Comparator and Comparable

Author: Rajeev Kumar Singh

java-arraylist-examples/src/IterateOverArrayListExample.java

@@ -11,7 +11,7 @@ public static void main(String[] args) {
         tvShows.add("Friends");
         tvShows.add("Prison break");
 
-        System.out.println("=== Iterate using Java 8 forEach loop ===");
+        System.out.println("=== Iterate using Java 8 forEach and lambda ===");
         tvShows.forEach(tvShow -> {
             System.out.println(tvShow);
         });

java-comparable-comparator-examples/src/ComparableExample.java

@@ -0,0 +1,27 @@
+import java.time.LocalDate;
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.Comparator;
+import java.util.List;
+
+public class ComparableExample {
+    public static void main(String[] args) {
+        List<Employee> employees = new ArrayList<>();
+
+        employees.add(new Employee(1010, "Rajeev", 100000.00, LocalDate.of(2010, 7, 10)));
+        employees.add(new Employee(1004, "Chris", 95000.50, LocalDate.of(2017, 3, 19)));
+        employees.add(new Employee(1007, "David", 134000.00, LocalDate.of(2017, 9, 28)));
+
+        System.out.println("Employees (Before Sorting) : " + employees);
+
+        // This will use the `compareTo()` method of the `Employee` class to compare two employees and sort them.
+        Collections.sort(employees);
+
+        System.out.println("Employees (After Sorting) : " + employees);
+
+        // This will use the same `compareTo()` method of the `Employee` class but will reverse the order
+        Collections.sort(employees, Comparator.reverseOrder());
+        System.out.println("Employees (After Sorting with reverseOrder)" + employees);
+    }
+
+}

java-comparable-comparator-examples/src/ComparatorExample.java

@@ -0,0 +1,39 @@
+import java.time.LocalDate;
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.Comparator;
+import java.util.List;
+
+public class ComparatorExample {
+    public static void main(String[] args) {
+        List<Employee> employees = new ArrayList<>();
+
+        employees.add(new Employee(1010, "Rajeev", 100000.00, LocalDate.of(2010, 7, 10)));
+        employees.add(new Employee(1004, "Chris", 95000.50, LocalDate.of(2017, 3, 19)));
+        employees.add(new Employee(1015, "David", 134000.00, LocalDate.of(2017, 9, 28)));
+        employees.add(new Employee(1009, "Steve", 100000.00, LocalDate.of(2016, 5, 18)));
+
+        System.out.println("Employees : " + employees);
+
+        // Sort employees by Name
+        Collections.sort(employees, Comparator.comparing(Employee::getName));
+        System.out.println("\nEmployees (Sorted by Name) : " + employees);
+
+        // Sort employees by Salary
+        Collections.sort(employees, Comparator.comparingDouble(Employee::getSalary));
+        System.out.println("\nEmployees (Sorted by Salary) : " + employees);
+
+        // Sort employees by JoiningDate
+        Collections.sort(employees, Comparator.comparing(Employee::getJoiningDate));
+        System.out.println("\nEmployees (Sorted by JoiningDate) : " + employees);
+
+        // Sort employees by descending order of Name
+        Collections.sort(employees, Comparator.comparing(Employee::getName).reversed());
+        System.out.println("\nEmployees (Sorted by Name Descending Order) : " + employees);
+
+        // Chaining multiple Comparators
+        // Sort by Salary. If Salary is same then sort by Name
+        Collections.sort(employees, Comparator.comparingDouble(Employee::getSalary).thenComparing(Employee::getName));
+        System.out.println("\nEmployees (Sorted by Salary and Id) : " + employees);
+    }
+}

