K

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

 
 
     public String toStringByPayerName(Payment[] payments) {
-        return null;
+        Arrays.sort(payments, new PaymentsSortByPayer());
+        StringBuilder sb = new StringBuilder();
+        for (Payment o : payments){
+            sb.append(o.getShortDescription() + "\n");
+        }
+
+        return sb.toString();
     }
 
     public String toStringById(Payment[] payments) {
-        return null;
+        Arrays.sort(payments, (Payment a, Payment b) -> Long.compare(a.getID(), b.getID()));
+        StringBuilder sb = new StringBuilder();
+        for (Payment o : payments){
+            sb.append(o.getShortDescription() + "\n");
+        }
+        return sb.toString();
     }
 }

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

 package com.zipcoder.payment;
 import java.util.Comparator;
+import java.util.function.Function;
 
-public class PaymentsSortByPayer {
+public class PaymentsSortByPayer implements Comparator<Payment> {
 
+
+    @Override
+    public int compare(Payment o1, Payment o2) {
+        return o1.getPayerName().compareTo(o2.getPayerName());
+    }
 }

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

     public void toStringByPayerName(){
         Payment[] payments = new Payment[2];
         Payment paypal = new Paypal(4L, "Tamara Mowry", "tamara@mowry.com");
-        Payment check = new Check(81L, "Tia Mowry", "11432543", "134344551")
+        Payment check = new Check(81L, "Tia Mowry", "11432543", "134344551");
 
         payments[0] = paypal;
         payments[1] = check;
     public void toStringByID(){
         Payment[] payments = new Payment[2];
         Payment paypal = new Paypal(120L, "Tamara Mowry", "tamara@mowry.com");
-        Payment check = new Check(81L, "Tia Mowry", "11432543", "134344551")
+        Payment check = new Check(81L, "Tia Mowry", "11432543", "134344551");
 
         payments[0] = paypal;
         payments[1] = check;
 
         PaymentPresenter presenter = new PaymentPresenter();
-        String expected = "Paypal Tamara Mowry tamara@mowry.com\nCheck Tia Mowry ***4551\n";
+        String expected = "Check Tia Mowry ***4551\nPaypal Tamara Mowry tamara@mowry.com\n";
 
         String actual = presenter.toStringById(payments);
         assertEquals(expected, actual);