How to Get the Array Class for a Given Class in Java

How to get the Array Class for a given Class in Java?

Since Java 12, there is the arrayType() method on java.lang.Class. With that:

Class<?> arrayOfFooClass = fooClass.arrayType();

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The implementation of Class.arrayType() just calls Arrays.newInstance(this, 0).getClass().

Java reflection getting array class of a given class

I think you want Array.newInstance(c, 0).getClass().

Getting class of array from class object

A simple method has been added to the API as late as JDK 12:

Class<?> arrayType = pojo.arrayType();

Before JDK 12, you needed a work-around like

Class<?> arrayType = Array.newInstance(pojo, 0).getClass();

using the old Array.newInstance(Class, int) method. This actually creates a zero length array of the type specified by pojo, then gets its Class.

Obtaining the array Class of a component type

One thing that comes to mind is:

java.lang.reflect.Array.newInstance(componentType, 0).getClass();

But it creates an unnecessary instance.

Btw, this appears to work:

Class clazz = Class.forName("[L" + componentType.getName() + ";");

Here is test. It prints true:

Integer[] ar = new Integer[1];
Class componentType = ar.getClass().getComponentType();
Class clazz = Class.forName("[L" + componentType.getName() + ";");

System.out.println(clazz == ar.getClass());

The documentation of Class#getName() defines strictly the format of array class names:

If this class object represents a class of arrays, then the internal form of the name consists of the name of the element type preceded by one or more '[' characters representing the depth of the array nesting.

The Class.forName(..) approach won't directly work for primitives though - for them you'd have to create a mapping between the name (int) and the array shorthand - (I)

Getting the class of the components of an array

Component Type

Use this:

array.getClass().getComponentType()

Returns the Class representing the
component type of an array. If this
class does not represent an array
class this method returns null.

Reference:

• Class.getComponentType()

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Safe / Unsafe casting

Is there a way I can cast to Class
from Class returned by
getComponentType() without getting a
compiler warning?

take this method:

public <T> void doSomething(final T[] array) throws Exception{
    final Class<? extends Object[]> arrayClass = array.getClass();
    final Class<?> componentType = arrayClass.getComponentType();
    final T newInstance = (T) componentType.newInstance();
}

Here's the generated byte code:

public void doSomething(java.lang.Object[] array) throws java.lang.Exception;
     0  aload_1 [array]
     1  invokevirtual java.lang.Object.getClass() : java.lang.Class [21]
     4  astore_2 [arrayClass]
     5  aload_2 [arrayClass]
     6  invokevirtual java.lang.Class.getComponentType() : java.lang.Class [25]
     9  astore_3 [componentType]
    10  aload_3 [componentType]
    11  invokevirtual java.lang.Class.newInstance() : java.lang.Object [30]
    14  astore 4 [newInstance]
    16  return

As you can see, the parameter type is erased to Object[], so the compiler has no way to know what T is. Yes, the compiler could use array.getClass().getComponentType(), but that would sometimes fail miserably because you can do stuff like this:

Object[] arr = new String[] { "a", "b", "c" };
Integer[] integerArray = (Integer[]) arr;
doSomething(integerArray);

(In this case array.getClass().getComponentType() returns String.class, but T stands for Integer. Yes, this is legal and does not generate compiler warnings.)

how to get a array class using classloader in java?

Assuming your ClassLoader implementation, MyClassLoader is properly implemented, you'd simply pass the canonical name of the array class to the loadClass method. For example, if you were trying to load the Class for the class com.something.Foo, you'd use

cl.loadClass("[Lcom.something.Foo");

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You have to know that the class name for Array types is the character [ appended X number of times, where X is the number of array dimensions, then the character L. You then append the qualified class name of the type of the array. And finally, you append the character ;.

Therefore

Class<?> clazz = Class.forName("[Lcom.something.Examples;");

would get you the Class instance returned by A[].class if it was loaded by the same ClassLoader.

Note that you won't be able to instantiate this type this way. You'll have to use Array.newInstance() passing in the Class object for A, for a one dimensional array, the Class object for A[] for a two dimensional array and so on.

Create array in class and access the array in another class

Something like this?

Room.java:

class Room {
  private Person[] persons;

  public Room(Person[] persons){
    this.persons = persons;
  }

  public void listAllPersons() {
    // loop over array and print details
  }
// add getters and setters as you need
}

Main class:

public static void main (String[] args) {

    Person persons[] = new Person[3];
    persons[0] = new Person("Me", 20);
    persons[1] = new Staff("Papa", 60, 300);
    persons[2] = new Person("Mama", 55);

    Room room = new Room(persons);
    room.listAllPersons();
}

Get type of array of class using Java reflection

I dont't know if it is optimal, but I do it in following way:

Array.newInstance(Class.forName("strClassName"), 0).getClass()

More over, strClassName must be the fully qualified name of the desired class.

How to get the class associated to an array type?

The class of new MySpecialClass[0] is MySpecialClass[].class so you can use:

example.mySpecialMethod(MySpecialClass[].class)

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