Static Method Summary

Class Methods (Static Methods)

• Methods associated with class rather than instances
• Declared using static keyword
• Can be called without creating class instance
• Cannot access instance fields/methods directly
• Cannot use this keyword
• Example:

  class Math {
    public static double sqrt(double x) {
      // compute square root
    }
  }
  

Static vs Non-Static Methods

• Static methods:

  • Belong to class
  • Can only access static members
  • Called through class name
  • No access to this
    • Static method exists without instantiating the class
    • this refers to a specific object instance, which static methods do not have

• Non-static methods:

  • Belong to instances
  • Can access all members
  • Called through instance
  • Has access to this

Non-Static from Static Context

• Static methods cannot access:
  • Instance fields
  • Instance methods
  • this reference
• Example of invalid code:

  class A {
    private int x;
    public static void foo() {
      this.x = 1;  // Error: cannot use `this`
      x = 1;       // Error: cannot access instance field
    }
  }
  

The main Method

• Special static method as program entry point
• Required signature:

  public static void main(String[] args)
  

• Characteristics:
  • Must be public
  • Must be static
  • Must return void
  • Must take String array parameter
  • Name must be main

Command Line Arguments

• Passed to String[] array of main method
• args parameter stores command line arguments
• Example:

  public static void main(String[] args) {
    // args[0] is first argument
    // args[1] is second argument
    // etc.
  }
  

Factory Methods

• Static methods that create objects
• Benefits:
• When object creation needs to be controlled
• Better naming than constructors
• Can return cached instances

• Can return different subtypes

• Example:

  public class Color {
    private final int red;
    private final int green;
    private final int blue;
  
    private Color(int r, int g, int b) {
      red = r;
      green = g;
      blue = b;
    }
  
    public static Color of(int r, int g, int b) {
      // Validation and caching logic here
      return new Color(r, g, b);
    }
  
  }
  

Best Practices

• Use static methods for:
  • Utility functions that perform general operations and don't depend on instance state (data)
    • Example:

      class MathUtils {
        public static int square(int x) {
          return x * x;
        }
      }
      

  • Accessing or managing static fields
    • Example:

      class Circle {
        private final int id;
        private static int lastId = 0;
      
        public Circle() {
          this.id = Circle.lastId;
          Circle.lastId += 1;
        }
      
        // Access static field
        public static int getNumOfCircles() {
          return Circle.lastId;
        }
      }
      

• Keep static methods independent of instance state
• Consider making utility classes final
• Document command line arguments clearly