inner interface examples added (#3151)

core-java/src/main/java/com/baeldung/interfaces/CommaSeparatedCustomers.java

@@ -0,0 +1,21 @@
+package com.baeldung.interfaces;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.stream.Collectors;
+
+public class CommaSeparatedCustomers implements Customer.List {
+
+    private List<Customer> customers = new ArrayList<Customer>();
+
+    @Override
+    public void Add(Customer customer) {
+        customers.add(customer);
+    }
+
+    @Override
+    public String getCustomerNames() {
+        return customers.stream().map(customer -> customer.getName()).collect(Collectors.joining(","));
+    }
+
+}

core-java/src/main/java/com/baeldung/interfaces/Customer.java

@@ -0,0 +1,19 @@
+package com.baeldung.interfaces;
+
+public class Customer {
+    public interface List {
+        void Add(Customer customer);
+
+        String getCustomerNames();
+    }
+
+    private String name;
+
+    public Customer(String name) {
+        this.name = name;
+    }
+
+    String getName() {
+        return name;
+    }
+}