Simple junit to demonstrate Predicates.

Apple.java

package de.kopis.java8;

public class Apple {
  private String color;
  private double weight;

  public Apple(String color, double weight) {
    this.color = color;
    this.weight = weight;
  }

  public String getColor() {
    return color;
  }

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

  public double getWeight() {
    return weight;
  }

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

SortAppleAnonymousClassTest.java

package de.kopis.java8;

import static org.junit.Assert.assertEquals;

import java.util.ArrayList;
import java.util.List;

import org.junit.Before;
import org.junit.Test;

public class SortApplesTest {

  private List<Apple> unsorted;

  @Before
  public void createApples() {
    unsorted = new ArrayList<>();
    for (int i = 1; i < 10; i++) {
      unsorted.add(new Apple("red", 1.23));
    }
    unsorted.add(new Apple("green", 2.34));
  }

  @Test
  public void canSortApplesByColor() {
    List<Apple> green = filterApples(unsorted, new ApplePredicate() {
      @Override
      public boolean test(Apple apple) {
        return apple.getColor().equals("green");
      }
    });
    assertEquals(1, green.size());
  }

  @Test
  public void canSortApplesByWeight() {
    List<Apple> green = filterApples(unsorted, new ApplePredicate() {
      @Override
      public boolean test(Apple apple) {
        return apple.getWeight() > 2.0;
      }
    });
    assertEquals(1, green.size());
  }

  private List<Apple> filterApples(List<Apple> unsorted, ApplePredicate colorPredicate) {
    List<Apple> sorted = new ArrayList<>();
    for (Apple apple : unsorted) {
      if (colorPredicate.test(apple)) {
        sorted.add(apple);
      }
    }
    return sorted;
  }
}

interface ApplePredicate {
  public boolean test(Apple apple);
}

SortAppleLambdaTest.java

package de.kopis.java8;

import static org.junit.Assert.assertEquals;

import java.util.ArrayList;
import java.util.List;

import org.junit.Before;
import org.junit.Test;

public class SortApplesTest {

  private List<Apple> unsorted;

  @Before
  public void createApples() {
    unsorted = new ArrayList<>();
    for (int i = 1; i < 10; i++) {
      unsorted.add(new Apple("red", 1.23));
    }
    unsorted.add(new Apple("green", 2.34));
  }

  @Test
  public void canSortApplesByColor() {
    List<Apple> green = filter(unsorted, (Apple apple) -> apple.getColor().equals("green"));
    assertEquals(1, green.size());
  }

  @Test
  public void canSortApplesByWeight() {
    List<Apple> green = filter(unsorted, (Apple apple) -> apple.getWeight() > 2.0);
    assertEquals(1, green.size());
  }

  private <T> List<T> filter(List<T> unsorted, Predicate<T> predicate) {
    List<T> sorted = new ArrayList<>();
    for (T element : unsorted) {
      if (predicate.test(element)) {
        sorted.add(element);
      }
    }
    return sorted;
  }
}

interface Predicate<T> {
  boolean test(T t);
}

SortAppleTest.java

package de.kopis.java8;

import static org.junit.Assert.assertEquals;

import java.util.ArrayList;
import java.util.List;

import org.junit.Before;
import org.junit.Test;

public class SortApplesTest {

  private List<Apple> unsorted;

  @Before
  public void createApples() {
    unsorted = new ArrayList<>();
    for (int i = 1; i < 10; i++) {
      unsorted.add(new Apple("red", 1.23));
    }
    unsorted.add(new Apple("green", 2.34));
  }

  @Test
  public void canSortApplesByColor() {
    List<Apple> green = filterApples(unsorted, new AppleColorPredicate("green"));
    assertEquals(1, green.size());
  }

  @Test
  public void canSortApplesByWeight() {
    List<Apple> green = filterApples(unsorted, new AppleMinimumWeightPredicate(2.0));
    assertEquals(1, green.size());
  }

  private List<Apple> filterApples(List<Apple> unsorted, ApplePredicate colorPredicate) {
    List<Apple> sorted = new ArrayList<>();
    for (Apple apple : unsorted) {
      if (colorPredicate.test(apple)) {
        sorted.add(apple);
      }
    }
    return sorted;
  }
}

interface ApplePredicate {
  public boolean test(Apple apple);
}

class AppleColorPredicate implements ApplePredicate {
  private final String color;

  public AppleColorPredicate(String color) {
    this.color = color;
  }

  public String getColor() {
    return color;
  }

  @Override
  public boolean test(Apple apple) {
    return apple.getColor().equals(color);
  }

}

class AppleMinimumWeightPredicate implements ApplePredicate {

  private final double weight;

  public double getWeight() {
    return weight;
  }

  public AppleMinimumWeightPredicate(double weight) {
    super();
    this.weight = weight;
  }

  @Override
  public boolean test(Apple apple) {
    return apple.getWeight() > weight;
  }

}