static Keyword in Java
The static keyword in Java is mainly used for memory management. The static keyword in Java is used to share the same variable or method of a given class. The users can apply static keywords with variables, methods, blocks, and nested classes. The static keyword belongs to the class than an instance of the class. The static keyword is used for a constant variable or a method that is the same for every instance of a class.
The static keyword is a non-access modifier in Java that is applicable for the following:
Note: To create a static member(block, variable, method, nested class), precede its declaration with the keyword static.
When a member is declared static, it can be accessed before any objects of its class are created, and without reference to any object. For example, in the below java program, we are accessing static method m1() without creating any object of the Test class.
If you need to do the computation in order to initialize your static variables, you can declare a static block that gets executed exactly once, when the class is first loaded.
Consider the following java program demonstrating the use of static blocks.
Static block initialized. from main Value of a : 10 Value of b : 40
For a detailed article on static blocks, see static blocks
When a variable is declared as static, then a single copy of the variable is created and shared among all objects at the class level. Static variables are, essentially, global variables. All instances of the class share the same static variable.
Important points for static variables:
- We can create static variables at the class level only. See here
- static block and static variables are executed in the order they are present in a program.
Below is the Java program to demonstrate that static block and static variables are executed in the order they are present in a program.
from m1 Inside static block Value of a : 20 from main
When a method is declared with the static keyword, it is known as the static method. The most common example of a static method is the main( ) method. As discussed above, Any static member can be accessed before any objects of its class are created, and without reference to any object. Methods declared as static have several restrictions:
- They can only directly call other static methods.
- They can only directly access static data.
- They cannot refer to this or super in any way.
Below is the java program to demonstrate restrictions on static methods.
prog.java:18: error: non-static variable b cannot be referenced from a static context b = 10; // compilation error ^ prog.java:22: error: non-static method m2() cannot be referenced from a static context m2(); // compilation error ^ prog.java:25: error: non-static variable super cannot be referenced from a static context System.out.println(super.a); // compiler error ^ prog.java:25: error: cannot find symbol System.out.println(super.a); // compiler error ^ symbol: variable a 4 errors
When to use static variables and methods?
Use the static variable for the property that is common to all objects. For example, in class Student, all students share the same college name. Use static methods for changing static variables.
Consider the following java program, that illustrates the use of static keywords with variables and methods.
name : Alice rollNo : 1 cllgName : XYZ name : Bob rollNo : 2 cllgName : XYZ
A class can be made static only if it is a nested class. We cannot declare a top-level class with a static modifier but can declare nested classes as static. Such types of classes are called Nested static classes. Nested static class doesn’t need a reference of Outer class. In this case, a static class cannot access non-static members of the Outer class.
Note: For static nested class, see a static nested class in java
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