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the order of object initialization in java

Have you ever thought about in which order objects in java initialize? If you haven’t, this article can you help to find it out.

Given: two classes: Child and Parent. According to their names, one inherits from another one. Each class involves static and ordinal init blocks, static and non-static fields. Let’s look at this case more closely.

Parent.java

public class Parent {
    private static final String staticField = staticMethod();
    static {
        System.out.println("parent static block");
    }

    {
        System.out.println("parent non-static block");
    }
    private final String field = initMethod();
    private String initMethod() {
        System.out.println("parent non static field");
        return "";
    }

    public static String staticMethod() {
        System.out.println("parent static field");
        return "";
    }

    public Parent() {
        System.out.println("parent constructor");
    }
}

Child.java

public final class Child extends Parent {
     private final static String staticField = staticMethod();
     private final String field = initMethod();

     static {
            System.out.println("child static block");
     }

     {
            System.out.println("child non-static block");
     }


     public static String staticMethod() {
            System.out.println("child static field");
            return null;
     }

     private String initMethod() {
            System.out.println("child non static field");
            return null;
     }

     public Child() {
            System.out.println("child constructor");
     }
}

If we run compile and run this code by this way:

public static void main(String[] args) throws Exception {
        new Child();
}

we will see the next output:

parent static field
parent static block
child static field
child static block
parent non-static block
parent non-static field
parent constructor
child non-static field
child non-static block
child constructor

To sum up, we can see that objects inits in the particular order:

  1. all static blocks and fields of a parent class
  2. the same happens in a child class
  3. all non-static blocks and fields of a parent class
  4. invocation of a parent constructor
  5. initialization of non-static fields and blocks in a child class
  6. invocation of a child constructor

As we can see, the order is quite logical and easy to remember.