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| Chapter 13 |

# 13. The java.lang.reflect Package

Contents:
Array
Constructor
Field
InvocationTargetException
Member
Method
Modifier

The package `java.lang.reflect` is new as of Java 1.1. It contains classes and interfaces that support the Reflection API. Reflection refers to the ability of a class to reflect upon itself, or look inside of itself, to see what it can do. The Reflection API makes it possible to:

- Discover the variables, methods, and constructors of any class.

- Create an instance of any class using any available constructor of that class, even if the class initiating the creation was not compiled with any information about the class to be instantiated.

- Access the variables of any object, even if the accessing class was not compiled with any information about the class to be accessed.

- Call the methods of any object, even if the calling class was not compiled with any information about the class that contains the methods.

- Create an array of objects that are instances of any class, even if the creating class was not compiled with any information about the class.

These capabilities are implemented by the `java.lang.Class` class and the classes in the `java.lang.reflect` package. Figure 13.1 shows the class hierarchy for the `java.lang.reflect` package.

#### Figure 13.1: The java.lang.reflect package

Java 1.1 currently uses the Reflection API for two purposes:

- The JavaBeans API supports a mechanism for customizing objects that is based on being able to discover their public variables, methods, and constructors. See the forthcoming Developing Java Beans from O'Reilly & Associates for more information about the JavaBeans API.

- The object serialization functionality in `java.io` is built on top of the Reflection API. Object serialization allows arbitrary objects to be written to a stream of bytes and then read back later as objects.

# Array

## Name

Array

## Synopsis

Class Name:

  `java.lang.reflect.Array`

Superclass:

  `java.lang.Object`

Immediate Subclasses:

  None

Interfaces Implemented:

  `java.lang.Cloneable`, `java.io.Serializable`

Availability:

  New as of JDK 1.1

## Description

The `Array` class provides static methods to manipulate arbitrary arrays in Java. There are methods to set and retrieve elements in an array, determine the size of an array, and create a new instance of an array.

The `Array` class is used to create array objects and manipulate their elements. The `Array` class is not used to represent array types. Because arrays in Java are objects, array types are represented by `Class` objects.

## Class Summary

    public final class java.lang.reflect.Array extends java.lang.Object {
      // Class Methods
      public static native Object get(Object array, int index);
      public static native boolean getBoolean(Object array, int index);
      public static native byte getByte(Object array, int index);
      public static native char getChar(Object array, int index);
      public static native double getDouble(Object array, int index);
      public static native float getFloat(Object array, int index);
      public static native int getInt(Object array, int index);
      public static native int getLength(Object array);
      public static native long getLong(Object array, int index);
      public static native short getShort(Object array, int index);
      public static Object newInstance(Class componentType, int length);
      public static Object newInstance(Class componentType, int[] dimensions);
      public static native void set(Object array, int index, Object value);
      public static native void setBoolean(Object array, int index, boolean z);
      public static native void setByte(Object array, int index, byte b);
      public static native void setChar(Object array, int index, char c);
      public static native void setDouble(Object array, int index, double d);
      public static native void setFloat(Object array, int index, float f);
      public static native void setInt(Object array, int index, int i);
      public static native void setLong(Object array, int index, long l);
      public static native void setShort(Object array, int index, short s);
    }

## Class Methods

### get

####

     public static native Object get(Object array, int index) throws IllegalArgumentException, ArrayIndexOutOfBoundsException

Parameters

  `array`

    An array object.

  `index`

    An index into the array.

Returns

  The object at the given index in the specified array.

Throws

  `IllegalArgumentException`

    If the given object is not an array.

  `ArrayIndexOutOfBoundsException`

    If the given index is invalid.

  `NullPointerException`

    If `array` is `null`.

Description

  This method returns the object at the given index in the array. If the array contains values of a primitive type, the value at the given index is wrapped in an appropriate object, and the object is returned.

