Extract similar fields from Transaction and NFT classes

src/main/java/pl/mikorosa/dziecoin/BlockData.java

@@ -0,0 +1,9 @@
+package pl.mikorosa.dziecoin;
+
+public abstract class BlockData {
+    protected String owner;
+
+    public String getOwner() {
+        return this.owner;
+    }
+}

src/main/java/pl/mikorosa/dziecoin/NFT.java

@@ -1,8 +1,7 @@
 package pl.mikorosa.dziecoin;
 
-public class NFT {
+public class NFT extends BlockData {
     private String contract;
-    private String owner;
     private String data;
 
     public NFT(String owner, String data) {
@@ -23,10 +22,6 @@ public class NFT {
         return contract;
     }
     
-    public String getOwner() {
-        return owner;
-    }
-
     public String getData() {
         return data;
     }

src/main/java/pl/mikorosa/dziecoin/Transaction.java

@@ -1,18 +1,17 @@
 package pl.mikorosa.dziecoin;
 
-public class Transaction {
-    private String sender;
+public class Transaction extends BlockData {
     private String recipient;
     private int amount;
 
     public Transaction(String sender, String recipient, int amount) {
-        this.sender = sender;
+        this.owner = sender;
         this.recipient = recipient;
         this.amount = amount;
     }
 
     public String getSender() {
-        return sender;
+        return owner;
     }
 
     public String getRecipient() {
@@ -26,7 +25,7 @@ public class Transaction {
     @Override
     public String toString() {
         return "Transaction{" +
-                "sender='" + sender + '\'' +
+                "sender='" + owner + '\'' +
                 ", recipient='" + recipient + '\'' +
                 ", amount=" + amount +
                 '}';