The Predicate Functional interface takes a single input and returns a boolean value. The predicate interface is located in java.util.function package. It has a Single Abstract Method (SAM) test(), which accepts the generic object type T and returns a boolean.
Java Predicate Example
Whenever we are creating a Lambda Expression, which takes a single input and returns a boolean value based on a certain condition, then the Predicate can be used as a target for the lambda expression.
Methods in Predicate Interface
- boolean test(T t) – This method takes a single generic argument and returns true or false
- default Predicate<T> and(Predicate<? super T> other) – This is a default method, returns a composed predicate that by performing short-circuiting logical AND of current predicate and another predicate.
- default Predicate<T> or(Predicate<? super T> other) – This is also a default method, returns a composed predicate that by performing short-circuiting logical OR of current predicate and another predicate.
- default Predicate<T> negate() – This also a default method, returns a predicate after performing logical negation(!) on the current predicate.
- static <T> Predicate<T> isEqual(Object targetRef) – This static method returns a predicate which test the equality of the arguments passed.
1. Java Predicate test() method example
If we have a lambda expression that takes a single input and evaluates it based on a condition and returns true or false based on the evaluation, then the Predicate interface is a perfect choice.
Let’s build a predicate which checks whether the number passed is greater than 3 or not.
package com.javainterviewpoint;
import java.util.function.Predicate;
public class NumberChecker
{
public static void main(String[] args)
{
Predicate checker = number -> number > 3;
System.out.println("Is 2 greater than 3 ? " + checker.test(2));
System.out.println("Is 8 greater than 3 ? " + checker.test(8));
}
}
In the above code, we have created a Predicate checker, which checks for the given number is greater than 3 or not.
Predicate checker = number -> number > 3;
We can invoke the checker predicate by passing an integer argument to the test() method.
Output:
Is 2 greater than 3 ? false Is 8 greater than 3 ? true
The Predicate is a perfect candidate for filtering the elements in a collection, the filter() method of the Stream takes a Predicate as its argument.
package com.javainterviewpoint;
import java.util.Arrays;
import java.util.List;
import java.util.function.Predicate;
public class NumberFilter
{
public static void main(String[] args)
{
List<Integer> numbers = Arrays.asList(12, 2, 4, 1, 2, 0, 9, 3, 5);
Predicate checker = number -> number > 3;
numbers.stream().filter(number -> checker.test(number))
.forEach(System.out::println);
}
}
The code snippet filters the numbers which are lesser than 3 and prints the numbers which are greater than 3.
Output:
12 4 9 5
The predicate can be used on custom objects as well. Let’s create a predicate that gives us the name of the Student with mark greater than 50.
Student.java
package com.javainterviewpoint;
public class Student
{
private int id;
private int mark;
private String name;
public Student()
{
super();
}
public Student(int id, int mark, String name)
{
super();
this.id = id;
this.mark = mark;
this.name = name;
}
public int getId()
{
return id;
}
public void setId(int id)
{
this.id = id;
}
public int getMark()
{
return mark;
}
public void setMark(int mark)
{
this.mark = mark;
}
public String getName()
{
return name;
}
public void setName(String name)
{
this.name = name;
}
}
StudentPredicate.java
package com.javainterviewpoint;
import java.util.ArrayList;
import java.util.List;
import java.util.function.Predicate;
public class StudentPredicate
{
public static void main(String[] args)
{
List<Student> studentList = new ArrayList<Student>();
studentList.add(new Student(1, 45, "Alice"));
studentList.add(new Student(2, 65, "Bob"));
studentList.add(new Student(3, 80, "Clair"));
studentList.add(new Student(4, 20, "Dom"));
Predicate markPredicate = mark -> mark > 50;
System.out.println("** Student with marks greater than 50 **");
studentList.stream()
.filter(student -> markPredicate.test(student.getMark()))
.forEach(student -> System.out.println(student.getName()));
}
}
In the above code, we have created a simple predicate which checks whether the mark is greater than 50 or not.
Output:
** Student with marks greater than 50 ** Bob Clair
2. Predicate interface and() method example
Let’s look into the above code where we checked whether the number is greater than 3. Suppose if we want the numbers which are greater than 3 but less than 10, then how do we achieve it?
