StreamEx tutorial (#2685)

libraries/src/main/java/com/baeldung/streamex/Role.java

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+package com.baeldung.streamex;
+
+public class Role {
+
+}

libraries/src/main/java/com/baeldung/streamex/StreamEX.java

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+package com.baeldung.streamex;
+
+import java.util.Arrays;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Objects;
+import java.util.Set;
+import java.util.stream.Collectors;
+import one.util.streamex.DoubleStreamEx;
+import one.util.streamex.EntryStream;
+import one.util.streamex.IntStreamEx;
+import one.util.streamex.StreamEx;
+
+public class StreamEX {
+
+    public static void main(String[] args) {
+        //Collector shortcut methods (toList, toSet, groupingBy, joining, etc.)
+        List<User> users = Arrays.asList(
+                new User("name"), new User(), new User());
+        users.stream()
+                .map(User::getName)
+                .collect(Collectors.toList());
+        List<String> userNames = StreamEx.of(users)
+                .map(User::getName)
+                .toList();
+        Map<Role, List<User>> role2users = StreamEx.of(users)
+                .groupingBy(User::getRole);
+        StreamEx.of(1, 2, 3).joining("; "); // "1; 2; 3"    
+        //Selecting stream elements of specific type
+        List usersAndRoles = Arrays.asList(new User(), new Role());
+        List<Role> roles = IntStreamEx.range(usersAndRoles.size())
+                .mapToObj(usersAndRoles::get)
+                .select(Role.class)
+                .toList();
+        System.out.println(roles);
+        //adding elements to Stream
+        List<String> appendedUsers = StreamEx.of(users)
+                .map(User::getName)
+                .prepend("(none)")
+                .append("LAST")
+                .toList();
+        System.out.println(appendedUsers);
+        //Removing unwanted elements and using the stream as Iterable:
+        for (String line : StreamEx.of(users).map(User::getName)
+                .nonNull()) {
+            System.out.println(line);
+        }
+        //Selecting map keys by value predicate:
+        Map<String, Role> nameToRole = new HashMap<>();
+        nameToRole.put("first", new Role());
+        nameToRole.put("second", null);
+        Set<String> nonNullRoles = StreamEx.
+                ofKeys(nameToRole, Objects::nonNull)
+                .toSet();
+        System.out.println(nonNullRoles);
+        //Operating on key-value pairs:
+        Map<User, List<Role>> users2roles = transformMap(role2users);
+        Map<String, String> mapToString = EntryStream.of(users2roles)
+                .mapKeys(String::valueOf)
+                .mapValues(String::valueOf)
+                .toMap();
+        //Support of byte/char/short/float types:
+        short[] src = {1, 2, 3};
+        char[] output = IntStreamEx.of(src)
+                .map(x -> x * 5)
+                .toCharArray();
+    }
+
+    public double[] getDiffBetweenPairs(double... numbers) {
+        return DoubleStreamEx.of(numbers)
+                .pairMap((a, b) -> b - a).toArray();
+    }
+
+    public static Map<User, List<Role>> transformMap(
+            Map<Role, List<User>> role2users) {
+        Map<User, List<Role>> users2roles = EntryStream.of(role2users)
+                .flatMapValues(List::stream)
+                .invert()
+                .grouping();
+        return users2roles;
+    }
+
+}

libraries/src/main/java/com/baeldung/streamex/User.java

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+package com.baeldung.streamex;
+
+public class User {
+
+    int id;
+    String name;
+    Role role = new Role();
+
+    public User() {
+    }
+
+    public User(String name) {
+        this.name = name;
+    }
+
+    public int getId() {
+        return id;
+    }
+
+    public void setId(int id) {
+        this.id = id;
+    }
+
+    public String getName() {
+        return name;
+    }
+
+    public void setName(String name) {
+        this.name = name;
+    }
+
+    public Role getRole() {
+        return role;
+    }
+
+    public void setRole(Role role) {
+        this.role = role;
+    }
+
+}