Static method in java

Question

I heard that static methods should use only static variables in java. But, main method is also static, right?

Answer 1

First of all, a technicality: it's NOT true that "main method is also static". You can define a non-static main method, with whatever signature you choose; it just won't be a valid Java application entry point.

With regards to "static methods should use only static variables", this is also NOT true. The key concept here is that static methods and fields are class-specific, not instance-specific. You simply can't access an instance variable/method if you don't actually have an instance of that class; it's a compilation error.

So to be precise, without an instance, you can't access instance fields/methods. You can access static fields/methods without an instance. If you need to access instance fields/methods from a static method, you have to get an instance of that class one way or another, either by simply instantiating it, or by getting a reference to it from a static field or method parameter.

Let's take a look at this simple example:

public static void main(String args[]) {
  System.out.println(args.length);
}

length is NOT a static field; it's an instance field of array instances, which args is. The static method main is able to get this instance (and thus access its instance methods and fields) because it's passed in as an argument.

Also, println is NOT a static method; it's an instance method of PrintStream instances. The static method main is able to get this instance by accessing the static field out of the class System.

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To summarize:

• A Java application entry point is a method that:
  • is named main
  • is public and static
  • returns void and takes a String[] argument as parameter
• A method named main doesn't have to be a Java application entry point
  • (but it's best to reserve this name for that purpose)

Furthermore,

• Instance fields/methods can only be accessed through an instance
• Static fields/methods can be accessed without an instance
• Static methods can get an instance of a class by one of the following ways:
  • creating a new instance
  • having it passed as an argument
  • accessing it through a static field of a class
  • accepting it as the return value of a static method of a class
  • catching it as a thrown Throwable