[collect-into-treeSet] Using Streams to Collect into a TreeSet (#12834)

* [collect-into-treeSet] Using Streams to Collect into a TreeSet

* [collect-into-treeSet] move to core-java-collections-set-2

core-java-modules/core-java-collections-set-2/pom.xml

@@ -0,0 +1,39 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+    <artifactId>core-java-collections-set-2</artifactId>
+    <version>0.0.1-SNAPSHOT</version>
+    <name>core-java-collections-set-2</name>
+    <packaging>jar</packaging>
+
+    <parent>
+        <groupId>com.baeldung.core-java-modules</groupId>
+        <artifactId>core-java-modules</artifactId>
+        <version>0.0.1-SNAPSHOT</version>
+    </parent>
+
+    <dependencies>
+    </dependencies>
+
+    <build>
+        <plugins>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-compiler-plugin</artifactId>
+                <version>${maven-compiler-plugin.version}</version>
+                <configuration>
+                    <source>${maven.compiler.source}</source>
+                    <target>${maven.compiler.target}</target>
+                </configuration>
+            </plugin>
+        </plugins>
+    </build>
+
+    <properties>
+        <maven.compiler.source>8</maven.compiler.source>
+        <maven.compiler.target>8</maven.compiler.target>
+    </properties>
+
+</project>

core-java-modules/core-java-collections-set-2/src/main/java/com/baeldung/treeset/collectintotreeset/Player.java

@@ -0,0 +1,36 @@
+package com.baeldung.treeset.collectintotreeset;
+
+public class Player implements Comparable<Player> {
+    private String name;
+    private int age;
+    private int numberOfPlayed;
+    private int numberOfWins;
+
+    public Player(String name, int age, int numberOfPlayed, int numberOfWins) {
+        this.name = name;
+        this.age = age;
+        this.numberOfPlayed = numberOfPlayed;
+        this.numberOfWins = numberOfWins;
+    }
+
+    public String getName() {
+        return name;
+    }
+
+    public int getAge() {
+        return age;
+    }
+
+    public int getNumberOfPlayed() {
+        return numberOfPlayed;
+    }
+
+    public int getNumberOfWins() {
+        return numberOfWins;
+    }
+
+    @Override
+    public int compareTo(Player o) {
+        return Integer.compare(age, o.age);
+    }
+}

core-java-modules/core-java-collections-set-2/src/test/java/com/baeldung/treeset/collectintotreeset/CollectIntoTreeSetUnitTest.java

@@ -0,0 +1,52 @@
+package com.baeldung.treeset.collectintotreeset;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import java.math.BigDecimal;
+import java.math.RoundingMode;
+import java.util.Comparator;
+import java.util.TreeSet;
+import java.util.stream.Collectors;
+import java.util.stream.Stream;
+
+import org.junit.jupiter.api.Test;
+
+public class CollectIntoTreeSetUnitTest {
+    Player kai = new Player("Kai", 26, 28, 7);
+    Player eric = new Player("Eric", 28, 30, 11);
+    Player saajan = new Player("Saajan", 30, 100, 66);
+    Player kevin = new Player("Kevin", 24, 50, 49);
+
+    @Test
+    void givenAStream_whenCollectIntoTreeSetWithNaturalOrder_thenGetExpectedResult() {
+        String kotlin = "Kotlin";
+        String java = "Java";
+        String python = "Python";
+        String ruby = "Ruby";
+        TreeSet<String> myTreeSet = Stream.of(ruby, java, kotlin, python)
+            .collect(Collectors.toCollection(TreeSet::new));
+        assertThat(myTreeSet).containsExactly(java, kotlin, python, ruby);
+    }
+
+    @Test
+    void givenAPlayerStream_whenCollectIntoTreeSet_thenGetExpectedResult() {
+        TreeSet<Player> myTreeSet = Stream.of(saajan, eric, kai, kevin)
+            .collect(Collectors.toCollection(TreeSet::new));
+        assertThat(myTreeSet).containsExactly(kevin, kai, eric, saajan);
+    }
+
+    @Test
+    void givenAPlayerStream_whenCollectIntoTreeSetWithNoOfWinsComparator_thenGetExpectedResult() {
+        TreeSet<Player> myTreeSet = Stream.of(saajan, eric, kai, kevin)
+            .collect(Collectors.toCollection(() -> new TreeSet<>(Comparator.comparingInt(Player::getNumberOfWins))));
+        assertThat(myTreeSet).containsExactly(kai, eric, kevin, saajan);
+    }
+
+    @Test
+    void givenAPlayerStream_whenCollectIntoTreeSetWithWinRateComparator_thenGetExpectedResult() {
+        TreeSet<Player> myTreeSet = Stream.of(saajan, eric, kai, kevin)
+            .collect(Collectors.toCollection(() -> new TreeSet<>(Comparator.comparing(player -> BigDecimal.valueOf(player.getNumberOfWins())
+                .divide(BigDecimal.valueOf(player.getNumberOfPlayed()), 2, RoundingMode.HALF_UP)))));
+        assertThat(myTreeSet).containsExactly(kai, eric, saajan, kevin);
+    }
+}

core-java-modules/pom.xml

@@ -34,6 +34,7 @@
         <module>core-java-collections-array-list</module>
         <module>core-java-collections-conversions</module>
         <module>core-java-collections-conversions-2</module>
+        <module>core-java-collections-set-2</module>
         <module>core-java-collections-list</module>
         <module>core-java-collections-list-2</module>
         <module>core-java-collections-list-3</module>