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Java code:
class CDemo{
public String Rnd(){
String str="";
str= String.valueOf(Math.random());
if(str.length()<=18){
//System.out.println(str.length());
this.Rnd();
}

//str=str.substring(5,15);
return str;
}

public void Display(){
for (int i = 1; i <= 10; i++) {
String str="";
str=Rnd();
System.out.println(str+" length:"+str.length());
}
}
};


public class Demo1_String_Test {
public static void main(String[] args) {
CDemo demo=new CDemo();
demo.Display();
}
};

---

  if(str.length() <=18){
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if(str.length() <=18){
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  if(str.length() <=18){
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  ÒÔÏÂÊÇÎҵĽá¹û
0.709153444358638 length:17
0.8676477453784845 length:18
0.8459151844354131 length:18
0.6034441577667381 length:1