An instance variable is a variable that is a member of an instance of a class (i.e., associated with something created with a new
), whereas a class variable is a member of the class itself.
Every instance of a class will have its own copy of an instance variable, whereas there is only one of each static (or class) variable, associated with the class itself.
What’s the difference between a class variable and an instance variable?
This test class illustrates the difference:
public class Test {
public static String classVariable = "I am associated with the class";
public String instanceVariable = "I am associated with the instance";
public void setText(String string){
this.instanceVariable = string;
}
public static void setClassText(String string){
classVariable = string;
}
public static void main(String[] args) {
Test test1 = new Test();
Test test2 = new Test();
// Change test1's instance variable
test1.setText("Changed");
System.out.println(test1.instanceVariable); // Prints "Changed"
// test2 is unaffected
System.out.println(test2.instanceVariable); // Prints "I am associated with the instance"
// Change class variable (associated with the class itself)
Test.setClassText("Changed class text");
System.out.println(Test.classVariable); // Prints "Changed class text"
// Can access static fields through an instance, but there still is only one
// (not best practice to access static variables through instance)
System.out.println(test1.classVariable); // Prints "Changed class text"
System.out.println(test2.classVariable); // Prints "Changed class text"
}
}