BAEL-2400: Adds examples for currying

patterns/design-patterns/README.md

@@ -19,3 +19,4 @@
 - [The Command Pattern in Java](http://www.baeldung.com/java-command-pattern)
 - [Java Constructors vs Static Factory Methods](https://www.baeldung.com/java-constructors-vs-static-factory-methods)
 - [The Adapter Pattern in Java](https://www.baeldung.com/java-adapter-pattern)
+- [Currying in Java](https://baeldung.com/currying-in-java)

patterns/design-patterns/src/main/java/com/baeldung/currying/Letter.java

@@ -0,0 +1,110 @@
+package com.baeldung.currying;
+
+import java.time.LocalDate;
+import java.util.Objects;
+import java.util.function.BiFunction;
+import java.util.function.Function;
+
+public class Letter {
+    private String returningAddress;
+    private String insideAddress;
+    private LocalDate dateOfLetter;
+    private String salutation;
+    private String body;
+    private String closing;
+
+    Letter(String returningAddress, String insideAddress, LocalDate dateOfLetter, String salutation, String body, String closing) {
+        this.returningAddress = returningAddress;
+        this.insideAddress = insideAddress;
+        this.dateOfLetter = dateOfLetter;
+        this.salutation = salutation;
+        this.body = body;
+        this.closing = closing;
+    }
+
+    Letter(String salutation, String body) {
+        this(null, null, LocalDate.now(), salutation, body, null);
+    }
+
+    @Override
+    public boolean equals(Object o) {
+        if (this == o) {
+            return true;
+        }
+        if (o == null || getClass() != o.getClass()) {
+            return false;
+        }
+        Letter letter = (Letter) o;
+        return Objects.equals(returningAddress, letter.returningAddress) &&
+          Objects.equals(insideAddress, letter.insideAddress) &&
+          Objects.equals(dateOfLetter, letter.dateOfLetter) &&
+          Objects.equals(salutation, letter.salutation) &&
+          Objects.equals(body, letter.body) &&
+          Objects.equals(closing, letter.closing);
+    }
+
+    @Override
+    public int hashCode() {
+        return Objects.hash(returningAddress, insideAddress, dateOfLetter, salutation, body, closing);
+    }
+
+    @Override
+    public String toString() {
+        return "Letter{" + "returningAddress='" + returningAddress + '\'' + ", insideAddress='" + insideAddress + '\''
+            + ", dateOfLetter=" + dateOfLetter + ", salutation='" + salutation + '\'' + ", body='" + body + '\''
+            + ", closing='" + closing + '\'' + '}';
+    }
+
+    static Letter createLetter(String salutation, String body) {
+        return new Letter(salutation, body);
+    }
+
+    static BiFunction<String, String, Letter> SIMPLE_LETTER_CREATOR = //
+        (salutation, body) -> new Letter(salutation, body);
+
+    static Function<String, Function<String, Letter>> SIMPLE_CURRIED_LETTER_CREATOR = //
+        saluation -> body -> new Letter(saluation, body);
+
+    static Function<String, Function<String, Function<LocalDate, Function<String, Function<String, Function<String, Letter>>>>>> LETTER_CREATOR = //
+      returnAddress
+          -> closing
+          -> dateOfLetter
+          -> insideAddress
+          -> salutation
+          -> body
+          -> new Letter(returnAddress, insideAddress, dateOfLetter, salutation, body, closing);
+
+    static AddReturnAddress builder() {
+        return returnAddress
+            -> closing
+            -> dateOfLetter
+            -> insideAddress
+            -> salutation
+            -> body
+            -> new Letter(returnAddress, insideAddress, dateOfLetter, salutation, body, closing);
+    }
+
+    interface AddReturnAddress {
+        Letter.AddClosing withReturnAddress(String returnAddress);
+    }
+
+    interface AddClosing {
+        Letter.AddDateOfLetter withClosing(String closing);
+    }
+
+    interface AddDateOfLetter {
+        Letter.AddInsideAddress withDateOfLetter(LocalDate dateOfLetter);
+    }
+
+    interface AddInsideAddress {
+        Letter.AddSalutation withInsideAddress(String insideAddress);
+    }
+
+    interface AddSalutation {
+        Letter.AddBody withSalutation(String salutation);
+    }
+
+    interface AddBody {
+        Letter withBody(String body);
+    }
+}

patterns/design-patterns/src/test/java/com/baeldung/currying/LetterUnitTest.java

@@ -0,0 +1,58 @@
+package com.baeldung.currying;
+
+import org.junit.Test;
+
+import java.time.LocalDate;
+
+import static com.baeldung.currying.Letter.LETTER_CREATOR;
+import static com.baeldung.currying.Letter.SIMPLE_CURRIED_LETTER_CREATOR;
+import static com.baeldung.currying.Letter.SIMPLE_LETTER_CREATOR;
+import static org.junit.Assert.assertEquals;
+
+public class LetterUnitTest {
+    private static final String BODY = "BODY";
+    private static final String SALUTATION = "SALUTATION";
+    private static final String RETURNING_ADDRESS = "RETURNING ADDRESS";
+    private static final String INSIDE_ADDRESS = "INSIDE ADDRESS";
+    private static final LocalDate DATE_OF_LETTER = LocalDate.of(2013, 3, 1);
+    private static final String CLOSING = "CLOSING";
+    private static final Letter SIMPLE_LETTER = new Letter(SALUTATION, BODY);
+    private static final Letter LETTER = new Letter(RETURNING_ADDRESS, INSIDE_ADDRESS, DATE_OF_LETTER, SALUTATION, BODY, CLOSING);
+
+    @Test
+    public void whenStaticCreateMethodIsCalled_thenaSimpleLetterIsCreated() {
+        assertEquals(SIMPLE_LETTER, Letter.createLetter(SALUTATION, BODY));
+    }
+
+    @Test
+    public void whenStaticBiFunctionIsCalled_thenaSimpleLetterIsCreated() {
+        assertEquals(SIMPLE_LETTER, SIMPLE_LETTER_CREATOR.apply(SALUTATION, BODY));
+    }
+
+    @Test
+    public void whenStaticSimpleCurriedFunctionIsCalled_thenaSimpleLetterIsCreated() {
+        assertEquals(SIMPLE_LETTER, SIMPLE_CURRIED_LETTER_CREATOR.apply(SALUTATION).apply(BODY));
+    }
+
+    @Test
+    public void whenStaticCurriedFunctionIsCalled_thenaLetterIsCreated() {
+        assertEquals(LETTER, LETTER_CREATOR
+            .apply(RETURNING_ADDRESS)
+            .apply(CLOSING)
+            .apply(DATE_OF_LETTER)
+            .apply(INSIDE_ADDRESS)
+            .apply(SALUTATION)
+            .apply(BODY));
+    }
+
+    @Test
+    public void whenStaticBuilderIsCalled_thenaLetterIsCreated() {
+        assertEquals(LETTER, Letter.builder()
+            .withReturnAddress(RETURNING_ADDRESS)
+            .withClosing(CLOSING)
+            .withDateOfLetter(DATE_OF_LETTER)
+            .withInsideAddress(INSIDE_ADDRESS)
+            .withSalutation(SALUTATION)
+            .withBody(BODY));
+    }
+}