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In the last two lessons we looked at static variables and their scope and static methods and their usage. In our final lesson on statics we investigate Java constants and how to use static initializer blocks.

Java Constants Top

We assign a java constant (a value that never changes) through the use of the static and final keywords. You must always assign a value to a static variable marked as final before you use it or the compiler complains. A static variable that is marked as final can never change once it has been assigned a value. So once assigned a value, trying to change a variable marked as final will cause a compiler error.

You can also use the final keyword to create final local variables, final instance variables, within parameter lists and for preventing inheritance/overriding.

So lets use some code to look at creating some Java constants.

package com.server2client;
/*
  Test class for Java Constants
*/ 
public class JavaConstants {
    public static final double EULERS_CONSTANT = 2.718281828459045;  // Base of natural logarithms 
    public static final int SQUARE_OF_THREE;  // 3 * 3 

    public static void main (String[] args) {
        System.out.println("Euler's Constant = " + EULERS_CONSTANT);
        System.out.println("Three squared + " + SQUARE_OF_THREE);
    }
}

The above screenshot shows the output of running the JavaConstants class. We got a compiler error as we forgot to initialize SQUARE_OF_THREE. Ok lets change the JavaConstants class.

package com.server2client;
/*
  Test class for Java Constants
*/ 
public class JavaConstants {
    public static final double EULERS_CONSTANT = 2.718281828459045;  // Base of natural logarithms 
    public static final int SQUARE_OF_THREE = 9;  // 3 * 3 

    public static void main (String[] args) {
        System.out.println("Euler's Constant = " + EULERS_CONSTANT);
        SQUARE_OF_THREE = 9;    // Here we set the value of SQUARE_OF_THREE again 
        System.out.println("Three squared + " + SQUARE_OF_THREE);
    }
}

The above screenshot shows the output of rerunning the JavaConstants class. We got a compiler error as we initialized SQUARE_OF_THREE, and then tried to assign a value to if after this which we can't do. Lets remove the second assignment:

package com.server2client;
/*
  Test class for Java Constants
*/ 
public class JavaConstants {
    public static final double EULERS_CONSTANT = 2.718281828459045;  // Base of natural logarithms 
    public static final int SQUARE_OF_THREE = 9;  // 3 * 3 

    public static void main (String[] args) {
        System.out.println("Euler's Constant = " + EULERS_CONSTANT);
        System.out.println("Three squared + " + SQUARE_OF_THREE);
    }
}

Save, compile and run the file in directory   c:\_ObjectsAndClasses in the usual way.

The above screenshot shows the output of running the JavaConstants class, it now works fine.

Static Initializer Blocks Top

You can only initialize static variables in two ways: when you declare them or in a static initializer block. Static initializer blocks run as soon as a class is loaded, before static variables and static methods and are used to action things we want done before anything else. Lets see how static initializer blocks work with our Java constants:

package com.server2client;
/*
  Test class for Java Constants
*/ 
public class JavaConstants2 {
    public static final double EULERS_CONSTANT; 
    public static final int SQUARE_OF_THREE;

    /*
      Static Initializer Block
    */ 
    static {
        EULERS_CONSTANT = 2.718281828459045;
        SQUARE_OF_THREE = 9;
    }
    
    public static void main (String[] args) {
        System.out.println("Euler's Constant = " + EULERS_CONSTANT);
        System.out.println("Three squared + " + SQUARE_OF_THREE);
    }
}

The above screenshot shows the output of running the JavaConstants2 class.

Java Constants Checklist Top

• There is no way to designate a constant in Java; so the convention is to use all uppercase letters and separate words using the underscore ( _ ) symbol when defining a constant.
• Although the static and final keywords can go either way round the offical documentation offers advice on the order of field modifiers as follows:
  Annotation public protected private static final transient volatile.
• You must initialize any static variable marked final before use, or the compiler throws an error. This can be done when declaring the static variable or in a static initailizer block.
• Once initialized any static variable marked final cannot have its value changed elsewhere or the compiler throws an error.

Related Quiz

Objects & Classes Quiz 14 - Java Constants

Lesson 14 Complete

In this lesson we investigated Java constants and how to use static initializer blocks.

What's Next?

In our final lesson of this section we go through enumerations which were introduced in Java5.

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• Basic Enumerations     >>