javastringobjecttostring

How do I print my Java object without getting "SomeType@2f92e0f4"?

I have a class defined as follows:

public class Person {
  private String name;

  // constructor and getter/setter omitted
}

I tried to print an instance of my class:

System.out.println(myPerson);

but I got the following output: com.foo.Person@2f92e0f4.

A similar thing happened when I tried to print an array of Person objects:

Person[] people = //...
System.out.println(people);

I got the output: [Lcom.foo.Person;@28a418fc

What does this output mean? How do I change this output so it contains the name of my person? And how do I print collections of my objects?

Note: this is intended as a canonical Q&A about this subject.

Solution

  • Background

    All Java objects have a toString() method, which is invoked when you try to print the object.

    System.out.println(myObject);  // invokes myObject.toString()

    This method is defined in the Object class (the superclass of all Java objects). The Object.toString() method returns a fairly ugly looking string, composed of the name of the class, an @ symbol and the hashcode of the object in hexadecimal. The code for this looks like:

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

    A result such as com.foo.MyType@2f92e0f4 can therefore be explained as:

    The name of array classes look a little different, which is explained well in the Javadocs for Class.getName(). For instance, [Ljava.lang.String means:

    Customizing the Output

    To print something different when you call System.out.println(myObject), you must override the toString() method in your own class. Here's a simple example:

    public class Person {

  private String name;
  
  // constructors and other methods omitted
  
  @Override
  public String toString() {
    return name;
  }
}

    Now if we print a Person, we see their name rather than com.foo.Person@12345678.

    Bear in mind that toString() is just one way for an object to be converted to a string. Typically this output should fully describe your object in a clear and concise manner. A better toString() for our Person class might be:

    @Override
public String toString() {
  return getClass().getSimpleName() + "[name=" + name + "]";
}

    Which would print, e.g., Person[name=Henry]. That's a really useful piece of data for debugging/testing.

    If you want to focus on just one aspect of your object or include a lot of jazzy formatting, you might be better to define a separate method instead, e.g. String toElegantReport() {...}.

    Auto-generating the Output

    Many IDEs offer support for auto-generating a toString() method, based on the fields in the class. See docs for Eclipse and IntelliJ, for example.

    Several popular Java libraries offer this feature as well. Some examples include:

    Printing groups of objects

    So you've created a nice toString() for your class. What happens if that class is placed into an array or a collection?

    Arrays

    If you have an array of objects, you can call Arrays.toString() to produce a simple representation of the contents of the array. For instance, consider this array of Person objects:

    Person[] people = { new Person("Fred"), new Person("Mike") };
System.out.println(Arrays.toString(people));

// Prints: [Fred, Mike]

    Note: this is a call to a static method called toString() in the Arrays class, which is different to what we've been discussing above.

    If you have a multi-dimensional array, you can use Arrays.deepToString() to achieve the same sort of output.

    Collections

    Most collections will produce a pretty output based on calling .toString() on every element.

    List<Person> people = new ArrayList<>();
people.add(new Person("Alice"));
people.add(new Person("Bob"));    
System.out.println(people);

// Prints [Alice, Bob]

    So you just need to ensure your list elements define a nice toString() as discussed above.