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Performance will also be affected by the scope of our objects and variables. Local variables remain on the stack and so can be accessed directly by the VM (a stack-based interpreter). Static and instance variables are kept on the heap, and can therefore take much longer to access.
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static int sValue = 1; int iValue = 2; void lotsOfVariables(int arg1, int arg2) { int value1; int value2; value1 = arg1; value2 = arg2; iValue = sValue; }
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In the above code snippet, sValue is a static and iValue is an instance variable; both are stored in the heap. value1 and value2 are local variables, arg1 and arg2 are method arguments, and all four are stored on the stack. The following table shows the performance difference in accessing static, instance, and local variables (see the Microbench MIDlet in the source code for the book, at www.symbian.com/books). In each case the executed code was of the form:
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value1 value2 value3 value4 = = = = value2; value3; value4; value1;
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where value<n> is either a static, an instance or a local variable. This code was repeated 16 times in each loop, giving 64 read/write operations, with the test looping one million times. Sun Wireless Toolkit 2.1 Static variable Instance variable Local variable 20.93 s 36.75 s 18.93 s Nokia 9210i Nokia 7650 Nokia 6600 Sony Ericsson P900 2.61 s 1.70 s 0.20 s
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As can be seen, accessing local variables can be an order of magnitude faster than accessing variables declared on the heap, and static variables are generally slower to access than instance variables.
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However, note that for Sun s Wireless Toolkit, access to static variables is faster than to instance variables. This illustrates something we said earlier: optimization behavior is platform-dependent. Good design encourages the use of getter and setter methods to access variables. As a simple example I might start with an implementation that stores a person s age, but later on change this to store their date of birth, calculating their age from the current date. I can make this change if I have used a getAge() method, but not if I have relied on a public age eld. But will getter and setter methods not be slower The following code is used to test the speed of getter and setter methods:
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private int instanceValue = 6; final int getInstanceVariable(){ return instanceValue; } final void setInstanceVariable(int value){ instanceValue = value; } long instanceMethodTest(int loop){ long timeStart = System.currentTimeMillis(); for(int i = loop; --i >= 0; ){ ... setInstanceVariable(getInstanceVariable()); ... } return System.currentTimeMillis() - timeStart; }
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The line setInstanceVariable(getInstanceVariable()); was repeated 64 times inside the loop. Similar code was used to test getter and setter methods for accessing a static, rather than an instance variable. In this case, the getter and setter methods and the variable being accessed were declared as static. Here are the results for a loop count of one million (in the case of WTK, extrapolated from a loop count of 100 000): Sun Wireless Toolkit 2.1 Static accessors Instance accessors 1362.55 s 1409.42 s Nokia 9210i Nokia 7650 Nokia 6600 Sony Ericsson P900 26.07 s 1.78 s
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Again we see platform-dependent differences in behavior. Sun s WTK, Nokia 9210i and Nokia 7650 are all KVM-based, and on all three the static getter and setter accessors are slower than the instance accessors. Of more interest, though, is comparing the time it takes to access an instance variable directly against accessing it via getter and setter methods. For KVM-based devices, getter and setter methods are very much slower (by about a factor of 20!) However, for CLDC HI-based devices (Nokia 6600 and Sony Ericsson P900), there is no difference. So for the newer devices, there is no excuse for not using getter and setter methods. What is happening All method calls are faster after their rst execution; the VM replaces lookup by name with a more ef cient lookup: virtual methods are dispatched using an index value into the method table for the class, while non-virtual methods are dispatched using a direct link to the method-block for the method. Both approaches offer a similar improvement; however, non-virtual methods can also be inlined. Public instance methods are virtual. Final methods may be virtual, but can never be overridden. So, depending on the type of object reference to make the call, inlining may still be allowed. Private methods are nonvirtual. Static methods are also non-virtual: they cannot be overridden by a derived class, only hidden. In addition, static methods do not have a this parameter, which saves a stack push. The VM attempts the actual inlining at runtime after the rst execution. It replaces the method call with an inline version of the method if the method body can be expressed in bytecodes that t into the method invocation bytespace. In practice this means that simpler getter and setter methods can be inlined by the VM. This optimization was not implemented in the KVM, which explains the poor performance of static methods on the earlier phones, but is present on the later CLDC HI-based phones.
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