Many compiler errors are generated because programmers don’t have a clear image on how long variables are available and when they can access them. The variable scope concept describes the life of a variable, or its availability area, after it was defined.
Other topics that are part of this Java tutorial are accessible through Java 6 Tutorial – Contents.
In order to understand this topic, let’s consider an example:
class Student{ public static int noStudents = 0; //static class variable public int age; //instance variable public int[] marks; //instance variable public Student() { noStudents++; //accessing static variables this.age = 0; marks = new int[10]; } public float getAverage() { float sum = 0; //local variable for(int i = 0;i < marks.length;) { //i is local variable in the for block sum+=marks[i]; i++; } return sum/marks.length; } } public class Main{ public static void main(String[] args) { Student s1 = new Student(); Student s2 = new Student(); //accessing static variables System.out.println("The are "+Student.noStudents+" students"); } }
Each type of variable has its own scope:
Instance variables (attributes) – these variables are part of an object, so, they are created when the object is created; they exists until the object is created; the object and its methods have access to its instance variables;
Static variables – these variables are part of a class; they are created when the class is loaded by the JVM;
Methods local variables – these variables are defined on the method stack and they exists as long as the method is executed (it is placed on the call stack); even the local variables can be accessed, you can’t use them from a nested method (Tutorial Java – #8 Understand Stack and Heap);
Block variables – these variables are defined inside blocks of code (between { and }) and can be used while the block is executed; typical blocks of code are for, while, initialization block.
What are common errors regarding variables scope – Cannot find symbol compiler error
Cannot find symbol compiler error is generated when you try to use a block variable outside the block:
public float getAverage() { float sum = 0; //local variable for(int i = 0;i < marks.length;) { //i is local variable in the for block sum+=marks[i]; i++; } //Cannot find symbol compiler error i = 0; //COMPILER ERROR return sum/marks.length; }
Another case in which you get the same Cannot find symbol compiler error is when you try to access a method local variable from a nested method:
public static void DoSomething() { int variable = 0; //local variable DoSomethingElse(); //nested method } public static void DoSomethingElse() { //Cannot find symbol compiler error variable++; //COMPILER ERROR }
You get the same error when you try to access instance variables or methods from a static context, like the main method.
public static void main(String[] args) { Student s1 = new Student(); Student s2 = new Student(); //accessing static variables System.out.println("The are "+Student.noStudents+" students"); //Cannot find symbol compiler error age = 23; //COMPILER ERROR getAverage(); //COMPILER ERROR }
Other topics that are part of this Java tutorial are accessible through Java 6 Tutorial – Contents.
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