My assignment is to make a program with an instance variable, a string, that should be input by the user. But I don't even know what an instance variable is. What is an instance variable?
How do I create one? What does it do?
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2docs.oracle.com/javase/tutorial/java/javaOO/classvars.htmlMaroun– Maroun2013-05-22 08:04:44 +00:00Commented May 22, 2013 at 8:04
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2 Answers
Instance variable is the variable declared inside a class, but outside a method: something like:
class IronMan {
/** These are all instance variables **/
public String realName;
public String[] superPowers;
public int age;
/** Getters and setters here **/
}
Now this IronMan Class can be instantiated in another class to use these variables. Something like:
class Avengers {
public static void main(String[] a) {
IronMan ironman = new IronMan();
ironman.realName = "Tony Stark";
// or
ironman.setAge(30);
}
}
This is how we use the instance variables. Shameless plug: This example was pulled from this free e-book here here.
Comments
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"
}
}
2 Comments
vikingsteve
correct. You can also think of an instance variable as a
field in an object. A relevant concept is encapsulation (see: private access modifier, getters and setters...)
Richard Tingle
Indeed, i have declared most things public for easy access, this is usually a bad idea