Translating JavaScript (UnityScript) to Java in android studio Random string generation

I am currently trying to learn java, but I do not know all the references inside java for random generation as well as length. I am trying to translate the following javascript/unityscript code to Java for use in android studio.

Origional javascript/unityscript (unity game engine is where i made it)

 // Javascript example follows
var characters : String = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";
var code : String = "";
var code1 : String = "";
var code2 : String = "";
var codet : String = "";
var codet1 : String = "";
var gui_result : UI.Text;

 function Generate () {
var code : String = "";
var code1 : String = "";
var code2 : String = "";
var codet : String = "";
var codet1 : String = "";

 for (var i : int = 0; i < 5; i++) {
     var a : int = Random.Range(0, characters.length);
     code = code + characters[a];
 }
      for (var o : int = 0; o < 5; o++) {
     var s : int = Random.Range(0, characters.length);
     code1 = code1 + characters[s];
     codet = code1 + " - " + code;
 }
      for (var p : int = 0; p < 5; p++) {
     var d : int = Random.Range(0, characters.length);
     code2 = code2 + characters[d];
     codet1 = codet + " - " + code2;
     gui_result.text = codet1;
 }

}

I understand that code is extremely poorly made, but I understand it because i laid it out to be simple and not efficient.

Here is my attempt at Java code but I got completely lost on where to define my variables and how to call the function I wanted with a button. It would be best if someone could lay it out as straight forward as above so I understand.

Java attempt but fail

     public void generate(View view) {
    String characters = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";
    String code = "";
    String code1 = "";
    String code2 = "";
    String codet = "";
    String codet1 = "";

    for (int i = 0; i < 5; i++){
        int a = new Random(0, characters.length());
        code = code + characters[a];
    }
}

All help is much appreciated!