In Java 8, Predicate is a functional interface, which accepts an argument and returns a boolean. Usually, it used to apply in a filter for a collection of objects.
@FunctionalInterface
public interface Predicate<T> {
boolean test(T t);
}
Further Reading
Java 8 BiPredicate Examples
1. Predicate in filter()
filter() accepts predicate as argument.
Java8Predicate.java
package com.mkyong.java8;
import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;
public class Java8Predicate {
public static void main(String[] args) {
List<Integer> list = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);
List<Integer> collect = list.stream().filter(x -> x > 5).collect(Collectors.toList());
System.out.println(collect); // [6, 7, 8, 9, 10]
}
}
Output
[6, 7, 8, 9, 10]
Java8Predicate.java
package com.mkyong.java8;
import java.util.Arrays;
import java.util.List;
import java.util.function.Predicate;
import java.util.stream.Collectors;
public class Java8Predicate {
public static void main(String[] args) {
Predicate<Integer> noGreaterThan5 = x -> x > 5;
List<Integer> list = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);
List<Integer> collect = list.stream()
.filter(noGreaterThan5)
.collect(Collectors.toList());
System.out.println(collect); // [6, 7, 8, 9, 10]
}
}
Output
[6, 7, 8, 9, 10]
2. Predicate.and()
2.1 Multiple filters.
Java8Predicate2.java
package com.mkyong.java8;
import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;
public class Java8Predicate2 {
public static void main(String[] args) {
List<Integer> list = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);
// multiple filters
List<Integer> collect = list.stream()
.filter(x -> x > 5 && x < 8).collect(Collectors.toList());
System.out.println(collect);
}
}
Output
[6, 7]
2.1 Replace with Predicate.and()
Java8Predicate2.java
package com.mkyong.java8;
import java.util.Arrays;
import java.util.List;
import java.util.function.Predicate;
import java.util.stream.Collectors;
public class Java8Predicate2 {
public static void main(String[] args) {
Predicate<Integer> noGreaterThan5 = x -> x > 5;
Predicate<Integer> noLessThan8 = x -> x < 8;
List<Integer> list = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);
List<Integer> collect = list.stream()
.filter(noGreaterThan5.and(noLessThan8))
.collect(Collectors.toList());
System.out.println(collect);
}
}
Output
[6, 7]
3. Predicate.or()
Java8Predicate3.java
package com.mkyong.java8;
import java.util.Arrays;
import java.util.List;
import java.util.function.Predicate;
import java.util.stream.Collectors;
public class Java8Predicate3 {
public static void main(String[] args) {
Predicate<String> lengthIs3 = x -> x.length() == 3;
Predicate<String> startWithA = x -> x.startsWith("A");
List<String> list = Arrays.asList("A", "AA", "AAA", "B", "BB", "BBB");
List<String> collect = list.stream()
.filter(lengthIs3.or(startWithA))
.collect(Collectors.toList());
System.out.println(collect);
}
}
Output
[A, AA, AAA, BBB]
4. Predicate.negate()
Find all elements not start with ‘A’.
Java8Predicate4.java
package com.mkyong.java8;
import java.util.Arrays;
import java.util.List;
import java.util.function.Predicate;
import java.util.stream.Collectors;
public class Java8Predicate4 {
public static void main(String[] args) {
Predicate<String> startWithA = x -> x.startsWith("A");
List<String> list = Arrays.asList("A", "AA", "AAA", "B", "BB", "BBB");
List<String> collect = list.stream()
.filter(startWithA.negate())
.collect(Collectors.toList());
System.out.println(collect);
}
}
Output
[B, BB, BBB]
5. Predicate.test() in function
Predicate in function.
