[21 Feb 2020] Java – Static Class, Block, Methods and Variables

Static keyword can be used with class, variable, method and block. Static members belong to the class instead of a specific instance, this means if you make a member static, you can access it without object. Let’s take an example to understand this:

Here we have a static method myMethod(), we can call this method without any object because when we make a member static it becomes class level. If we remove the static keyword and make it non-static then we must need to create an object of the class in order to call it.

Static members are common for all the instances(objects) of the class but non-static members are separate for each instance of class.

class SimpleStaticExample
{
    // This is a static method
    static void myMethod()
    {
        System.out.println("myMethod");
    }
 
    public static void main(String[] args)
    {
          /* You can see that we are calling this
           * method without creating any object. 
           */
           myMethod();
    }
}

Output:

myMethod

Static Block

Static block is used for initializing the static variables.This block gets executed when the class is loaded in the memory. A class can have multiple Static blocks, which will execute in the same sequence in which they have been written into the program.

Example 1: Single static block

As you can see that both the static variables were intialized before we accessed them in the main method.

class JavaExample{
   static int num;
   static String mystr;
   static{
      num = 97;
      mystr = "Static keyword in Java";
   }
   public static void main(String args[])
   {
      System.out.println("Value of num: "+num);
      System.out.println("Value of mystr: "+mystr);
   }
}

Output:

Value of num: 97
Value of mystr: Static keyword in Java

Example 2: Multiple Static blocks

Lets see how multiple static blocks work in Java. They execute in the given order which means the first static block executes before second static block. That’s the reason, values initialized by first block are overwritten by second block.

class JavaExample2{
   static int num;
   static String mystr;
   //First Static block
   static{
      System.out.println("Static Block 1");
      num = 68;
      mystr = "Block1";
  } 
  //Second static block
  static{
      System.out.println("Static Block 2");
      num = 98;
      mystr = "Block2";
  }
  public static void main(String args[])
  {
      System.out.println("Value of num: "+num);
      System.out.println("Value of mystr: "+mystr);
   }
}

Output:

Static Block 1
Static Block 2
Value of num: 98
Value of mystr: Block2

Java Static Variables

A static variable is common to all the instances (or objects) of the class because it is a class level variable. In other words you can say that only a single copy of static variable is created and shared among all the instances of the class. Memory allocation for such variables only happens once when the class is loaded in the memory.
Few Important Points:

Static variables are also known as Class Variables.
Unlike non-static variables, such variables can be accessed directly in static and non-static methods.

Example 1: Static variables can be accessed directly in Static method

Here we have a static method disp() and two static variables var1 and var2. Both the variables are accessed directly in the static method.

class JavaExample3{
  static int var1;
  static String var2;
  //This is a Static Method
  static void disp(){
      System.out.println("Var1 is: "+var1);
      System.out.println("Var2 is: "+var2);
  }
  public static void main(String args[]) 
  {
      disp();
  }
}

Output:

Var1 is: 0
Var2 is: null

Example 2: Static variables are shared among all the instances of class

In this example, String variable is non-static and integer variable is Static. As you can see in the output that the non-static variable is different for both the objects but the static variable is shared among them, thats the reason the changes made to the static variable by object ob2 reflects in both the objects.

class JavaExample{
   //Static integer variable
   static int var1=77; 
   //non-static string variable
   String var2;

   public static void main(String args[])
   {
 JavaExample ob1 = new JavaExample();
 JavaExample ob2 = new JavaExample();
 /* static variables can be accessed directly without
  * any instances. Just to demonstrate that static variables
  * are shared, I am accessing them using objects so that 
  * we can check that the changes made to static variables
  * by one object, reflects when we access them using other
  * objects
  */
        //Assigning the value to static variable using object ob1
 ob1.var1=88;
 ob1.var2="I'm Object1";
        /* This will overwrite the value of var1 because var1 has a single 
         * copy shared among both the objects.
         */
        ob2.var1=99;
 ob2.var2="I'm Object2";
 System.out.println("ob1 integer:"+ob1.var1);
 System.out.println("ob1 String:"+ob1.var2);
 System.out.println("ob2 integer:"+ob2.var1);
 System.out.println("ob2 STring:"+ob2.var2);
   }
}

Output:

ob1 integer:99
ob1 String:I'm Object1
ob2 integer:99
ob2 STring:I'm Object2

For more details on refer: Java – static variable

Java Static Methods

Static Methods can access class variables(static variables) without using object(instance) of the class, however non-static methods and non-static variables can only be accessed using objects.
Static methods can be accessed directly in static and non-static methods.
Syntax:
Static keyword followed by return type, followed by method name.

static return_type method_name();

Example 1: static method main is accessing static variables without object

class JavaExample{
   static int i = 10;
   static String s = "Beginnersbook";
   //This is a static method
   public static void main(String args[]) 
   {
       System.out.println("i:"+i);
       System.out.println("s:"+s);
   }
}

Output:

i:10
s:Beginnersbook

Example 2: Static method accessed directly in static and non-static method

class JavaExample{
  static int i = 100;
  static String s = "Beginnersbook";
  //Static method
  static void display()
  {
     System.out.println("i:"+i);
     System.out.println("i:"+s);
  }

  //non-static method
  void funcn()
  {
      //Static method called in non-static method
      display();
  }
  //static method
  public static void main(String args[])
  {
   JavaExample obj = new JavaExample();
   //You need to have object to call this non-static method
   obj.funcn();
   
      //Static method called in another static method
      display();
   }
}

Output:

i:100
i:Beginnersbook
i:100
i:Beginnersbook

Static Class

A class can be made static only if it is a nested class.

Nested static class doesn’t need reference of Outer class
A static class cannot access non-static members of the Outer class

We will see these two points with the help of an example:

Static class Example

class JavaExample{
   private static String str = "BeginnersBook";

   //Static class
   static class MyNestedClass{
 //non-static method
 public void disp() {

    /* If you make the str variable of outer class
     * non-static then you will get compilation error
     * because: a nested static class cannot access non-
     * static members of the outer class.
     */
    System.out.println(str); 
 }

   }
   public static void main(String args[])
   {
       /* To create instance of nested class we didn't need the outer
 * class instance but for a regular nested class you would need 
 * to create an instance of outer class first
        */
 JavaExample.MyNestedClass obj = new JavaExample.MyNestedClass();
 obj.disp();
   }
}

Output:

BeginnersBook

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