filter(), building complex Predicates, and combining Predicates.

The full source code is available over on GitHub.