Hi, I should want to assign a letter to a substring in a String,

for example, with two string:

1. String KeyList = "abcdefghilmnopqrstuvzABCDEFGHILMNOPQRSTUVZ";
2. String[] RecordList = { "example", "B", "C",};

I want record a substring to compose a word recorded in an index of the string RecordList:

1. RecordList[0].substring(0,1) = KeyList.charAt(x);

The full code I use is:

1. PFont AndaleMono;
    
   void setup() {
     size(800, 600);
     smooth();
     stroke(200);
     background(255);
     AndaleMono = loadFont("AndaleMono-11.vlw");
     fill(230);
     rect(50,35,200,20);
   }
   
   void draw() {
     smooth();
   
   }
   
   String KeyList = "abcdefghilmnopqrstuvzABCDEFGHILMNOPQRSTUVZ";
   String[] RecordList = {
     "example",
     "B",
     "C",
     };
   
     void keyPressed() {
      
   for (int x = 0; x < 42; x++)  {
     if (key == KeyList.charAt(x)) {
       textFont(AndaleMono,11);
       fill(50);
       textAlign(LEFT);
       text(KeyList.charAt(x),52,50);
       RecordList[0].substring(0,1) = KeyList.charAt(x);
      
     } else {
   
     }
   }
   }

This code give me an error "The left-hand side of an assignment must be a variable". What the problem? Or how could I record a substring of a string in another string?

Thank you so much for answering...