Displaying objects #

The class Object #

Java has a native class called Object with 11 instance methods. Every Java class implicitly extends Object, including user-defined classes. Therefore any Java class inherits these methods.

The method toString #

toString is one of the methods of the class Object. Here is (part of) its specification:

/** Returns a string representation of the object.
 * In general, the toString method returns a string that "textually represents" this object.
 */
public String toString();

This is the preferred way to display information about an object. For instance, the debugger of your IDE probably relies on this method to display a string that represents an object.

This method is also called by many (native or not) methods and operators. For instance, the following program

Unicorn myUnicorn = new Unicorn("green");
System.out.println(myUnicorn);
String message = "Hello " + myUnicorn;

is equivalent to

Unicorn myUnicorn = new Unicorn("green");
System.out.println(myUnicorn.toString());
String message = "Hello " + myUnicorn.toString();

Overriding toString #

The default implementation of toString (in the class Object) displays very little information about the object:

public String toString() {
    return getClass().getName() + "@" +
      Integer.toHexString(hashCode());
}

Warning. The output of hashCode() in this example is not the reference (a.k.a. “memory address”) of the object.

So a common practice consists in overriding toString. For instance, here is a possible implementation of the method toString within a class Unicorn:

public class Unicorn {

    String color;
    int health;

    ...

    @Override
    public String toString() {
        return "Unicorn{" +
            "color=" + color + ", " +
            "health=" + health +
        "}";
    }
}

Or better (if MobileUnit is a superclass of Unicorn and Butterfly, as in our previous example):

public class MobileUnit extends Unit {

    String color;

    ...

    @Override
    public String toString() {
        return getClass().getName() + "{" +
            "color=" + color + ", " +
            "health=" + health +
        "}";
    }
}

Hint. Your IDE can generate such a method.

Recursion #

A naive implementation of toString (e.g. generated by an IDE) may have unexpected effects.

public class Column {

    Unit[] units;
    // null if this is the left-most column
    Column leftAdjacentColumn;
    // null if this is the right-most column
    Column rightAdjacentColumn;

    public Column(Unit[] units){
        this.units = units;
    }

    @Override
    public String toString() {
        return "Column{" +
            "units=" + units + ", " +
            "left=" + leftAdjacentColumn + ", " +
            "right=" + rightAdjacentColumn +
        "}";
    }
}

Unit[] a1 = new Unit[]{ new Unicorn("green") };
Column c1 = new Column(a1);
Unit[] a2 = new Unit[]{ new Butterfly("yellow") };
Column c2 = new Column(a2);
c1.rightAdjacentColumn = c2;
c2.leftAdjacentColumn = c1;

System.out.println(c1);