Need some help with encryption decryption

Can you help me fill in the blanks in the most basic code you can think of. Please
public class ToolKit {
/* Some Code Removed For Brevity */
public static String lowerCase(String str) {
String lowerT="";
for (int i=0; i<str.length(); i++) {
String ltr=str.substring(i, i+1);
int caseIndex=UPPER_ALPHABET.indexOf(ltr);
if(caseIndex==-1) {
lowerT+=ltr;
} else {
lowerT+=LOWER_ALPHABET.substring(caseIndex, caseIndex+1);
}
}
return lowerT;
}
public static int randomNumber(int min, int max) {
if (max < min) {
int temp=min;
min = max;
max = temp;
}
return ((int)(Math.random()*(max- min + 1)) + min);
}
public static String generateCode(String str) {
String code=ALL_CHARACTERS;
/* Your code HERE */
return code;
}
public static String encrypt(String str,String code) {
// Sample Run:
// str="ABCDEF";
// code="CDABEF"
// encrypted="CDABEF"
String encrypted = "";
/* Your code HERE */
return encrypted;
}
public static String decrypt(String str, String code) {
// str="CDABEF";
// code="CDABEF"
// decrypted="ABCDEF"
String decrypted="";
/* Your code HERE */
return decrypted;
}
public final static String UPPER_ALPHABET="ABCDEFGHIJKLMNOPQRSTUVWXYZ";
public final static String LOWER_ALPHABET="abcdefghijklmnopqrstuvwxyz";
public final static String PUNCTUATION=" ,.!@#$%^&*()[]{};:'\"<>\\-_=+";
public final static String ALL_CHARACTERS=UPPER_ALPHABET+LOWER_ALPHABET+PUNCT UATION