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:
com.foo.MyType – the name of the class, i.e. the class is MyType in the package com.foo.
@ – joins the string together
2f92e0f4 the hashcode of the object.
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:
[ – an single-dimensional array (as opposed to [[ or [[[ etc.)
L – the array contains a class or interface
java.lang.String – the type of objects in the array
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:
ToStringBuilder from Apache Commons Lang
MoreObjects.ToStringHelper from Google Guava
@ToString annotation from Project Lombok
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
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.
I think apache provides a better util class which provides a function to get the string
ReflectionToStringBuilder.toString(object)