25.

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instanceof

test class of object

Jasmin Syntax

     instanceof <type> 

<type> is the name of a Java class, e.g. java/lang/String. Alternatively, to test for array references, <type> can be the type descriptor of an array, e.g. [[Ljava/lang/String;

Stack

Before	After
objectref	int-result
...	...

Description

The instanceof instruction is used to implement the Java language's instanceof operator, which tests whether an object reference or array belongs to a given class.

instanceof takes a single parameter, <type>. <type> is either the name of a Java class, or it is the type descriptor of an array.

At runtime, <type> is resolved (Chapter 7 describes how classes are resolved). Next, instanceof pops objectref (a reference to an object) off the top of the operand stack. If objectref is an instance of <type> or one of <type>'s subclasses, the int 1 is pushed onto the stack, otherwise the int 0 is pushed onto the stack. If objectref is null, the result is always 0. If <type> is an interface, int-result will be 1 if objectref implements that interface, and 0 if objectref does not implement the interface.

Example

 ; using instanceof to test for a String:     aload_1        ; push object reference in local variable 1 onto stack     instanceof java/lang/String;      ; test if item on stack is a String     ifne HaveString                   ; if so, goto HaveString     return                            ; otherwise, return HaveString:     ; if this point is reached, local variable 1 holds a string ; this example uses instanceof to test if local variable 1 holds ; an integer array     aload_1                  ; push local variable 1 onto the stack     instanceof [I            ; test if the top item on the stack is an integer array     ifne HaveIntegerArray    ; if so, jump to HaveIntegerArray     return                   ; simply return if local variable 1 is not an int array HaveIntegerArray:                 ; if this point is reached, local variable 1 holds an integer array ; you can also use instanceof to test that objects implement a given interface, ; e.g.     aload_1                  ; push local variable 1 onto the stack     instanceof java/lang/Runnable  ; test if it implements the Runnable interface     ifne HaveARunnable       ; if so, jump to HaveARunnable     return                   ; otherwise return HaveARunnable:                 ; if this point is reached, local variable 1 holds a reference to an object     ; that implements the Runnable interface. 

Bytecode

In bytecode, immediately following the instanceof opcode is a 16-bit unsigned short integer. This integer is the index of an entry in the constant pool of the current class. The entry is tagged a CONSTANT_Class entry. The name field of the CONSTANT_Class entry is the same as the string given by <type> parameter in Jasmin.

Type	Description
u1	instanceof opcode = 0xC1 (193)
u2	index

See Also

checkcast

Chapter 7 gives a pseudo-code implementation of instanceof.

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Java Virtual Machine, by Jon Meyer and Troy Downing, O'Reilly Associates