In this tutorial we will discuss Java 8 Predicate on how to use it Java with examples.
1. What is Java Predicate?
A java Predicate is a functional interface whose functional method is test(Object), that takes an argument and returns boolean value. This is a functional interface and can therefore be used as the assignment target for a lambda expression or method reference.
Simple use case of predicate can be identifying the given number is even or odd, let’s have a look into example without Predicate and with Predicate.
2. Example without Predicate
public class NoPredicateTest {
boolean test(int number) {
if(number % 2 == 0)
return true;
return false;
}
public static void main(String[] args) {
NoPredicateTest nptest = new NoPredicateTest();
System.out.println(nptest.test(10));//true
System.out.println(nptest.test(11));//false
}
}
Output :
true false
3. Examples with Java Predicate
public class PredicateTest {
public static void main(String[] args) {
Predicate<Integer> p = number -> (number % 2 == 0);
System.out.println(p.test(10));//true
System.out.println(p.test(11));//false
}
}
Output :
true false
Following example shows you filter Stream elements using Predicate and iterate list items using forEach method.. Stream filter method takes predicate argument as input.
public class PredicateStreamFilterTest {
public static void main(String[] args) {
List<String> names = Arrays.asList(
"Peter",
"Martin",
"Alex",
"Philip",
"Piyush",
"Mike"
);
Predicate<String> p = name -> name.startsWith("P");
names.stream()
.filter(p)
.forEach(System.out::println);
}
}
Output :
Peter Philip Piyush
4. Java Predicate methods
| Modifier and Type | Method and Description |
|---|---|
default Predicate | and(Predicate<? super T> other) – Returns a composed predicate that represents a short-circuiting logical AND of this predicate and another. |
static Predicate | isEqual(Object targetRef) – Returns a predicate that tests if two arguments are equal according to Objects.equals(Object, Object). |
default Predicate | negate() – Returns a predicate that represents the logical negation of this predicate. |
default Predicate | or(Predicate<? super T> other) – Returns a composed predicate that represents a short-circuiting logical OR of this predicate and another. |
boolean | test(T t) – Evaluates this predicate on the given argument. |
4.1. Predicate.and example
Following example demonstrates that, return true if given number divisible by 2 and greater than 10 using Predicate and also prints names from list if name starts with “P” and does not end with “p”.
public class PredicateAndTest {
public static void main(String[] args) {
Predicate<Integer> p1 = number -> (number % 2 == 0);
Predicate<Integer> p2 = number -> (number > 10);
// number should be divisible by 2 and greater than 10
System.out.println(p1.and(p2).test(10));//false
System.out.println(p1.and(p2).test(12));//true
List<String> names = Arrays.asList(
"Peter",
"Martin",
"Alex",
"Philip",
"Piyush",
"Mike"
);
Predicate<String> p3 = name -> name.startsWith("P");
Predicate<String> p4 = name -> !name.endsWith("p");
// find a name starts with "P" and not ends with "p"
names.stream()
.filter(p3.and(p4))
.forEach(System.out::println);
}
}
Output :
false true Peter Piyush
4.2. Predicate.isEqual example
Following example demonstrates that how predicate isEqual(Object) method works.
public class PredicateIsEqualTest {
public static void main(String[] args) {
System.out.println(Predicate.isEqual(10).test(12)); // false
System.out.println(Predicate.isEqual(10).test(10)); // true
List<String> names = Arrays.asList(
"Peter",
"Martin",
"Alex",
"Philip",
"Piyush",
"Mike"
);
String MATCH = "Mike";
// find a name that is equals "Mike"
names.stream()
.filter(Predicate.isEqual(MATCH))
.forEach(System.out::println);
}
}
Output :
false true Mike
4.3. Predicate.negate example
Following example demonstrates how Predicate negate() method works.
public class PredicateNegateTest {
public static void main(String[] args) {
Predicate<Integer> p1 = number -> (number % 2 == 0);
// number should not divisible by 2
System.out.println(p1.negate().test(10));//false
System.out.println(p1.negate().test(13));//true
List<String> names = Arrays.asList(
"Peter",
"Martin",
"Alex",
"Philip",
"Piyush",
"Mike"
);
Predicate<String> p2 = name -> name.startsWith("P");
// find a name not starts with "P"
names.stream()
.filter(p2.negate())
.forEach(System.out::println);
}
}
Output :
false true Martin Alex Mike
4.4. Predicate.or example
Following example shows you how java Predicate or method works.
public class PredicateOrTest {
public static void main(String[] args) {
Predicate<Integer> p1 = number -> (number % 2 == 0);
Predicate<Integer> p2 = number -> (number > 10);
// number should be divisible by 2 or greater than 10
System.out.println(p1.or(p2).test(11)); // true
System.out.println(p1.or(p2).test(12)); // true
List<String> names = Arrays.asList(
"Peter",
"Martin",
"Alex",
"Philip",
"Piyush",
"Mike"
);
Predicate<String> p3 = name -> name.startsWith("A");
Predicate<String> p4 = name -> name.endsWith("p");
// find a name starts with "A" or not ends with "p"
names.stream()
.filter(p3.or(p4))
.forEach(System.out::println);
}
}
Output :
Rtrue true Alex Philip
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