Breaking A Lock Code

I'm suppose to read the following code and devise a program that can break the lock.

public class InsecurePasswordLock {
private char[] secret;
private boolean unlocked;
public InsecurePasswordLock() {
// secrets are usually 30 to 50 upper case characters long
// but here's a hard coded example one
secret = "ASECRETTHATYOUWILLNOTGUESSTODAY".toCharArray();

[Code]....

So, I believe that I can try various password lengths until I get a return from open that !=-1. But I'm not sure how to implement this. This is what I have so far to get the length:

public char[] breakLock(InsecurePasswordLock lock) {
int checkCount=1;
int length=0;
while(checkCount!=-1){
for(int i=0;i<=25;i++){
char[] key = new char[i];
if(InsecurePasswordLock.open(key)!=-1){ // I get a fault at this line for calling a static method
length=i;
checkCount=-1;
}
}
}

I get the error:

Cannot make a static reference to the non-static method open(char[]) from the type InsecurePasswordLock..

How can I keep querying the open method to get my length??