lookupswitch : match key in table and jump : index : visitLookupSwitchInsn()

Description

This is used to perform an efficient compare-and-jump, as might be needed for a switch statement. The table used by lookupswitch is given after the lookupswitch opcode in bytecode.

lookupswitch works as follows. First, an int, item, is taken from the top of the stack. Then, lookupswitch searches the table looking for an entry whose <key> field matches item. If a match is found, execution branches to the address of the corresponding <label>. If no match is found, execution branches to <labelDefault>.

Example

; this is like the Java code:
;      switch (i) {
;           case 1:     return(10);
;           case 10:    return(100);
;           default:    return(0);
;      }
    iload_1    ; push local variable 1 onto the stack (i.e. i)
    ; switch based on the value on the stack
    lookupswitch
        1       : Label1
        10      : Label2
        default : Dlabel
Label1:               ; local variable 1 holds the value 1
    bipush 10    
    ireturn    ; return 10
Label2:               ; local variable 1 holds the value 10
    bipush 100
    ireturn    ; return 100
Dlabel:               ; local variable 1 holds something else 
    bipush 0
    return    ; return 0

Notes

1. Addresses are measured in bytes from the start of the bytecode (i.e. address 0 is the first byte in the bytecode of the currently executing method).

2. Addresses given in the table are relative - the values in the table are added to the current pc (i.e. the address of this lookupswitch instruction) to obtain the new value for pc.

3. Keys in the table must be sorted in increasing order, so <key1> is less than <key2>, and so on up to <keyN>. Jasmin performs this sorting automatically.

4. Even though offsets in the table are given as 32-bit ints, the maximum address in a Java method is limited by other factors to 65535.

See also

tableswitch

Stack

Before	After
item	...
...	...

Bytecode

Type	Description
u1	lookupswitch opcode = 0xAB (171)
-	...0-3 bytes of padding ...
s4	default_offset
s4	n
s4	key_1
s4	offset_1
s4	key_2
s4	offset_2
...	...
s4	key_n
s4	offset_n