dup : duplicate top single-word item on the stack : index : visitInsn()

Description

This pops the top single-word value off the operand stack, and then pushes that value twice - i.e. it makes an extra copy of the top item on the stack.

This instruction cannot be used to duplicate two-word items (longs or doubles) - use dup2 instead.

Example

; This is like the java expression:
;
;   StringBuffer x = new StringBuffer();
;
; Make a new StringBuffer object and leave a reference to it
; on the stack:
new java/lang/StringBuffer
; [ Stack now contains: objectref ]
; Duplicate the object reference:
dup
; [ Stack now contains: objectref objectref ]
; Invoke the object's initializer:
invokespecial java/lang/StringBuffer/<init>()V
; [ Stack now contains: objectref ]
; Store the objectref in local variable 1.
astore_1
; [ Stack is now empty. ]

See also

dup2 dup_x1 dup2_x1 dup2_x2

Stack

Before	After
item	item
...	item
	...

Bytecode

Type	Description
u1	dup opcode = 0x59 (89)