### getBoolean

####

     public static native boolean getBoolean(Object array, int index) throws IllegalArgumentException, ArrayIndexOutOfBoundsException

Parameters

  `array`

    An array object.

  `index`

    An index into the array.

Returns

  The `boolean` value at the given index in the specified array.

Throws

  `IllegalArgumentException`

    If the given object is not an array, or the object at the given index cannot be converted to a `boolean`.

  `ArrayIndexOutOfBoundsException`

    If the given index is invalid.

  `NullPointerException`

    If `array` is `null`.

Description

  This method returns the object at the given index in the array as a `boolean` value.

### getByte

####

     public static native byte getByte(Object array, int index) throws IllegalArgumentException, ArrayIndexOutOfBoundsException

Parameters

  `array`

    An array object.

  `index`

    An index into the array.

Returns

  The `byte` value at the given index in the specified array.

Throws

  `IllegalArgumentException`

    If the given object is not an array, or the object at the given index cannot be converted to a `byte`.

  `ArrayIndexOutOfBoundsException`

    If the given index is invalid.

  `NullPointerException`

    If `array` is `null`.

Description

  This method returns the object at the given index in the array as a `byte` value.

### getChar

####

     public static native char getChar(Object array, int index) throws IllegalArgumentException, ArrayIndexOutOfBoundsException

Parameters

  `array`

    An array object.

  `index`

    An index into the array.

Returns

  The `char` value at the given index in the specified array.

Throws

  `IllegalArgumentException`

    If the given object is not an array, or the object at the given index cannot be converted to a `char`.

  `ArrayIndexOutOfBoundsException`

    If the given index is invalid.

  `NullPointerException`

    If `array` is `null`.

Description

  This method returns the object at the given index in the array as a `char` value.

### getDouble

####

     public static native double getDouble(Object array, int index) throws IllegalArgumentException, ArrayIndexOutOfBoundsException

Parameters

  `array`

    An array object.

  `index`

    An index into the array.

Returns

  The `double` value at the given index in the specified array.

Throws

  `IllegalArgumentException`

    If the given object is not an array, or the object at the given index cannot be converted to a `double`.

  `ArrayIndexOutOfBoundsException`

    If the given index is invalid.

  `NullPointerException`

    If `array` is `null`.

Description

  This method returns the object at the given index in the array as a `double` value.

### getFloat

####

     public static native float getFloat(Object array, int index) throws IllegalArgumentException, ArrayIndexOutOfBoundsException

Parameters

  `array`

    An array object.

  `index`

    An index into the array.

Returns

  The `float` value at the given index in the specified array.

Throws

  `IllegalArgumentException`

    If the given object is not an array, or the object at the given index cannot be converted to a `float`.

  `ArrayIndexOutOfBoundsException`

    If the given index is invalid.

  `NullPointerException`

    If `array` is `null`.

Description

  This method returns the object at the given index in the array as a `float` value.

### getInt

####

     public static native int getInt(Object array, int index) throws IllegalArgumentException, ArrayIndexOutOfBoundsException

Parameters

  `array`

    An array object.

  `index`

    An index into the array.

Returns

  The `int` value at the given index in the specified array.

Throws

  `IllegalArgumentException`

    If the given object is not an array, or the object at the given index cannot be converted to a `int`.

  `ArrayIndexOutOfBoundsException`

    If the given index is invalid.

  `NullPointerException`

    If `array` is `null`.

Description

  This method returns the object at the given index in the array as a `int` value.

### getLength

####

     public static native int getLength(Object array) throws IllegalArgumentException

Parameters

  `array`

    An array object.

Returns

  The length of the specified array.

Throws

  `IllegalArgumentException`

    If the given object is not an array.

Description

  This method returns the length of the array.

### getLong

####

     public static native long getLong(Object array, long index) throws IllegalArgumentException, ArrayIndexOutOfBoundsException

Parameters

  `array`

    An array object.

  `index`

    An index into the array.

