How to encrypt and Decrypt random string in Java?

I'm trying to practice my java skills and i'm writing a program that does the following:

• Implement a symmetric cryptosystem.
• Generate a 16 bit value key
• Messages are randomly generated string with an even number of characters.
• Encrypt and Decrypt a method.
• Implement a brute force method decryption of attack for this cryptosystem using the randomly generated String.
• Also I can only call methods in Main

So in my code I'm pretty much done. The problem that I'm having is when I generate the random String. The String is not going through the Encryption and Decryption methods in my code. The methods I have for encryption and Decryption would be perfect if it passed the String along the way.

The method I have in this code is completely diffrent then other encryption methods. This encryption method works but I just need to pass the String.

        private static String msg;

private static String msgE;

private static String msgD;

private static int key;
//This is the String for the program. So Upper is the chracters in upper letters
private static String upper="ABCDEFGHIJKLMNOPQRSTUVWXYZ";
//This is for the chracters in the lower case
private static String lower=upper.toLowerCase();
private static String space=" ";
//Alphanum is upper and lower and space combined and this is what I'll be using for this project. 
private static String alphanum=upper+lower+space;


public static void main(String[] args){

//TODO: You can only call methods in main method

key = generateKey();

msg = generateMsg();

msgE = encryption(key,msg);

bruteForce(msgE);

}

private static int generateKey() {

//TODO: implement step a (randomly generate 16-bit key)
//So in Decimal a key would be 2^16-1 
//So i'm using random rand to generate a key which is going to be a int and in power of 2^16 -1
    Random rand=new Random();
    return rand.nextInt((int) (Math.pow(2, 16)-1));


}

private static String generateMsg() {

//TODO: implement step b (randonly generate a string with an even number of characters)
//For this method i'm going to generate a string which is random 
//This string is going to use the above alphanum.
StringBuilder builder =new StringBuilder();
//The while loop states while the builder length is divisible by 2 then you can print the random string

while(builder.length()%2!=0) {
    //The chracter is using the random rand and alphanum length to generate the String
int character=(int)(Math.random()*alphanum.length());
//Builder append is shortering the string into something more simple. 
    builder.append(alphanum);
}

return builder.toString() ;


}

private static String encryption(int key, String msg) {

//TODO: implement step c (encrypt the message)
//To encrypt the string we're going to use the key we generated before
/*The String Key is going to take the key and put that into a string*/
//then the loop is going to go through and put the String Key to generate a key
String empty="";
int xor;
char temp;

for(int i=0;i<msg.length();i++) {
xor=msg.charAt(i)^key;
temp=(char)xor;
empty=empty+temp;
}
// This will return a encrypted message.
msg=empty;
return msg;

}

private static void decryption(int key, String msgE) {

//TODO: implement step d (decryption)
//For decryption we're going to use the key we got before and go through the loop
//We're going to go through the loop and put the String into String Key
//Then we're going to return the String with the key.
String empty="";
    int xor;
    char temp;
    for(int i=0;i<msgE.length();i++) {
        xor=msgE.charAt(i)^key;
        temp=(char)xor;
        empty=empty+temp;
    }

     msgE=empty;


}

private static void bruteForce(String msgE) {

//TODO: implement bruteForce algorithm, you may need the above decryption(key,msgE) method

decryption(key, msgE);
boolean isEnglish=msgE.matches("[a-zA-Z]");
if(isEnglish) {

System.out.println("This string is English: "+msgE);
}else {
    System.out.println("This String is not English at all: "+msgE);

    }
}}