// Constructor.

var Interface = function(name, methods) {
    if(arguments.length != 2) {
        throw new Error("Interface constructor called with " + arguments.length
          + "arguments, but expected exactly 2.");
    }
   
    this.name = name;
    this.methods = [];
    for(var i = 0, len = methods.length; i < len; i++) {
        if(typeof methods[i] !== 'string') {
            throw new Error("Interface constructor expects method names to be "
              + "passed in as a string.");
        }
        this.methods.push(methods[i]);       
    }   
};   

// Static class method.

Interface.ensureImplements = function(object) {
    if(arguments.length < 2) {
        throw new Error("Function Interface.ensureImplements called with " +
          arguments.length  + "arguments, but expected at least 2.");
    }

    for(var i = 1, len = arguments.length; i < len; i++) {
        var interface = arguments[i];
        if(interface.constructor !== Interface) {
            throw new Error("Function Interface.ensureImplements expects arguments "  
              + "two and above to be instances of Interface.");
        }
       
        for(var j = 0, methodsLen = interface.methods.length; j < methodsLen; j++) {
            var method = interface.methods[j];
            if(!object[method] || typeof object[method] !== 'function') {
                throw new Error("Function Interface.ensureImplements: object "
                  + "does not implement the " + interface.name
                  + " interface. Method " + method + " was not found.");
            }
        }
    }
};

/*

// Example usage:

// Interfaces.

var Composite = new Interface('Composite', ['add', 'remove', 'getChild']);
var FormItem = new Interface('FormItem', ['save']);

// CompositeForm class

var CompositeForm = function(id, method, action) { // implements Composite, FormItem
 ...
};

...

function addForm(formInstance) {
    Interface.ensureImplements(formInstance, Composite, FormItem);
    // This function will throw an error if a required method is not implemented, halting execution.
    // All code beneath this line will be executed only if the checks pass.
    ...
}

*/