Java Wallet Lab, converting array of int's to string using StringBuilder

Question

I'm having a difficult time figuring out a way to convert a array of ints to a String just using String builder. In my toString Method, Am I using the right for loop?

public class Wallet {
  // max possible # of banknotes in a wallet
  private static final int MAX = 10;
  private int contents[];
  private int count; // number of banknotes stored in contents[]

  public Wallet() {
    count = 0;
    contents = new int[MAX];
  }

  public Wallet(int a[]) {
    contents = new int[MAX];
    for (int i = 0; i < a.length; i++) {
      contents[i] = a[i];
      count++;
    }
  }

  public String toString() {
    StringBuilder builder = new StringBuilder("Wallet[" + contents + "]");
    for (int s : contents) {
      builder.append(s);
    }
    return builder.toString();
  }
}

Here is the walletTester:

public class WalletTester {
  public static void main(String args[]) {
    // create a new Wallet object using an array
    int a[] = {5, 50, 10, 5};
    Wallet myWallet = new Wallet(a);

    // show the contents of myWallet
    System.out.println("myWallet contains: " + myWallet.toString());

    // print the value of myWallet
    System.out.println("\nvalue of myWallet is: $" + myWallet.value());

    // transfer all the banknotes from myWallet to yourWallet!
    Wallet yourWallet = new Wallet();
    yourWallet.add(1);
    yourWallet.transfer(myWallet);
    System.out.println("\nnow myWallet contains: " + myWallet.toString());
    System.out.println("yourWallet contains: " + yourWallet.toString());
  }
}

When this program is ran, I get this output:

myWallet contains: Wallet[[I@d0a7f3]550105000000
now myWallet contains: Wallet[[I@d0a7f3]5555000000
yourWallet contains: Wallet[[I@99588e]15505500000

yourWallet with $5s removed is: Wallet[[I@99588e]1555500000

The output is suppose to be something similiar to this:

Wallet[5, 50, 10, 5]
Wallet[]
Wallet[1, 5, 10, 50, 5]