Can we execute code before the main(String[]) method ?

One of the questions you might get at an interview or certification or a simple test concerning the Java language is the following: Can we execute code before the main(String[]) method?

The answer is yes. If we use a static block of code in the class. The code in the static block is executed before the object itself is created.

All the declarations inside this static block will only be executed once when the class is loaded, so before the object is created in the main method.

Compile and run the following code.

public class AClass {
    public AClass() {
        System.out.println("call costuctor AClass()");
    }
    public void method1() {
        System.out.println("call method1");
    }
    public static void main(String[] args) {
        AClass instance = new AClass();
        instance.method1();
    }
}

you will get the following output

call costuctor AClass()
call method1

Now change the code as follows

public class AClass {
    static {
        System.out.println("call static block");
    }
    public AClass() {
        System.out.println("call costuctor AClass()");
    }
    public void method1() {
        System.out.println("call method1");
    }
    public static void main(String[] args) {
        AClass instance = new AClass();
        instance.method1();
    }
}

you obtain the following output

call static block
call costuctor AClass()
call method1

As you can see, the code in static { } was executed even before the constructor itself, so when no instance of the class existed yet.

Still a variation of the code just above, the changes are limited to the main method, so I’ll just quote that

    public static void main(String[] args) {
        AClass instance = new AClass();
        instance.method1();
        
        AClass instanceBis = new AClass();
        instanceBis.method1();
    }

and the output produced

call static block
call costuctor AClass()
call method1
call costuctor AClass()
call method1

Again, although two instances of the AClass class have been created, the execution of the static block is limited to a single occurrence first when the class is loaded. And that’s all for today. As usual, if you have a question about this post, want a more in-depth discussion, or have a topic that you would like me to cover in future posts, you can write to me.

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