vmspec: Managing Arrays

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3.6 Managing Arrays

newarray

Allocate new array


newarray = 188
atype

Stack: ..., size => result

size must be an integer. It represents the number of elements in the new array.

atype is an internal code that indicates the type of array to allocate. Possible values for atype are as follows:
T_BOOLEAN 4
T_CHAR 5
T_FLOAT 6
T_DOUBLE 7
T_BYTE 8
T_SHORT 9
T_INT 10
T_LONG 11

A new array of atype, capable of holding size elements, is allocated, and result is a reference to this new object. Allocation of an array large enough to contain size items of atype is attempted. All elements of the array are initialized to zero.

If size is less than zero, a NegativeArraySizeException is thrown. If there is not enough memory to allocate the array, an OutOfMemoryError is thrown.

anewarray

Allocate new array


anewarray = 189
indexbyte1
indexbyte2

of references to objects

Stack: ..., size=> result

size must be an integer. It represents the number of elements in the new array.

indexbyte1 and indexbyte2 are used to construct an index into the constant pool of the current class. The item at that index is resolved. The resulting entry must be a class.

A new array of the indicated class type and capable of holding size elements is allocated, and result is a reference to this new object. Allocation of an array large enough to contain size items of the given class type is attempted. All elements of the array are initialized to null.

If size is less than zero, a NegativeArraySizeException is thrown. If there is not enough memory to allocate the array, an OutOfMemoryError is thrown.

anewarray is used to create a single dimension of an array of object references. For example, to create

new Thread[7]

the following code is used:

bipush 7
anewarray <Class "java.lang.Thread">

anewarray can also be used to create the first dimension of a multi-dimensional array. For example, the following array declaration:

new int[6][]

is created with the following code:

bipush 6
anewarray <Class "[I">

See CONSTANT_Class in the "Class File Format" chapter for information on array class names.

multianewarray

Allocate new multi-dimensional array


multianewarray = 197
indexbyte1
indexbyte2
dimensions

Stack: ..., size1 size2...sizen => result

Each size must be an integer. Each represents the number of elements in a dimension of the array.

indexbyte1 and indexbyte2 are used to construct an index into the constant pool of the current class. The item at that index is resolved. The resulting entry must be an array class of one or more dimensions.

dimensions has the following aspects:

It must be an integer >=1.
It represents the number of dimensions being created. It must be the number of dimensions of the array class.
It represents the number of elements that are popped off the stack. All must be integers greater than or equal to zero. These are used as the sizes of the dimension. For example, to create

 new int[6][3][]

the following code is used:

 bipush 6
 bipush 3
 multianewarray <Class "[[[I"> 2

If any of the size arguments on the stack is less than zero, a NegativeArraySizeException is thrown. If there is not enough memory to allocate the array, an OutOfMemoryError is thrown.

The result is a reference to the new array object.

Note: More explanation needed about how this is an array of arrays.

Note: It is more efficient to use newarray or anewarray when creating a single dimension.

See CONSTANT_Class in the "Class File Format" chapter for information on array class names.

arraylength


arraylength = 190

Get length of array

Stack: ..., objectref => ..., length

objectref must be a reference to an array object. The length of the array is determined and replaces objectref on the top of the stack.

If the objectref is null, a NullPointerException is thrown.

iaload

Load integer from array


iaload = 46

Stack: ..., arrayref, index => ..., value

arrayref must be a reference to an array of integers. index must be an integer. The integer value at position number index in the array is retrieved and pushed onto the top of the stack.