5.2 Comparing References

The third group of instructions is used to compare references to see if they are equal (if_acmpeq) or not equal (if_acmpne). Two references are equal only if they refer to the same object or if they are both null. For example,

 aload_0 aload_0 if_acmpeq success           ; This test always goes to success,                             ; since the same reference is loaded                             ; twice aload_1 dup if_acmpeq success           ; This test also goes to success,                             ; since the duped value is equal                             ; to the original new Cookie dup invokevirtual Cookie/<init> ()V   ; Make a cookie new Cookie dup invokevirtual Cookie/<init> ()V   ; Make another one if_acmpeq success                 ; This comparison fails, since                                   ; it compares two different                                   ; Cookies new Cookie() dup invokevirtual Cookie/<init>()V    ; Make yet another cookie aconst_null                       ; Put null on the stack if_acmpeq nullcookie              ; This test fails, because                                   ; no object is equal to null                                   ; Get the chocolate chip                                   ; cookie getstatic Cookie/chocolateChip LCookie; aconst_null if_acmpeq nocookie                ; If the chocolateChip Cookie is                                   ; null, go to nocookie 

Since comparison to null is something JVM programs do frequently, there are two special instructions (ifnull and ifnonnull) that compare a single object to null. The last example can be rewritten

 getstatic Cookie/chocolateChip LCookie; ifnull nocookie                   ; If the chocolateChip Cookie is                                   ; null, go to nocookie 

Table 5.1 summarizes the branching instructions.

Table 5.1. Branch instructions

Mnemonic	Argument	Stack	Effect
goto	label		Control transfers to label.
if_acmpeq	label	ref1 ref2	If ref1 references the same object as ref2, control branches to label.
if_acmpne	label	ref1 ref2	If ref1 does not reference the same object as ref2, control branches to label.
ifnonnull	label	ref1	If ref1 is not null, control branches to label.
ifnull	label	ref2	If ref1 is null, control branches to label.
if_icmpeq	label	int1 int2	If int1 == int2, control branches to label.
if_icmpne	label	int1 int2	If int1!= int2, control branches to label.
if_icmplt	label	int1 int2	If int1 < int2, control branches to label.
if_icmpge	label	int1 int2	If int1 >= int2, control branches to label.
if_icmpgt	label	int1 int2	If int1 >= int2, control branches to label.
if_icmple	label	int1 int2	If int1 <= int2, control branches to label.
ifeq	label	int1	If int1 == 0, control branches to label.
ifne	label	int1	If int1!= 0, control branches to label.
iflt	label	int1	If int1 < 0, control branches to label.
ifge	label	int1	If int1 >= 0, control branches to label.
ifgt	label	int1	If int1 > 0, control branches to label.
ifle	label	int1	If int1 <= 0, control branches to label.