These code are easy sample(test) for static field in Java.

Base.java

package staticFieldTest;

public abstract class Base {

    public static int staticCounter = 0;
    public int NonStaticCounter = 0;

    public void countUp(){
        staticCounter++;
        NonStaticCounter++;
    }
    
    public void showCounter(){
        System.out.println("staticCounter: " + staticCounter);
        System.out.println("NonStaticCounter: " + NonStaticCounter);
    }
}

Main.java

package staticFieldTest;

public class Main {

    public static void main(String[] args) {
        Base sub1 = new Sub1();
        Base sub2 = new Sub2();
        
        sub1.countUp();
        sub2.countUp();
        
        sub1.showCounter(); 
        sub2.showCounter();
        // Result:
        // - staticCounter: 2
        // - NonStaticCounter: 1
        // "staticCounter" is shared even if it is in base class and extended to both sub1 and sub2. 
    }

}

Sub1.java

package staticFieldTest;

public class Sub1 extends Base {
    // Just extends Base.
}

Sub2.java

package staticFieldTest;

public class Sub2 extends Base {
    // Just extends Base.
}