Java – Static Classes In Java

javastatic-classes

Is there anything like static class in java?

What is the meaning of such a class. Do all the methods of the static class need to be static too?

Is it required the other way round, that if a class contains all the static methods, shall the class be static too?

What are static classes good for?

Best Solution

Java has static nested classes but it sounds like you're looking for a top-level static class. Java has no way of making a top-level class static but you can simulate a static class like this:

  • Declare your class final - Prevents extension of the class since extending a static class makes no sense
  • Make the constructor private - Prevents instantiation by client code as it makes no sense to instantiate a static class
  • Make all the members and functions of the class static - Since the class cannot be instantiated no instance methods can be called or instance fields accessed
  • Note that the compiler will not prevent you from declaring an instance (non-static) member. The issue will only show up if you attempt to call the instance member

Simple example per suggestions from above:

public class TestMyStaticClass {
     public static void main(String []args){
        MyStaticClass.setMyStaticMember(5);
        System.out.println("Static value: " + MyStaticClass.getMyStaticMember());
        System.out.println("Value squared: " + MyStaticClass.squareMyStaticMember());
        // MyStaticClass x = new MyStaticClass(); // results in compile time error
     }
}

// A top-level Java class mimicking static class behavior
public final class MyStaticClass {
    private MyStaticClass () { // private constructor
        myStaticMember = 1;
    }
    private static int myStaticMember;
    public static void setMyStaticMember(int val) {
        myStaticMember = val;
    }
    public static int getMyStaticMember() {
        return myStaticMember;
    }
    public static int squareMyStaticMember() {
        return myStaticMember * myStaticMember;
    }
}

What good are static classes? A good use of a static class is in defining one-off, utility and/or library classes where instantiation would not make sense. A great example is the Math class that contains some mathematical constants such as PI and E and simply provides mathematical calculations. Requiring instantiation in such a case would be unnecessary and confusing. See the Math class and source code. Notice that it is final and all of its members are static. If Java allowed top-level classes to be declared static then the Math class would indeed be static.