Java8Predicate5.java
package com.mkyong.java8;
import java.util.Arrays;
import java.util.List;
import java.util.function.Predicate;
import java.util.stream.Collectors;
public class Java8Predicate5 {
public static void main(String[] args) {
List<String> list = Arrays.asList("A", "AA", "AAA", "B", "BB", "BBB");
System.out.println(StringProcessor.filter(
list, x -> x.startsWith("A"))); // [A, AA, AAA]
System.out.println(StringProcessor.filter(
list, x -> x.startsWith("A") && x.length() == 3)); // [AAA]
}
}
class StringProcessor {
static List<String> filter(List<String> list, Predicate<String> predicate) {
return list.stream().filter(predicate::test).collect(Collectors.toList());
}
}
Output
[A, AA, AAA]
[AAA]
6. Predicate Chaining
We can chain predicates together.
Java8Predicate6.java
package com.mkyong.java8;
import java.util.function.Predicate;
public class Java8Predicate6 {
public static void main(String[] args) {
Predicate<String> startWithA = x -> x.startsWith("a");
// start with "a" or "m"
boolean result = startWithA.or(x -> x.startsWith("m")).test("mkyong");
System.out.println(result); // true
// !(start with "a" and length is 3)
boolean result2 = startWithA.and(x -> x.length() == 3).negate().test("abc");
System.out.println(result2); // false
}
}
Output
true
false
7. Predicate in Object
Hosting.java
package com.mkyong.java8;
public class Hosting {
private int Id;
private String name;
private String url;
public Hosting(int id, String name, String url) {
Id = id;
this.name = name;
this.url = url;
}
//... getters and setters, toString()
}
HostingRespository.java
package com.mkyong.java8;
import java.util.List;
import java.util.function.Predicate;
import java.util.stream.Collectors;
public class HostingRespository {
public static List<Hosting> filterHosting(List<Hosting> hosting,
Predicate<Hosting> predicate) {
return hosting.stream()
.filter(predicate)
.collect(Collectors.toList());
}
}
Java8Predicate7.java
package com.mkyong.java8;
import java.util.Arrays;
import java.util.List;
import java.util.function.Predicate;
public class Java8Predicate7 {
public static void main(String[] args) {
Hosting h1 = new Hosting(1, "amazon", "aws.amazon.com");
Hosting h2 = new Hosting(2, "linode", "linode.com");
Hosting h3 = new Hosting(3, "liquidweb", "liquidweb.com");
Hosting h4 = new Hosting(4, "google", "google.com");
List<Hosting> list = Arrays.asList(new Hosting[]{h1, h2, h3, h4});
List<Hosting> result = HostingRespository.filterHosting(list, x -> x.getName().startsWith("g"));
System.out.println("result : " + result); // google
List<Hosting> result2 = HostingRespository.filterHosting(list, isDeveloperFriendly());
System.out.println("result2 : " + result2); // linode
}
public static Predicate<Hosting> isDeveloperFriendly() {
return n -> n.getName().equals("linode");
}
}
Output
result : [Hosting{Id=4, name='google', url='google.com'}]
result2 : [Hosting{Id=2, name='linode', url='linode.com'}]
Done.
Thanks!
Great post.
Thank you so much, really usefull.
greaterThan5 instead of noGreaterThan5, variable name does not match implementation.
for readability isGreaterThan5, isLessThan8 is better
otherwise great thanks
Hi mkyong.
Can we create a predicate that checks the strings that starts with a prefix(this is going to be dynamic, it comes as input from user). So the predicate should check for the strings that starts with the prefix provided ie we need the prefix also as a extra parameter in predicate. Is it possible? If yes can you please give me code of it?
Thanks in advance.
Satya
superb no words to say. But it is in pdf form it will very useful for further
Just what I was looking for, thanks.
Thank you so much for the explanation!
I was a old schooler , I read your article a while ago but for the first time I had used it in real time, the code looked clean and simple with these predicates.Your examples helped me understand it with ease. I feel guilty if I don’t ack you on this. Appreciate your efforts.
Thanks for your kind words; it means a lot to me.
Hi Mkyong,
I’ve gotten so much out of your post over the years. Thank you so much!
Awesome, you eased my struggle in understanding conditions in lambda expressions.