toString

Every object has a toString method, which is defined in java.lang.Object.

Methods like println can count on being able to call toString on any object.

• Object's default toString returns the object's classname, an @, and a number (the object's hashcode).

• This provides no information about the object's state - the current values of its fields.

Drill

StringAndStringBuilder/com.example.stringstringbuilder.drills.UserCommentDriver * Run UserCommentDriver and examine the default toString output of UserComment.

toString is important for log messages, debugging, output in character user interfaces (console programs), etc.

• You should always provide a toString in classes you create, overriding Object's default, for classes that represent data (as opposed to a driver class with nothing but a main.)

Your toString should include all interesting state information of your object.

In general, the toString method returns a string that "textually represents" this object. The result should be a concise but informative representation that is easy for a person to read. It is recommended that all subclasses override this method.¹

public class Sphere {
  private double radius;
  @Override
  public String toString() {
    return "A sphere, with a radius of " + radius;
  }
//...
}

As your class definition evolves and you add more instance fields, you must decide which need to be included in your toString.

public class Sphere {
  private double radius;
  private String color;
  @Override
  public String toString() {
    return "A " + color + " sphere, with a radius of " + radius;
  }
//...
}

* Again, simple String concatenation causes unnecessary object creation and garbage collection, so consider using StringBuilder.

public class Sphere {
  private double radius;
  private String color;
  @Override
  public String toString() {
    StringBuilder builder = new StringBuilder(100);
    builder.append("A ").append(color).append(" sphere, with a radius of ")
           .append(radius);
    return builder.toString();
  }
//...
}

Practice Exercise

Should I always use StringBuilder?

In simple cases of building text from a just few literals and variables there may be little difference between using String concatenation and using a StringBuilder. For code that runs often, such as inside a loop or a frequently-called method like toString, or for text built from many variables and literals, accumulation of temporary String objects from concatenation can result in additional garbage collection that impacts performance. Also, on devices with limited memory and processing power - say, an Android phone or a piece of test equipment - every optimization counts.

Don't feel guilty for using "Hello" + " World" in your code, but consider StringBuilder anywhere text manipulation is important.

[1] https://docs.oracle.com/javase/8/docs/api/java/lang/Object.html#toString

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