[BAEL-3210] - Java Copy Constructor (#7706)

* [BAEL-3210] - Copy constructors in Java

* [BAEL-3210] - Copy constructors in Java

* Change indentation space based on code style instructions.

core-java-modules/core-java-lang-oop-2/src/main/java/com/baeldung/copyconstructor/Employee.java

@@ -0,0 +1,30 @@
+package com.baeldung.copyconstructor;
+
+import java.util.Date;
+
+public class Employee {
+
+    protected int id;
+    protected String name;
+    protected Date startDate;
+
+    public Employee(int id, String name, Date startDate) {
+        this.id = id;
+        this.name = name;
+        this.startDate = startDate;
+    }
+
+    public Employee(Employee employee) {
+        this.id = employee.id;
+        this.name = employee.name;
+        this.startDate = new Date(employee.startDate.getTime());
+    }
+
+    Date getStartDate() {
+        return startDate;
+    }
+
+    public Employee copy() {
+        return new Employee(this);
+    }
+}

core-java-modules/core-java-lang-oop-2/src/main/java/com/baeldung/copyconstructor/Manager.java

@@ -0,0 +1,31 @@
+package com.baeldung.copyconstructor;
+
+import java.util.Date;
+import java.util.List;
+import java.util.stream.Collectors;
+
+public class Manager extends Employee {
+
+    private List<Employee> directReports;
+
+    public Manager(int id, String name, Date startDate, List<Employee> directReports) {
+        super(id, name, startDate);
+        this.directReports = directReports;
+    }
+
+    public Manager(Manager manager) {
+        super(manager.id, manager.name, manager.startDate);
+        this.directReports = manager.directReports.stream()
+          .collect(Collectors.toList());
+    }
+
+    @Override
+    public Employee copy() {
+        return new Manager(this);
+    }
+
+    List<Employee> getDirectReport() {
+        return this.directReports;
+    }
+
+}

core-java-modules/core-java-lang-oop-2/src/test/java/com/baeldung/copyconstructor/EmployeeUnitTest.java

@@ -0,0 +1,36 @@
+package com.baeldung.copyconstructor;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertNotEquals;
+
+import java.util.Date;
+
+import org.junit.Test;
+
+public class EmployeeUnitTest {
+    @Test
+    public void givenCopyConstructor_whenDeepCopy_thenDistinct() {
+        Date d1 = new Date(123);
+        Employee e1 = new Employee(1, "Baeldung", d1);
+        Employee e2 = new Employee(e1);
+        assertEquals(d1, e1.getStartDate());
+        assertEquals(d1, e2.getStartDate());
+
+        d1.setTime(456);
+        assertEquals(d1, e1.getStartDate());
+        assertNotEquals(d1, e2.getStartDate());
+    }
+
+    @Test
+    public void givenCopyMethod_whenCopy_thenDistinct() {
+        Date d1 = new Date(123);
+        Employee e1 = new Employee(1, "Baeldung", d1);
+        Employee e2 = e1.copy();
+        assertEquals(d1, e1.getStartDate());
+        assertEquals(d1, e2.getStartDate());
+
+        d1.setTime(456);
+        assertEquals(d1, e1.getStartDate());
+        assertNotEquals(d1, e2.getStartDate());
+    }
+}

core-java-modules/core-java-lang-oop-2/src/test/java/com/baeldung/copyconstructor/ManagerUnitTest.java

@@ -0,0 +1,67 @@
+package com.baeldung.copyconstructor;
+
+import static org.junit.Assert.assertArrayEquals;
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertTrue;
+
+import java.util.ArrayList;
+import java.util.Date;
+import java.util.List;
+
+import org.junit.Test;
+
+public class ManagerUnitTest {
+    @Test
+    public void givenCopyConstructor_whenDeepCopy_thenDistinct() {
+        Date startDate = new Date(123);
+        Employee e1 = new Employee(1, "Baeldung", startDate);
+        Employee e2 = new Employee(e1);
+        List<Employee> directReports = new ArrayList<Employee>();
+        directReports.add(e1);
+        directReports.add(e2);
+
+        Manager m1 = new Manager(1, "Baeldung Manager", startDate, directReports);
+        Manager m2 = new Manager(m1);
+        List<Employee> directReports1 = m1.getDirectReport();
+        List<Employee> directReports2 = m2.getDirectReport();
+        assertEquals(directReports1.size(), directReports2.size());
+        assertArrayEquals(directReports1.toArray(), directReports2.toArray());
+
+        // clear m1's direct reports list. m2's list should not be affected
+        directReports.clear();
+        directReports1 = m1.getDirectReport();
+        directReports2 = m2.getDirectReport();
+        assertEquals(0, directReports1.size());
+        assertEquals(2, directReports2.size());
+
+    }
+
+    @Test
+    public void givenCopyMethod_whenCopy_thenDistinct() {
+        Date startDate = new Date(123);
+        Employee e1 = new Employee(1, "Baeldung", startDate);
+        Employee e2 = new Employee(e1);
+        List<Employee> directReports = new ArrayList<Employee>();
+        directReports.add(e1);
+        directReports.add(e2);
+
+        // a Manager object whose declaration type is Employee.
+        Employee source = new Manager(1, "Baeldung Manager", startDate, directReports);
+        Employee clone = source.copy();
+
+        // after copy, clone should be still a Manager object.
+        assertTrue(clone instanceof Manager);
+        List<Employee> directReports1 = ((Manager) source).getDirectReport();
+        List<Employee> directReports2 = ((Manager) clone).getDirectReport();
+        assertEquals(directReports1.size(), directReports2.size());
+        assertArrayEquals(directReports1.toArray(), directReports2.toArray());
+
+        // clear source's direct reports list. clone's list should not be affected
+        directReports.clear();
+        directReports1 = ((Manager) source).getDirectReport();
+        directReports2 = ((Manager) clone).getDirectReport();
+        assertEquals(0, directReports1.size());
+        assertEquals(2, directReports2.size());
+
+    }
+}