我在几个方法上遇到多个“错误：缺少返回语句”错误。我做错了什么，该如何解决？

我目前正在尝试修改名为Coin的类，该类实现了称为Lockable的接口以锁定方法，直到输入密码以将其解锁为止。当我尝试编译代码时，出现以下错误：

C:\Users\mrowl_000\Dropbox\APCS\Ch. 5\Ch. 5 Projects\Three\Coin.java:112: error: missing return statement
   }
   ^
C:\Users\mrowl_000\Dropbox\APCS\Ch. 5\Ch. 5 Projects\Three\Coin.java:137: error: missing return statement
   }
   ^
2 errors

Tool completed with exit code 1

我的代码如下：

import java.util.Random;

public class Coin implements Lockable
{

   private final int HEADS = 0;
   private final int TAILS = 1;
   private int password;
   private boolean isUnlocked = true;
   private int face;

   public void setKey(int key)
    {
        password = key;
    }

    public void lock(int userKey)
    {
        int userpass = userKey;
        int accessKey = password;

        if (userpass != accessKey)
        {
            System.out.println("The following methods have been locked.");
            isUnlocked = false;
        }
        else
        {
            System.out.println("The following methods have been unlocked.");
            isUnlocked = true;
        }
    }

    public void unlock(int userKey)
    {
        int userpass = userKey;

        if (userpass != password)
        {
            System.out.println("The following methods have been locked.");
            isUnlocked = false;
        }
        else
        {
            System.out.println("The following methods have been unlocked.");
            isUnlocked = true;
        }
    }

    public boolean locked()
    {
        return isUnlocked;
    }

   //END LOCKABLE

   //-----------------------------------------------------------------
   //  Sets up the coin by flipping it initially.
   //-----------------------------------------------------------------
   public Coin ()
   {
      boolean result = locked();

            if (result == false)
            {
                System.out.println("The coin method is locked");
            }
            else
            {
                flip();
        }
   }

   //-----------------------------------------------------------------
   //  Flips the coin by randomly choosing a face value.
   //-----------------------------------------------------------------
   public void flip ()
   {
       boolean result = locked();

            if (result == false)
            {
                System.out.println("The coin method is locked");
            }
            else
            {
                face = (int) (Math.random() * 2);
        }
   }

   //-----------------------------------------------------------------
   //  Returns true if the current face of the coin is heads.
   //-----------------------------------------------------------------
   public boolean isHeads ()
   {
      boolean result = locked();

            if (result == false)
            {
                System.out.println("The coin method is locked");
            }
            else
            {
                return (face == HEADS);
        }
   } // <---Error Here

   //-----------------------------------------------------------------
   //  Returns the current face of the coin as a string.
   //-----------------------------------------------------------------
  public String toString()
   {
      boolean result = locked();

            if (result == false)
            {
                System.out.println("The coin method is locked");
            }
            else
            {
                String faceName;

                if (face == HEADS)
                   faceName = "Heads";
                else
                   faceName = "Tails";


                return faceName;
        }
   } // <---Error Here
}

如果需要，这里是称为Lockable的接口：

public interface Lockable
{
   public void setKey (int value);
   public void lock(int key);
   public void unlock(int key);
   public boolean locked();
}

我在错误发生的地方加了注释。有人会这么友善地指出我哪里出问题了吗？