We can use the and() method to chain the predicates, where one predicates whether the number is greater than 3, and the other predicate checks whether the number is less than 10.
package com.javainterviewpoint;
import java.util.function.Predicate;
public class NumberChecker
{
public static void main(String[] args)
{
Predicate checker1 = number -> number > 3;
Predicate checker2 = checker1.and(number -> number < 10);
System.out.println("Does 1 satisfies the condition ? " + checker2.test(1));
System.out.println("Does 7 satisfies the condition ? " + checker2.test(7));
System.out.println("Does 11 satisfies the condition ? " + checker2.test(11));
}
}
We have used the and() method to chain the checker1 and checker2 predicates; also note that the and() method performs short-circuiting logical AND operation, so only when checker1 is true it checks checker2.
Output:
Does 1 satisfies the condition ? false Does 7 satisfies the condition ? true Does 11 satisfies the condition ? false
3. Predicate interface or() method example
The or() methods also chain two predicates; the only difference is that it performs the short-circuiting logical OR operation.
Let’s chain the above predicate with the or() method and check the results.
package com.javainterviewpoint;
import java.util.function.Predicate;
public class NumberChecker
{
public static void main(String[] args)
{
Predicate checker1 = number -> number > 3;
Predicate checker2 = checker1.or(number -> number < 10);
System.out.println("Does 1 satisfies the condition ? " + checker2.test(1));
System.out.println("Does 7 satisfies the condition ? " + checker2.test(7));
System.out.println("Does 11 satisfies the condition ? " + checker2.test(11));
}
}
Output:
Does 1 satisfies the condition ? true Does 7 satisfies the condition ? true Does 11 satisfies the condition ? true
While using the and() method, the results are false, true, and false, whereas for the or() method, it is true, true, and true.
Since the or() method performs short-circuiting logical OR operation, only if checker1 is false, it checks checker2, and so the below condition will be checked.
- Since is 1 is less than 3, checker2 will be checked, and 1 is less than 10, so we get true.
- 7 is greater than 3, so we get true. checker2 will not be checked
- Similarly, 11 is greater than 3, so we will get true.
4. Predicate functional interface negate() method example
Suppose if we already have a predicate, and we want to perform a NOT operation on it, then we can use the negate() method.
Let’s take the checker1 predicate, which checks whether the number is greater than 3 or not. If we want the numbers less than 3, then we can call the negate() method on the checker1 predicate.
package com.javainterviewpoint;
import java.util.function.Predicate;
public class NumberChecker
{
public static void main(String[] args)
{
Predicate checker1 = number -> number > 3;
System.out.println("*** Checker 1 Predicate without negate**** ");
System.out.println("Does 1 satisfies the condition ? " + checker1.test(1));
System.out.println("Does 2 satisfies the condition ? " + checker1.test(2));
System.out.println("Does 6 satisfies the condition ? " + checker1.test(6));
Predicate checker1Negate = checker1.negate();
System.out.println("*** Checker 1 Predicate with negate**** ");
System.out.println("Does 1 satisfies the condition ? " + checker1Negate.test(1));
System.out.println("Does 2 satisfies the condition ? " + checker1Negate.test(2));
System.out.println("Does 6 satisfies the condition ? " + checker1Negate.test(6));
}
}
Output:
*** Checker 1 Predicate without negate**** Does 1 satisfies the condition ? false Does 2 satisfies the condition ? false Does 6 satisfies the condition ? true *** Checker 1 Predicate with negate**** Does 1 satisfies the condition ? true Does 2 satisfies the condition ? true Does 6 satisfies the condition ? false
We can see that the results get inversed when we negate.
5. Java Predicate isEqual() method example
The isEqual() method is a static method that checks whether the two arguments are equal or not.
package com.javainterviewpoint;
import java.util.function.Predicate;
public class IsEqualPredicate
{
public static void main(String[] args)
{
Predicate helloPredicate = Predicate.isEqual("Hello");
System.out.println(helloPredicate.test("Hello"));
System.out.println(helloPredicate.test("Morning"));
}
}
Output:
true false
Happy Learning!!
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