Java: when to use static methods

Question

I am wondering when to use static methods? Say if I have a class with a few getters and setters, a method or two, and I want those methods only to be invokable on an instance object of the class. Does this mean I should use a static method?

e.g

Obj x = new Obj();
x.someMethod

or

Obj.someMethod

(is this the static way?)

I'm rather confused!

Answer 1

I am wondering when to use static methods?

1. A common use for static methods is to access static fields.

2. But you can have static methods, without referencing static variables. Helper methods without referring static variable can be found in some java classes like java.lang.Math

   public static int min(int a, int b) {
       return (a <= b) ? a : b;
   }
   

3. The other use case, I can think of these methods combined with synchronized method is implementation of class level locking in multi threaded environment.

Say if I have a class with a few getters and setters, a method or two, and I want those methods only to be invokable on an instance object of the class. Does this mean I should use a static method?

If you need to access method on an instance object of the class, your method should should be non static.

Oracle documentation page provides more details.

Not all combinations of instance and class variables and methods are allowed:

1. Instance methods can access instance variables and instance methods directly.
2. Instance methods can access class variables and class methods directly.
3. Class methods can access class variables and class methods directly.
4. Class methods cannot access instance variables or instance methods directly—they must use an object reference. Also, class methods cannot use the this keyword as there is no instance for this to refer to.

Answer 2

In eclipse you can enable a warning which helps you detect potential static methods. (Above the highlighted line is another one I forgot to highlight)

Answer 3

A static method is one type of method which doesn't need any object to be initialized for it to be called. Have you noticed static is used in the main function in Java? Program execution begins from there without an object being created.

Consider the following example:

 class Languages 
 {
     public static void main(String[] args) 
     {
         display();
     }

     static void display() 
     {
         System.out.println("Java is my favorite programming language.");
     }
  }

Answer 4

A static method has two main purposes:

1. For utility or helper methods that don't require any object state. Since there is no need to access instance variables, having static methods eliminates the need for the caller to instantiate the object just to call the method.
2. For the state that is shared by all instances of the class, like a counter. All instance must share the same state. Methods that merely use that state should be static as well.

Answer 5

If you apply static keyword with any method, it is known as static method.

1. A static method belongs to the class rather than object of a class.
2. A static method invoked without the need for creating an instance of a class.
3. static method can access static data member and can change the value of it.

//Program of changing the common property of all objects(static field).

class Student9{  
 int rollno;  
 String name;  
 static String college = "ITS";  

 static void change(){  
 college = "BBDIT";  
 }  

 Student9(int r, String n){  
 rollno = r;  
 name = n;  
 }  

 void display (){System.out.println(rollno+" "+name+" "+college);}  

public static void main(String args[]){  
Student9.change();  

Student9 s1 = new Student9 (111,"Indian");  
Student9 s2 = new Student9 (222,"American");  
Student9 s3 = new Student9 (333,"China");  

s1.display();  
s2.display();  
s3.display();  
}  }

O/P: 111 Indian BBDIT 222 American BBDIT 333 China BBDIT

Answer 6

No, static methods aren't associated with an instance; they belong to the class. Static methods are your second example; instance methods are the first.