静态和非静态方法/子例程之间的区别

Question

I was reading a java book, where I came across this statement:

So, every subroutine is contained either in a class or in an object

I'm really confused why does it say "class or in an object"

I would like some explanation.

Answer 1

Let's try this example

public class Demo {

    public static void classMethod() {
        System.out.println("Call to static method");
    }

    public void objectMethod() {
        System.out.println("Call to object method");
    }

    public static void main(String[] args) {
        Demo demo = null;
        demo.classMethod();
        //demo.objectMethod();// throws NPE if uncommented
    }
}

This code will work (even if the demo variable is null ) because static method classMethod is contained within the class Demo . The commented line will throw a NullPointerException because the method objectMethod is not contained in the class but in the object so will need an instance of Demo class to call it.

Answer 2

Subroutine is a method written inside a class. We use them to do various tasks. That statement states that these methods/subroutines are written in an object or a class.

If we have an object instantiated, it will create new methods for every non-static method for that object which were defined in the class of the object. Hence those non-static methods/subroutines are in the object.

But if the class is a static class, we can't have any objects from it. But we can use subroutines/methods of that class. So, they are in a Class

That's what your statement says.

EDIT:

I thought to give an example for this.

public class ExampleClass {

  public String getNonStaticString() {
    return "This String is From Non-Static Method";
  }

  public static String getStaticString() {
    return "This String is From Static Method"
  }
}

Then, if you need to get the static String, all you have to do is

String staticString = ExampleClass.getStaticString();

Note that I havn't created an object from the ExampleClass Here. I just used the method.

But, if you need to get the String from the non-static method, you should instantiate an object first.

ExampleClass exampleObject = new ExampleClass();
String nonStaticString = exampleObject.getNonStaticString();

Answer 3

static methods also known as class method. Static method associated only with the class and not with any specific instance of that class(object).

Answer 4

So, every subroutine is contained either in a class or in an object

The statement is technically not 100% correct.

First of all, subroutines in java are commonly called methods. The following two terms are often used interchangeably:

Method : A subroutine working with an object instance, this .

Function : A subroutine not working with an object instance.

---

Here is an example scenario which should you get an idea what that means:

 public class Circle { //region static code //we cannot call "this" in a static context, main(String[]) is no exception here public static void main(String[] args) { Circle a = new Circle(0, 0, 10); Circle b = new Circle(10, 10, 2); System.out.println("a = " + a); System.out.println("b = " + b); System.out.println("circumference of a = " + a.getCircumference()); System.out.println("circumference of b = " + b.getCircumference()); System.out.println("area of a = " + a.getArea()); System.out.println("area of b = " + b.getArea()); System.out.println("distance of a, b = " + distance(a, b)); System.out.println("a, b intersect = " + (intersects(a, b) ? "yes" : "no")); } //we cannot call "this" in a static context, but we have the circles a, b as parameters we can use to calculate their distance public static double distance(Circle a, Circle b) { return Math.sqrt(squared(ax - bx) + squared(ay - by)); } //we cannot call "this" in a static context, but we have the circles a, b as parameters we can use to check for an intersection public static boolean intersects(Circle a, Circle b) { return a.radius + b.radius > distance(a, b); } //we cannot call "this" in a static context, but we have the number x as parameter we can use to calculate the square of public static double squared(double x) { return x * x; } //we cannot call "this" in a static context, but we have the number radius as parameter we can use to check if its value is in range public static void checkRadius(double radius) { if(radius < 0) { throw new IllegalArgumentException("radius must be >= 0"); } } //endregion //region member / instance code private double x; private double y; private double radius; public Circle(double x, double y, double radius) { checkRadius(radius); this.x = x; this.y = y; this.radius = radius; } //region getters and setters //we may refer to the instance variables with or without "this", sometimes it is necessary to clarify - see: setX(double) public double getX() { return x; } //we may refer to the instance variables with or without "this", but in this case we have two variables with name "x" //if we write "x", the parameter is taken. for the circle's x coordinate, we need to clarify with "this.x" public void setX(double x) { this.x = x; } public double getY() { return y; } public void setY(double y) { this.y = y; } public double getRadius() { return radius; } public void setRadius(double radius) { checkRadius(radius); this.radius = radius; } //endregion //we may refer to the instance variables with or without "this", sometimes it is necessary to clarify - see: setX(double) public double getCircumference() { return 2 * Math.PI * radius; } public double getArea() { return Math.PI * squared(radius); } //we may refer to the instance variables with or without "this", sometimes it is necessary to clarify - see: setX(double) @Override public String toString() { return "circle at [" + x + ", " + y + "] with radius " + radius; } //endregion } 

Output :

 a = circle at [0.0, 0.0] with radius 10.0 b = circle at [10.0, 10.0] with radius 2.0 circumference of a = 62.83185307179586 circumference of b = 12.566370614359172 area of a = 314.1592653589793 area of b = 12.566370614359172 distance of a, b = 14.142135623730951 a, b intersect = no