Returns

  The `long` value at the given index in the specified array.

Throws

  `IllegalArgumentException`

    If the given object is not an array, or the object at the given index cannot be converted to a `long`.

  `ArrayIndexOutOfBoundsException`

    If the given index is invalid.

  `NullPointerException`

    If `array` is `null`.

Description

  This method returns the object at the given index in the array as a `long` value.

### getShort

####

     public static native short getShort(Object array, short index) throws IllegalArgumentException, ArrayIndexOutOfBoundsException

Parameters

  `array`

    An array object.

  `index`

    An index into the array.

Returns

  The `short` value at the given index in the specified array.

Throws

  `IllegalArgumentException`

    If the given object is not an array, or the object at the given index cannot be converted to a `short`.

  `ArrayIndexOutOfBoundsException`

    If the given index is invalid.

  `NullPointerException`

    If `array` is `null`.

Description

  This method returns the object at the given index in the array as a `short` value.

### newInstance

####

     public static Object newInstance(Class componentType, int length) throws NegativeArraySizeException

Parameters

  `componentType`

    The type of each element in the array.

  `length`

    The length of the array.

Returns

  An array object that contains elements of the given component type and has the specified length.

Throws

  `NegativeArraySizeException`

    If `length` is negative.

  `NullPointerException`

    If `componentType` is `null`.

Description

  This method creates a single-dimension array with the given length and component type.

####

     public static Object newInstance(Class componentType, int[] dimensions) throws IllegalArgumentException, NegativeArraySizeException

Parameters

  `componentType`

    The type of each element in the array.

  `dimensions`

    An array that specifies the dimensions of the array to be created.

Returns

  An array object that contains elements of the given component type and has the specified number of dimensions.

Throws

  `IllegalArgumentException`

    If `dimensions` has zero dimensions itself, or if it has too many dimensions (typically 255 array dimensions are supported).

  `NegativeArraySizeException`

    If `length` is negative.

  `NullPointerException`

    If `componentType` is `null`.

Description

  This method creates a multidimensional array with the given dimensions and component type.

### set

####

     public static native void set(Object array, int index, Object value) throws IllegalArgumentException, ArrayIndexOutOfBoundsException

Parameters

  `array`

    An array object.

  `index`

    An index into the array.

  `value`

    The new value.

Throws

  `IllegalArgumentException`

    If the given object is not an array, or if it represents an array of primitive values, and the given value cannot be unwrapped and converted to that primitive type.

  `ArrayIndexOutOfBoundsException`

    If the given index is invalid.

  `NullPointerException`

    If `array` is `null`.

Description

  This method sets the object at the given index in the array to the specified value. If the array contains values of a primitive type, the given value is automatically unwrapped before it is put in the array.

### setBoolean

####

     public static native void setBoolean(Object array, int index, boolean z) throws IllegalArgumentException, ArrayIndexOutOfBoundsException

Parameters

  `array`

    An array object.

  `index`

    An index into the array.

  `z`

    The new value.

Throws

  `IllegalArgumentException`

    If the given object is not an array, or if the given value cannot be converted to the component type of the array.

  `ArrayIndexOutOfBoundsException`

    If the given index is invalid.

  `NullPointerException`

    If `array` is `null`.

Description

  This method sets the element at the given index in the array to the given `boolean` value.

### setByte

####

     public static native void setByte(Object array, int index, byte b) throws IllegalArgumentException, ArrayIndexOutOfBoundsException

Parameters

  `array`

    An array object.

  `index`

    An index into the array.

  `b`

    The new value.

Throws

  `IllegalArgumentException`

    If the given object is not an array, or if the given value cannot be converted to the component type of the array.

  `ArrayIndexOutOfBoundsException`

    If the given index is invalid.

  `NullPointerException`

    If `array` is `null`.

Description

  This method sets the element at the given index in the array to the given `byte` value.

