How to load and change existing .java source code at runtime

quickstart.java.TailorExistingCode.java

package quickstart.java;

import varcode.java.adhoc.Export;
import varcode.java.meta._Java;
import varcode.java.meta._class;

public class TailorExistingCode 
{
    public class Existing {
        public int count;
        
        public Existing() {
        }                
    }

    public static void main( String[] args )
    {
        /* load the _class model from the Existing java nested class */
        _class _c = _Java.Nested._classFrom( Existing.class );
        
        /* change the _class */
        _c.setName( "Tailored" ); //rename from "Existing" to "Tailored"
        
        _c.field( "private final String message;"); //add a new field 
        
        _c.getConstructors().getAt( 0 ) //add a param to the constructor
            .addParameter( "String", "message" )
            .setBody( "this.message = message;" ); //modify ctor body    
        
        _c.method( "public final String toString()", //add a toString method
            "return this.message;" );
        
        Export.TEMP_DIR.file( _c ); //write the .java code to the temp dir
        
        /* compile, load and create a new "ad hoc" instance of the class */
        Object adHocInstance = _c.instance( "Tailor" ); //new Tailored("Tailor")
        
        System.out.println( adHocInstance ); //prints "Tailor"       
    }
}

will print
"Tailor"
...to the console, and will write out the following .java source file to {java.temp.dir}\Tailored.java

public class Tailored
{
    public int count;
    private final String message;
    public Tailored( String message )
    {
        this.message = message;
    }

    public final String toString(  )
    {
        return this.message;
    }
}