finished

src/main/java/com/zipcoder/payment/PaymentPresenter.java

 public class PaymentPresenter {
 
     public String toString(Payment[] payments) {
-        if (payments == null || payments.length == 0) {
-            return "";
-        }
+        Arrays.sort(payments, new PaymentSortByShortDescription());
+        return createArrayString(payments);
+    }
 
-        StringBuilder paymentStringBuilder = new StringBuilder();
+    public String toStringByPayerName(Payment[] payments) {
         Arrays.sort(payments, new PaymentSortByPayer());
-        for (Payment payment : payments) {
-            paymentStringBuilder.append(payment.getShortDescription() + "\n");
-        }
-        return paymentStringBuilder.toString();
+        return createArrayString(payments);
     }
 
-    public String toStringByPayerName(Payment[] payments) {
-        if (payments == null || payments.length == 0) {
-            return "";
-        }
+    public String toStringById(Payment[] payments) {
+        Arrays.sort(payments, new PaymentSortById());
+        return createArrayString(payments);
+    }
 
+    private String createArrayString(Payment[] payments) {
         StringBuilder paymentStringBuilder = new StringBuilder();
-        return null;
+        for (Payment payment : payments) {
+            paymentStringBuilder.append(payment.getShortDescription() + "\n");
+        }
+        return paymentStringBuilder.toString();
     }
 }

src/main/java/com/zipcoder/payment/PaymentSortById.java

+package com.zipcoder.payment;
+
+import java.util.Comparator;
+
+public class PaymentSortById implements Comparator<Payment> {
+    public int compare(Payment o1, Payment o2) {
+        return o1.getId().compareTo(o2.getId());
+    }
+}

src/main/java/com/zipcoder/payment/PaymentSortByShortDescription.java

+package com.zipcoder.payment;
+
+import java.util.Comparator;
+
+public class PaymentSortByShortDescription implements Comparator<Payment> {
+    public int compare(Payment o1, Payment o2) {
+        return o1.getShortDescription().compareTo(o2.getShortDescription());
+    }
+}

src/test/java/com/zipcoder/payment/PaymentPresenterTest.java

         String actual = presenter.toStringByPayerName(payments);
         assertEquals(expected, actual);
     }
+
+    @Test
+    public void toStringByIdTest() {
+        Payment[] payments = new Payment[2];
+        Payment paypal = new PayPal(120L, "Tamara Mowry", "tamara@mowry.com");
+        Payment check = new Check(81L, "Tia Mowry", "11432543", "134344551");
+
+        payments[0] = paypal;
+        payments[1] = check;
+
+        PaymentPresenter presenter = new PaymentPresenter();
+        String expected = "Check Tia Mowry ***4551\nPaypal Tamara Mowry tamara@mowry.com\n";
+
+        String actual = presenter.toStringById(payments);
+        assertEquals(expected, actual);
+    }
 }