Java value passing and reference passing small example

public class Example{
    String str=new String("hello");
    //char[]ch={'a','b'};
    public static void main(String args[]){
        Example ex=new Example();
        //ex.change(ex.str,ex.ch);
        //ex.change();
        ex.change(ex.str);
        System.out.print(ex.str+" and ");
        //System.out.print(ex.ch);
     }
        //public void change(String str,char ch[]){
    public void change(String str){
        //public void change(){
        System.out.println("Local variable parameter value (local value equals global value)"+str);
        str="test ok";
      System.out.println("Local variable parameter value 2 (changed)"+str);
      System.out.println("Global parameters (unchanged)"+this.str);
      //ch[0]='c';
    }
}