java-comparable-comparator-examples/src/Employee.java

@@ -0,0 +1,68 @@
+import java.time.LocalDate;
+
+class Employee implements Comparable<Employee> {
+    private int id;
+    private String name;
+    private double salary;
+    private LocalDate joiningDate;
+
+    public Employee(int id, String name, double salary, LocalDate joiningDate) {
+        this.id = id;
+        this.name = name;
+        this.salary = salary;
+        this.joiningDate = joiningDate;
+    }
+
+    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 double getSalary() {
+        return salary;
+    }
+
+    public void setSalary(double salary) {
+        this.salary = salary;
+    }
+
+    public LocalDate getJoiningDate() {
+        return joiningDate;
+    }
+
+    public void setJoiningDate(LocalDate joiningDate) {
+        this.joiningDate = joiningDate;
+    }
+
+    // Compare Two Employees based on their ID
+    /**
+     * @param   anotherEmployee - The Employee to be compared.
+     * @return  A negative integer, zero, or a positive integer as this employee
+     *          is less than, equal to, or greater than the supplied employee object.
+    */
+    @Override
+    public int compareTo(Employee anotherEmployee) {
+        return this.getId() - anotherEmployee.getId();
+    }
+
+    @Override
+    public String toString() {
+        return "Employee{" +
+                "id=" + id +
+                ", name='" + name + '\'' +
+                ", salary=" + salary +
+                ", joiningDate=" + joiningDate +
+                '}';
+    }
+}

java-hashmap-examples/src/IterateOverHashMap.java

@@ -11,7 +11,7 @@ public static void main(String[] args) {
         employeeSalary.put("Mark", 95000.00);
         employeeSalary.put("Steven", 134000.00);
 
-        System.out.println("=== Iterating over a HashMap using Java 8 forEach loop ===");
+        System.out.println("=== Iterating over a HashMap using Java 8 forEach and lambda ===");
         employeeSalary.forEach((employee, salary) -> {
             System.out.println(employee + " => " + salary);
         });
@@ -24,7 +24,7 @@ public static void main(String[] args) {
             System.out.println(entry.getKey() + " => " + entry.getValue());
         }
 
-        System.out.println("\n=== Iterating over the HashMap's entrySet using Java 8 forEach loop ===");
+        System.out.println("\n=== Iterating over the HashMap's entrySet using Java 8 forEach and lambda ===");
         employeeSalary.entrySet().forEach(entry -> {
             System.out.println(entry.getKey() + " => " + entry.getValue());
         });

java-hashset-examples/src/IterateOverHashSetExample.java

@@ -12,7 +12,7 @@ public static void main(String[] args) {
         programmingLanguages.add("PHP");
         programmingLanguages.add("Ruby");
 
-        System.out.println("=== Iterate over a HashSet using Java 8 forEach loop ===");
+        System.out.println("=== Iterate over a HashSet using Java 8 forEach and lambda ===");
         programmingLanguages.forEach(programmingLanguage -> {
             System.out.println(programmingLanguage);
         });

java-linkedhashmap-examples/src/IterateOverLinkedHashMapExample.java

@@ -11,13 +11,13 @@ public static void main(String[] args) {
         userCityMapping.put("David", "Paris");
         userCityMapping.put("Jesse", "California");
 
-        System.out.println("=== Iterating over a LinkedHashMap using Java 8 forEach loop ===");
+        System.out.println("=== Iterating over a LinkedHashMap using Java 8 forEach and lambda ===");
         userCityMapping.forEach((user, city) -> {
             System.out.println(user + " => " + city);
         });
 
 
-        System.out.println("\n=== Iterating over the LinkedHashMap's entrySet using Java 8 forEach loop ===");
+        System.out.println("\n=== Iterating over the LinkedHashMap's entrySet using Java 8 forEach and lambda ===");
         userCityMapping.entrySet().forEach(entry -> {
             System.out.println(entry.getKey() + " => " + entry.getValue());
         });

java-linkedlist-examples/src/IterateOverLinkedListExample.java

@@ -11,7 +11,7 @@ public static void main(String[] args) {
         humanSpecies.add("Homo Erectus");
         humanSpecies.add("Home Habilis");
 
-        System.out.println("=== Iterate over a LinkedList using Java 8 forEach loop ===");
+        System.out.println("=== Iterate over a LinkedList using Java 8 forEach and lambda ===");
         humanSpecies.forEach(name -> {
             System.out.println(name);
         });