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Compliant Solution (immutable)
If In this solution the Foo class is unchanged, but the Helper class is immutable, then it . In this case, the Helper class is guaranteed to be fully constructed before becoming visible. The object must be truly immutable; it is not sufficient for the program to refrain from modifying the object.
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public class Helper {
private final int n;
public Helper(int n) {
this.n = n;
}
// other fields & methods, all fields are final
}
class Foo {
private Helper helper = null;
public Helper getHelper() {
if (helper == null) {
synchronized(this) {
if (helper == null) {
helper = new Helper(); // If the helper is null, create a new instance
}
}
}
return helper; // If helper is non-null, return its instance
}
}
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Compliant Solution (static initialization)
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