diff --git a/src/main/scala/com/github/fluency03/blockchain/core/KeyContainer.scala b/src/main/scala/com/github/fluency03/blockchain/core/KeyContainer.scala
index 3fd74c5..accbfb0 100644
--- a/src/main/scala/com/github/fluency03/blockchain/core/KeyContainer.scala
+++ b/src/main/scala/com/github/fluency03/blockchain/core/KeyContainer.scala
@@ -13,6 +13,7 @@ case class KeyContainer() {
 
   private[this] val keyPair: KeyPair = Secp256k1.generateKeyPair()
 
+  // TODO (Chang): change it to actual address of a PublicKey
   lazy val address: String = keyPair.getPublic.toHex
 
   lazy val publicKeyHex: String = keyPair.getPublic.toHex
diff --git a/src/main/scala/com/github/fluency03/blockchain/core/Transaction.scala b/src/main/scala/com/github/fluency03/blockchain/core/Transaction.scala
index 9b46674..920430c 100644
--- a/src/main/scala/com/github/fluency03/blockchain/core/Transaction.scala
+++ b/src/main/scala/com/github/fluency03/blockchain/core/Transaction.scala
@@ -49,6 +49,7 @@ object Transaction {
 
   def createCoinbase(blockIndex: Int): TxIn = TxIn(Outpoint("", blockIndex), "")
 
+  // TODO (Chang): correct coinbase format
   def createCoinbaseTx(blockIndex: Int, miner: String, timestamp: Long): Transaction =
     Transaction(Seq(createCoinbase(blockIndex)), Seq(TxOut(miner, COINBASE_AMOUNT)), timestamp)
 
diff --git a/src/main/scala/com/github/fluency03/blockchain/core/Wallet.scala b/src/main/scala/com/github/fluency03/blockchain/core/Wallet.scala
index 4546f5e..fbe7036 100644
--- a/src/main/scala/com/github/fluency03/blockchain/core/Wallet.scala
+++ b/src/main/scala/com/github/fluency03/blockchain/core/Wallet.scala
@@ -1,41 +1,46 @@
 package com.github.fluency03.blockchain.core
 
-trait Wallet {
-
-}
+import com.github.fluency03.blockchain.core.KeyContainer.balanceOfKey
 
+import scala.collection.mutable
 
+trait Wallet {
 
-case class RandomWallet() extends Wallet {
+  def getKey(str: String): Option[KeyContainer]
 
+  def newKey(): KeyContainer
 
+  def balance(uTxOs: mutable.Map[Outpoint, TxOut]): Long
 
 }
 
 
+case class RandomWallet() extends Wallet {
 
-object RandomWallet {
-
-
-
-}
-
-
+  private[this] val keys = mutable.Map.empty[String, KeyContainer]
 
+  override def getKey(str: String): Option[KeyContainer] = keys.get(str)
 
-case class SeededWallet() extends Wallet {
+  override def newKey(): KeyContainer = {
+    val aNewKey = KeyContainer()
+    keys += (aNewKey.publicKeyHex -> aNewKey)
+    aNewKey
+  }
 
+  override def balance(uTxOs: mutable.Map[Outpoint, TxOut]): Long =
+    keys.values.map(k => balanceOfKey(k, uTxOs)).sum
 
 
 }
 
 
+// TODO (Chang): To be implemented
+case class SeededWallet() extends Wallet {
 
-object SeededWallet {
+  override def getKey(str: String): Option[KeyContainer] = ???
 
+  override def newKey(): KeyContainer = ???
 
+  override def balance(uTxOs: mutable.Map[Outpoint, TxOut]): Long = ???
 
 }
-
-
-