### setChar

####

     public static native void setChar(Object array, int index, char c) throws IllegalArgumentException, ArrayIndexOutOfBoundsException

Parameters

  `array`

    An array object.

  `index`

    An index into the array.

  `c`

    The new value.

Throws

  `IllegalArgumentException`

    If the given object is not an array, or if the given value cannot be converted to the component type of the array.

  `ArrayIndexOutOfBoundsException`

    If the given index is invalid.

  `NullPointerException`

    If `array` is `null`.

Description

  This method sets the element at the given index in the array to the given `char` value.

### setDouble

####

     public static native void setDouble(Object array, int index, double d) throws IllegalArgumentException, ArrayIndexOutOfBoundsException

Parameters

  `array`

    An array object.

  `index`

    An index into the array.

  `d`

    The new value.

Throws

  `IllegalArgumentException`

    If the given object is not an array, or if the given value cannot be converted to the component type of the array.

  `ArrayIndexOutOfBoundsException`

    If the given index is invalid.

  `NullPointerException`

    If `array` is `null`.

Description

  This method sets the element at the given index in the array to the given `double` value.

### setFloat

####

     public static native void setFloat(Object array, int index, float f) throws IllegalArgumentException, ArrayIndexOutOfBoundsException

Parameters

  `array`

    An array object.

  `index`

    An index into the array.

  `f`

    The new value.

Throws

  `IllegalArgumentException`

    If the given object is not an array, or if the given value cannot be converted to the component type of the array.

  `ArrayIndexOutOfBoundsException`

    If the given index is invalid.

  `NullPointerException`

    If `array` is `null`.

Description

  This method sets the element at the given index in the array to the given `float` value.

### setInt

####

     public static native void setInt(Object array, int index, int i) throws IllegalArgumentException, ArrayIndexOutOfBoundsException

Parameters

  `array`

    An array object.

  `index`

    An index into the array.

  `i`

    The new value.

Throws

  `IllegalArgumentException`

    If the given object is not an array, or if the given value cannot be converted to the component type of the array.

  `ArrayIndexOutOfBoundsException`

    If the given index is invalid.

  `NullPointerException`

    If `array` is `null`.

Description

  This method sets the element at the given index in the array to the given `int` value.

### setLong

####

     public static native void setLong(Object array, int index, long l) throws IllegalArgumentException, ArrayIndexOutOfBoundsException

Parameters

  `array`

    An array object.

  `index`

    An index into the array.

  `l`

    The new value.

Throws

  `IllegalArgumentException`

    If the given object is not an array, or if the given value cannot be converted to the component type of the array.

  `ArrayIndexOutOfBoundsException`

    If the given index is invalid.

  `NullPointerException`

    If `array` is `null`.

Description

  This method sets the element at the given index in the array to the given `long` value.

### setShort

####

     public static native void setShort(Object array, int index, short s) throws IllegalArgumentException, ArrayIndexOutOfBoundsException

Parameters

  `array`

    An array object.

  `index`

    An index into the array.

  `s`

    The new value.

Throws

  `IllegalArgumentException`

    If the given object is not an array, or if the given value cannot be converted to the component type of the array.

  `ArrayIndexOutOfBoundsException`

    If the given index is invalid.

  `NullPointerException`

    If `array` is `null`.

Description

  This method sets the element at the given index in the array to the given `short` value.

## Inherited Methods

Method | Inherited From | Method | Inherited From
--- | --- | --- | ---
`clone()` | `Object` | `equals(Object)` | `Object`
`finalize()` | `Object` | `getClass()` | `Object`
`hashCode()` | `Object` | `notify()` | `Object`
`notifyAll()` | `Object` | `toString()` | `Object`
`wait()` | `Object` | `wait(long)` | `Object`
`wait(long, int)` | `Object` |  |

## See Also

`ArrayIndexOutOfBoundsException`, `Class`, `IllegalArgumentException`, `NegativeArraySizeException`, `NullPointerException`, `Object`

Void |  | Constructor