Programming Tutorials

String Conversion and toString() in Java

By: Mashoud in Java Tutorials on 2007-09-02  

When Java converts data into its string representation during concatenation, it does so by calling one of the overloaded versions of the string conversion method valueOf() defined by String. valueOf() is overloaded for all the simple types and for type Object. For the simple types, valueOf() returns a string that contains the human-readable equivalent of the value with which it is called. For objects, valueOf() calls the toString() method on the object. We will look more closely at valueOf() later in this chapter. Here, let's examine the toString() method, because it is the means by which you can determine the string representation for objects of classes that you create.

Every class implements toString() because it is defined by Object. However, the default implementation of toString() is seldom sufficient. For most important classes that you create, you will want to override toString() and provide your own string representations. Fortunately, this is easy to do. The toString() method has this general form:

String toString()

To implement toString(), simply return a String object that contains the human-readable string that appropriately describes an object of your class.

By overriding toString() for classes that you create, you allow the resulting strings to be fully integrated into Java's programming environment. For example, they can be used in print() and println() statements and in concatenation expressions. The following program demonstrates this by overriding toString() for the Box class:

// Override toString() for Box class.
class Box {
    double width;
    double height;
    double depth;

    Box(double w, double h, double d) {
        width = w;
        height = h;
        depth = d;
    }

    public String toString() {
        return "Dimensions are " + width + " by " +
                depth + " by " + height + ".";
    }
}

class toStringDemo {
    public static void main(String args[]) {
        Box b = new Box(10, 12, 14);
        String s = "Box b: " + b; // concatenate Box object
        System.out.println(b); // convert Box to string
        System.out.println(s);
    }
}

The output of this program is shown here:

Dimensions are 10 by 14 by 12.
Box b: Dimensions are 10 by 14 by 12.

As you can see, Box's toString() method is automatically invoked when a Box object is used in a concatenation expression or in a call to println().