Trail: The Reflection API
Lesson: Arrays and Enumerated Types
Section: Arrays

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Identifying Array Types

Array types may be identified by invoking Class.isArray(). To obtain a Class use one of the methods described in Retrieving Class Objects section of this trail.

The ArrayFind example identifies the fields in the named class that are of array type and reports the component type for each of them.


import java.lang.reflect.Field;
import java.lang.reflect.Type;
import static java.lang.System.out;

public class ArrayFind {
    public static void main(String... args) {
	boolean found = false;
 	try {
	    Class<?> cls = Class.forName(args[0]);
	    Field[] flds = cls.getDeclaredFields();
	    for (Field f : flds) {
 		Class<?> c = f.getType();
		if (c.isArray()) {
		    found = true;
		    out.format("%s%n"
                               + "           Field: %s%n"
			       + "            Type: %s%n"
			       + "  Component Type: %s%n",
			       f, f.getName(), c, c.getComponentType());
		}
	    }
	    if (!found) {
		out.format("No array fields%n");
	    }

        // production code should handle this exception more gracefully
 	} catch (ClassNotFoundException x) {
	    x.printStackTrace();
	}
    }
}

The syntax for the returned value of Class.get*Type() is described in Class.getName(). The number of '[' characters at the beginning of the type name indicates the number of dimensions (that is, depth of nesting) of the array.

Samples of the output follows. User input is in italics. An array of primitive type byte:

$java ArrayFind java.nio.ByteBuffer
final byte[] java.nio.ByteBuffer.hb
           Field: hb
            Type: class [B
  Component Type: byte

An array of reference type StackTraceElement:

$ java ArrayFind java.lang.Throwable
private java.lang.StackTraceElement[] java.lang.Throwable.stackTrace
           Field: stackTrace
            Type: class [Ljava.lang.StackTraceElement;
  Component Type: class java.lang.StackTraceElement

predefined is a one-dimensional array of reference type java.awt.Cursor and cursorProperties is a two-dimensional array of reference type String:

$ java ArrayFind java.awt.Cursor
protected static java.awt.Cursor[] java.awt.Cursor.predefined
           Field: predefined
            Type: class [Ljava.awt.Cursor;
  Component Type: class java.awt.Cursor
static final java.lang.String[][] java.awt.Cursor.cursorProperties
           Field: cursorProperties
            Type: class [[Ljava.lang.String;
  Component Type: class [Ljava.lang.String;

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