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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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.

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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" }
}

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