Java String.replace()方法"无效"的原因及解决方式

public class Demo1 {
  public static void main(String[] args) {
    String aa="abcd";
    aa.replace("a","f");
    System.out.println("输出结果是"+aa);
  }
}

public class Demo1 {
  public static void main(String[] args) {
    String aa="abcd";
    String replaceStr = aa.replace("a", "f");
    System.out.println("输出结果是"+replaceStr);
  }
}

public static void main(String[] args){
   String str = "aaa";
   String ret = str.replace("aa","b");
   System.out.println(str);
   System.out.println(ret);
}
aaa
ba