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Java in a Nutshell

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26. The java.lang.reflect Package

The java.lang.reflect package contains the classes and interfaces that, along with java.lang.Class, comprise the Java Reflection API. This package is new in Java 1.1. Figure 26.1 shows the class hierarchy.

The Constructor, Field, and Method classes represent constructors, fields, and methods of a class. Because these types all represent members of a class, they each implement the Member interface, which defines a simple set of methods that can be invoked for any class member. These classes allow information about the class members to be obtained, and allow methods and constructors to be invoked and fields to be queried and set.

Class member modifiers are represented as integers that specify a number of bit flags. The Modifier class defines static methods that are useful in interpreting the meanings of these flags. The Array class defines static methods for creating arrays and reading and writing array elements.

See Chapter 12, Reflection for examples of using these classes.

26.1 java.lang.reflect.Array (JDK 1.1)

This class contains methods that allow you to set and query the values of array elements, to determine the length of an array, and to create new instances of arrays. Note that the Array class is used for manipulating array values, not array types; Java data types, including array types, are represented by java.lang.Class. Since the Array class represents a Java value, unlike the Field, Method, and Constructor classes which represent class members, the Array class is significantly different (despite some surface similarities) from those other classes in this package. Most notably, all the methods of Array are static and apply to all array values, rather than applying only to a specific field, method, or constructor.

The get() method returns the value of the specified element of the specified array as an Object. If the array elements are of a primitive type, the value is converted to a wrapper object before being returned. You can also use getInteger() and related methods to query array elements and return them as specific primitive types. The set() method and its primitive type variants perform the opposite operation. Also, the getLength() method returns the length of the array.

The newInstance() methods create new arrays. One version of this method is passed the number of elements in the array and the type of those elements. The other version of this method creates multidimensional arrays. Besides just specifying the component type of the array, it is passed an array of numbers. The length of this array specifies the number of dimensions for the array to be created, and the values of each array element specify the size of each dimension of the created array.

public final class Array extends Object {
    // No Constructor
    // Class Methods
            public static native Object get(Object array, int index) throws IllegalArgumentException, ArrayIndexOutOfBoundsException;
            public static native boolean getBoolean(Object array, int index) throws IllegalArgumentException, ArrayIndexOutOfBoundsException;
            public static native byte getByte(Object array, int index) throws IllegalArgumentException, ArrayIndexOutOfBoundsException;
            public static native char getChar(Object array, int index) throws IllegalArgumentException, ArrayIndexOutOfBoundsException;
            public static native double getDouble(Object array, int index) throws IllegalArgumentException, ArrayIndexOutOfBoundsException;
            public static native float getFloat(Object array, int index) throws IllegalArgumentException, ArrayIndexOutOfBoundsException;
            public static native int getInt(Object array, int index) throws IllegalArgumentException, ArrayIndexOutOfBoundsException;
            public static native int getLength(Object array) throws IllegalArgumentException;
            public static native long getLong(Object array, int index) throws IllegalArgumentException, ArrayIndexOutOfBoundsException;
            public static native short getShort(Object array, int index) throws IllegalArgumentException, ArrayIndexOutOfBoundsException;
            public static Object newInstance(Class componentType, int length) throws NegativeArraySizeException;
            public static Object newInstance(Class componentType, int[] dimensions) throws IllegalArgumentException, NegativeArraySizeException;
            public static native void set(Object array, int index, Object value) throws IllegalArgumentException, ArrayIndexOutOfBoundsException;
            public static native void setBoolean(Object array, int index, boolean z) throws IllegalArgumentException, ArrayIndexOutOfBoundsException;
            public static native void setByte(Object array, int index, byte b) Xsthrows IllegalArgumentException, ArrayIndexOutOfBoundsException;
            public static native void setChar(Object array, int index, char c) throws IllegalArgumentException, ArrayIndexOutOfBoundsException;
            public static native void setDouble(Object array, int index, double d) throws IllegalArgumentException, ArrayIndexOutOfBoundsException;
            public static native void setFloat(Object array, int index, float f) throws IllegalArgumentException, ArrayIndexOutOfBoundsException;
            public static native void setInt(Object array, int index, int i) throws IllegalArgumentException, ArrayIndexOutOfBoundsException;
            public static native void setLong(Object array, int index, long l) throws IllegalArgumentException, ArrayIndexOutOfBoundsException;
            public static native void setShort(Object array, int index, short s) throws IllegalArgumentException, ArrayIndexOutOfBoundsException;
}


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