Initialization On Demand Holder 习惯用法

Question

This is the implementation mostly seen on the web

private static class LazySomethingHolder {
  public static Something something = new Something();
}

public static Something getInstance() {
  return LazySomethingHolder.something;
}

Is the following simpler variation correct, if not what is wrong with it? Is the problem specific to java or also present in C++?

public static Something getInstance() {
  private static Something something = new Something();
  return something;
}

Answer 1

You cannot have static local variables in Java.

Simpler alternatives are

private static final Something something = new Something();

public static Something getInstance() {
  return something;
}

or my preference if for.

enum Something {
    INSTANCE;
}

The only problem with these patterns is that if you have multiple instances you want lazily loaded, you need to have a class each, or loading one will mean loading them all.

Answer 2

Java (unlike c++) doesn't have local static variables, so what you describe is simply not possible. In c++ you would most likely write (known as meyer singleton):

public static Something& getInstance() {
    static Something something{};
    return something;
}