问题
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.
回答1:
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, i.e. the class isMyTypein the packagecom.foo.@- joins the string together2f92e0f4the 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 interfacejava.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<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.
回答2:
I think apache provides a better util class which provides a function to get the string
ReflectionToStringBuilder.toString(object)
回答3:
Every class in Java has the toString() method in it by default, which is called if you pass some object of that class to System.out.println(). By default, this call 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);
}
}
回答4:
In Eclipse,
Go to your class,
Right click->source->Generate toString();
It will override the toString() method and will print the object of that class.
回答5:
I prefer to use a utility function which uses GSON to de-serialize the Java object into JSON string.
/**
* This class provides basic/common functionalities to be applied on Java Objects.
*/
public final class ObjectUtils {
private static final Gson GSON = new GsonBuilder().setPrettyPrinting().create();
private ObjectUtils() {
throw new UnsupportedOperationException("Instantiation of this class is not permitted in case you are using reflection.");
}
/**
* This method is responsible for de-serializing the Java Object into Json String.
*
* @param object Object to be de-serialized.
* @return String
*/
public static String deserializeObjectToString(final Object object) {
return GSON.toJson(object);
}
}
回答6:
In intellij you can auto generate toString method by pressing alt+inset and then selecting toString() here is an out put for a test class:
public class test {
int a;
char b;
String c;
Test2 test2;
@Override
public String toString() {
return "test{" +
"a=" + a +
", b=" + b +
", c='" + c + '\'' +
", test2=" + test2 +
'}';
}
}
As you can see, it generates a String by concatenating, several attributes of the class, for primitives it will print their values and for reference types it will use their class type (in this case to string method of Test2).
回答7:
By default, every Object in Java has the toString() method which outputs the ObjectType@HashCode.
If you want more meaningfull information then you need to override the toString() method in your class.
public class Person {
private String name;
// constructor and getter/setter omitted
// overridding toString() to print name
public String toString(){
return name;
}
}
Now when you print the person object using System.out.prtinln(personObj); it will print the name of the person instead of the classname and hashcode.
In your second case when you are trying to print the array, it prints [Lcom.foo.Person;@28a418fc the Array type and it's hashcode.
If you want to print the person names, there are many ways.
You could write your own function that iterates each person and prints
void printPersonArray(Person[] persons){
for(Person person: persons){
System.out.println(person);
}
}
You could print it using Arrays.toString(). This seems the simplest to me.
System.out.println(Arrays.toString(persons));
System.out.println(Arrays.deepToString(persons)); // for nested arrays
You could print it the java 8 way (using streams and method reference).
Arrays.stream(persons).forEach(System.out::println);
There might be other ways as well. Hope this helps. :)
回答8:
If you Directly print any object of Person It will the ClassName@HashCode to the Code.
in your case com.foo.Person@2f92e0f4 is getting printed . Where Person is a class to which object belongs and 2f92e0f4 is hashCode of the Object.
public class Person {
private String name;
public Person(String name){
this.name = name;
}
// getter/setter omitted
@override
public String toString(){
return name;
}
}
Now if you try to Use the object of Person then it will print the name
Class Test
{
public static void main(String... args){
Person obj = new Person("YourName");
System.out.println(obj.toString());
}
}
回答9:
If you look at the Object class (Parent class of all classes in Java) the toString() method implementation is
public String toString() {
return getClass().getName() + "@" + Integer.toHexString(hashCode());
}
whenever you print any object in Java then toString() will be call. Now it's up to you if you override toString() then your method will call other Object class method call.
回答10:
Arrays.deepToString(arrayOfObject)
Above function print array of object of different primitives.
[[AAAAA, BBBBB], [6, 12], [2003-04-01 00:00:00.0, 2003-10-01 00:00:00.0], [2003-09-30 00:00:00.0, 2004-03-31 00:00:00.0], [Interim, Interim], [2003-09-30, 2004-03-31]];
来源：https://stackoverflow.com/questions/37403097/java-how-to-pass-an-object-as-a-parameter-to-different-classes