Java Lambda - Predicate example

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Predicate represents a predicate, which is boolean-valued function, of one argument.

Method

1. Predicate test
2. Predicate and
3. Predicate negate
4. Predicate or
5. Predicate isEqual

Example

The following example shows how to use Predicate.

import java.util.function.Predicate;
/*from   ww  w.  j  av  a  2  s.  c om*/
public class Main {

  public static void main(String[] args) {
    Predicate<String> i  = (s)-> s.length() > 5;
   
    System.out.println(i.test("java2s.com "));
  }
}

The code above generates the following result.

Example 2

The following code shows how to create Predicate from method reference and lambda.

import java.util.*;
import java.util.function.Predicate;
//w  w  w  . j a v a 2  s  .  c om
public class Main {

  public static void main(String... args) {

    List<Box> inventory = Arrays.asList(new Box(80, "green"), new Box(
        155, "green"), new Box(120, "red"));

    List<Box> greenApples = filter(inventory,Main::isGreenApple);
    System.out.println(greenApples);

    List<Box> heavyApples = filter(inventory,Main::isHeavyApple);
    System.out.println(heavyApples);

    List<Box> greenApples2 = filter(inventory,(Box a) -> "green".equals(a.getColor()));
    System.out.println(greenApples2);

    List<Box> heavyApples2 = filter(inventory,(Box a) -> a.getWeight() > 150);
    System.out.println(heavyApples2);

    List<Box> weirdApples = filter(inventory,(Box a) -> a.getWeight() < 80 || "brown".equals(a.getColor()));
    System.out.println(weirdApples);
  }


  public static boolean isGreenApple(Box apple) {
    return "green".equals(apple.getColor());
  }

  public static boolean isHeavyApple(Box apple) {
    return apple.getWeight() > 150;
  }

  public static List<Box> filter(List<Box> inventory,
      Predicate<Box> p) {
    List<Box> result = new ArrayList<>();
    for (Box apple : inventory) {
      if (p.test(apple)) {
        result.add(apple);
      }
    }
    return result;
  }

}

class Box {
  private int weight = 0;
  private String color = "";

  public Box(int weight, String color) {
    this.weight = weight;
    this.color = color;
  }

  public Integer getWeight() {
    return weight;
  }

  public void setWeight(Integer weight) {
    this.weight = weight;
  }

  public String getColor() {
    return color;
  }

  public void setColor(String color) {
    this.color = color;
  }

  public String toString() {
    return "Apple{" + "color='" + color + '\'' + ", weight=" + weight + '}';
  }
}

The code above generates the following result.

Example 3

The following code shows how to return Predicate.

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.function.Function;
import java.util.function.Predicate;
//www . ja va  2  s .c om
public class Main {
   public static void main(String[] args) {
      List<Student> employees = Arrays.asList(
            new Student(1, 3, "John"),
            new Student(2, 3, "Jane"),
            new Student(3, 4, "Jack")
      );

      // with predicate
      System.out.println(findStudents(employees, createCustomPredicateWith(10_000)));

      // with function definition, both are same
      Function<Double, Predicate<Student>> customFunction = threshold -> (e -> e.gpa > threshold);
      System.out.println(findStudents(employees, customFunction.apply(10_000D)));
   }

   private static Predicate<Student> createCustomPredicateWith(double threshold) {
      return e -> e.gpa > threshold;
   }

   private static List<Student> findStudents(List<Student> employees, Predicate<Student> condition) {
      List<Student> result = new ArrayList<>();

      for (Student e : employees) {
         if (condition.test(e)) {
            result.add(e);
         }
      }

      return result;
   }

}
class Student {
      public int id;
      public long gpa;
      public String name;

      Student(int id, long g, String name) {
         this.id = id;
         this.gpa = g;
         this.name = name;
      }

      @Override
      public String toString() {
         return id + ">" + name + ": " + gpa;
      }
   }

The code above generates the following result.

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