How does this static code work? [duplicate]

Question

Code:

public static void main ( String[] args){
      String a = new String("Hello");
      String b = " pardner.";
      System.out.println(a+b);
      System.out.println("a.equals(\"Hello\") --> " + (a.equals("Hello")));
      System.out.println("a --> " + a);
}

static {
      try {
          Field value = String.class.getDeclaredField("value");
          value.setAccessible(true);
          value.set("Hello", value.get("Howdy"));
      } catch (Exception e) { }
}

Result:

Howdy pardner.
a.equals("Hello") --> true
a --> Howdy

How does this code change "Hello" to "Howdy" when printing?

Answer 1

First, String literals composed of the same characters resolve to the same instance. So in

String one = "hello";
String two = "hello";

both variables are referring to the same object.

Second, static initializer blocks are executed when a class is first loaded (and initialized). This occurs before any class methods are invoked, ie. before main.

Third, your Java version's implementation of String, presumably, uses a char\[\] field to store the string of characters. This field is named value.

You're using reflection to retrieve this char[] for the String object referenced by the String literal "Howdy".

Field value = String.class.getDeclaredField("value");
...
value.get("Howdy")

and assigning it to the char[] field of the String object referenced by the String literal "Hello"

value.set("Hello", value.get("Howdy"));

Now, when your main method executes

String a = new String("Hello");

the String literal "Hello" is referencing the same object for which you set the char[] field previously. This char[] contains the characters 'H', 'o', 'w', 'd', and 'y' since it was taken from the String object referenced by the literal "Howdy".

These characters are copied into a new String object created here

String a = new String("Hello"); 

Answer 2

The first thing that happens is your static block executes. By reflection, it changes the value of String a to "Howdy" (actually it changes the String "Hello" to "Howdy" but that has the same effect). However, you get

a.equals("Hello") --> true

because the compiler has already replaced the value with true. I ran javap -v and got

31: ldc           #75                 // String a.equals(\"Hello\") --> true

So that is exactly what has happened. As I noted in the comments, if you change String a to

final String a = "Hello";

The last line changes to

a --> Hello

for the same reason.