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Greetable.sol

pragma solidity ^0.4.0;


contract Greetable {
    function Greetable() public {}
    function greet() constant public returns (bytes32);
}

Greeter1.sol

pragma solidity ^0.4.0;

import './Greetable.sol';

contract Greeter1 is Greetable {
    
    bytes32 constant greeting = "Hi!";
    
    function Greeter1() public {
    }
    
    function greet() constant public returns (bytes32) {
        return greeting;
    }
    
}

Greeter2.sol

pragma solidity ^0.4.0;

import './Greetable.sol';

contract Greeter2 is Greetable {
    
    bytes32 constant greeting = "Hello!";
    
    function Greeter2() public {
    }
    
    function greet() constant public returns (bytes32) {
        return greeting;
    }
}

GreeterContainer.sol

pragma solidity ^0.4.0;


import './Greetable.sol';


contract GreeterContainer is Greetable {

    Greetable greeter;

    address owner;

    function GreeterContainer(address greeter_address) public {
        greeter = Greetable(greeter_address);
        owner = msg.sender;
    }

    function setGreeter(address _greeter_address) public {
        require(owner == msg.sender);
        greeter = Greetable(_greeter_address);
    }

    function greet() constant public returns (bytes32) {
        return greeter.greet();
    }

    function greetString() constant public returns (string) {
        return bytes32ToString(greeter.greet());
    }

    // bytes32 to string converter
    function bytes32ToString(bytes32 x) constant internal returns  (string) {
        bytes memory bytesString = new bytes(32);
        uint charCount = 0;
        for (uint j = 0; j < 32; j++) {
            byte char = byte(bytes32(uint(x) * 2 ** (8 * j)));
            if (char != 0) {
                bytesString[charCount] = char;
                charCount++;
            }
        }
        bytes memory bytesStringTrimmed = new bytes(charCount);
        for (j = 0; j < charCount; j++) {
            bytesStringTrimmed[j] = bytesString[j];
        }
        return string(bytesStringTrimmed);
    }

}