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Q: What about class initialization?

JavaFAQ Home » Java IAQ by Peter Norvig Go to all tips in Java IAQ by Peter Norvig


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Q: What about class initialization?

Answer: It is important to distinguish class initialization from instance creation. An instance is created when you call a constructor with new. A class C is initialized the first time it is actively used. At that time, the initialization code for the class is run, in textual order. There are two kinds of class initialization code: static initializer blocks (static { ... }), and class variable initializers (static String var = ...).

Active use is defined as the first time you do any one of the following:

  1. Create an instance of C by calling a constructor;
  2. Call a static method that is defined in C (not inherited);
  3. Assign or access a static variable that is declared (not inherited) in C. It does not count if the static variable is initialized with a constant expression (one involving only primitive operators (like + or ||), literals, and static final variables), because these are initialized at compile time.

Here is an example:

Program:
class A {
    static String a1 = ABC.echo(" 1: a1");
    static String a2 = ABC.echo(" 2: a2");
}

class B extends A {
    static String b1 = ABC.echo(" 3: b1");
    static String b2;
    static { 
        ABC.echo(" 4: B()"); 
        b1 = ABC.echo(" 5: b1 reset"); 
        a2 = ABC.echo(" 6: a2 reset"); 
    }
}

class C extends B {
    static String c1; 
    static { c1 = ABC.echo(" 7: c1"); }
    static String c2;
    static String c3 = ABC.echo(" 8: c3");

    static { 
        ABC.echo(" 9: C()"); 
        c2 = ABC.echo("10: c2");
        b2 = ABC.echo("11: b2");
    }
}

public class ABC {
    static String echo(String arg) {
        System.out.println(arg);
        return arg;
    }

    public static void main(String[] args) { 
        new C(); 
    }
}

Output:
 1: a1
 2: a2
 3: b1
 4: B()
 5: b1 reset
 6: a2 reset
 7: c1
 8: c3
 9: C()
10: c2
11: b2


This tip is reprinted on JavaFAQ.nu by by courtesy of Peter Norvig I am thankful for his important contributions to my site - 21 Infrequently Answered Java Questions. Alexandre Patchine


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