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.
Background
All Java objects have a toString() method, which is invoked when you try and 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, ie 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, eg, 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, eg 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<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.
我认为apache提供了一个更好的util类，它提供了一个获取字符串的函数
ReflectionToStringBuilder.toString(object)
Every class in java has toString() method in it by default, which is called by System.out.println() if you pass some object of a class to it. When you try to print object of a class, the System.out.println() method will call toString() of the class which returns the className@hashcode of that object.
{
SomeClass sc = new SomeClass();
// Class @ followed by hashcode of object in Hexadecimal
System.out.println(sc);
}
You can override the toString method of a class to get different output. See this example
class A {
String s = "I am just a object";
@Override
public String toString()
{
return s;
}
}
class B {
public static void main(String args[])
{
A obj = new A();
System.out.println(obj);
}
}
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