1 Object Oriented Programming Lecture IX Some notes on Java Performance with aspects on execution time and memory consumption

2 Last Lecture We talked about refactoring strategies –refactoring methods –refactoring by inheritance –refactoring by delegation Netbeans and GUI forms for Graphic User Interfaces

3 Today’s Talk Improving performance in Java programs –time consumption –space consumption Reference to the material in todays lecture: –Peter Sestoft, “Java performance: Reducing time and space consumption”

4 Java Compilers Java compilers cannot do all kinds of optimization that we are used to in other languages Compared to a language as C –Java is semantically more restricted and many things have to be done at run-time operations on objects bound at run-time, dynamic data structures (eg Vector) etc. It many times cannot be predicted –what is the type of an object or –which object that is being accessed untill the code is executed

5 What can we do? With some knowledge about the –semantics of the language –run-time mechanisms in the JVM –how classes in the API are implemented, we can in some cases avoid expensive run-time JVM operations by how we write the code

6 Time consumption

7 Iterative computing on invariant expressions Loops and loop-invariant expressions –avoid recomputing loop bounds for expressions when we know the value is static One example is the size() method for(int i=0; i<size()+1; i++){ … }

8 Iterative computing on invariant expressions Instead of calling size() everytime the bound is checked: –compute the loop bound once, and bind it to a local variable for(int i=0,stop=size()+1; i<stop; i++){ … }

9 Computing subexpressions Avoid recomputing the same subexpression more than once ”Bad” example: if(shapes.elementAt(i).isCircle()); if(shapes.elementAt(i).isSquare());

10 Computing subexpressions Instead, compute the subexpression once and bind the value to a local variable ”Good” example: Shape shape = shapes.elementAt(i); if(shape.isCircle()); if(shape.isSquare());

11 Computing on Arrays in loop expressions For every access in an array, an index (out-of- bounds) check is required to be performed double[] rowsum = new double[n]; for(int i=0; i<n; i++){ for(int j=0; j<m; j++){ rowsum[i]+= arr[i][j]; } }

12 Computing on Arrays in loop expressions What we can do in this example, is to compute the outer index once double[] rowsum = new double[n]; for(int i=0; i<n; i++){ double[] arri = arr[i]; //copy reference double sum = 0.0; for(int j=0; j<m; j++) sum += arr[j];//avoid checking i index rowsum[i] = sum; }

13 Constant Fields and Variables Declare –constant fields as final static –Variables as final The compiler can inline such code and precompute constant expressions Examples: final static int x = 10; final Object a;//assigned only once

14 Long If-Else chains Avoid construction of long if- else chains: if(exp){... }else if(exp){... }else if(exp){... }... else{... } Replace with a switch-case expression instead switch(exp){ case a: statement 1 case b: statement 2 Default: statement n }

15 What about ”clever one-liners” Example: int year = 0; double sum = 200.0; double[] balance = new double[100]; while((balance[year++] = sum *= 1.05) < ); Nothing is gained by such expressions, except that you get a piece of code which noone understands

16 Fields and Variables There is no run-time overhead for variables declared as local –local variables also improves clarity of the code In a method –it is faster to access local variables and parameters than accessing static values or instance (class global) variables In loops –it might be more efficient to copy a global variable into a locally created variable, and use the local inside the loop

17 String Manipulation Avoid dynamically building strings using the concatenation (+) operator: String s = ””; for(int i=0; i<n; i++){ S += ”#” + i; } This loop has quadratic execution time and can easily cause heap fragmentation

18 String Manipulation Instead, use a StringBuilder object StringBuilder sbuf = new StringBuilder; for(int i=0; i<n; i++){ sbuf.append(”#”).append(i); } However, for a sequence, using ’+’ is ok : –String s = ”(” + x + ”, ” + y + ”)”;

19 Initialized Array variables Local method variables are initialized upon method entrance –avoid declaring initialized arrays in methods When initialized array variables or tables need to be used, declare them once as instance global final static int arr = {31,28,31,30,...};

20 Methods Method calls can be executed faster if methods are declared using private, final or static For example, accessor methods (like getSize ) can often be declared final –there is usually no need for overriding such methods in inheriting sub classes

21 Input and Output For program input and output (such as file access) –use buffered streams from java.io BufferedReader, BufferedWriter, BufferedInputStream, BufferedOutputStream Using buffered streams can speed up input/output up by a factor of 20

22 Bulk Array Operations Bulk operations means performing operations on whole, or sub blocks of data For Arrays, there are some special operations which are usually faster than using for -loops –one reason is that only a single bound check is required

23 Bulk Array operations static void arrayCopy(src, si, dst, di,n) –In java.lang.System Many bulk methods in java.util.Arrays –equality (compare elements) –fill (initialize elements) –searching –sorting

24 Collection Classes The collection classes in java.util are well designed and implemented and can improve the speed of a program When using lists (eg LinkedList, ArrayList… ) –Avoid indexed position access, insertion or removal of elements Those methods do linear search –Example System.out.println(list.get(i));

25 Scientific Computing There are several open source libraries available for different fields of computing For scientific computing –Colt open source library Linear algebra, matrix computation, statistics, random number generators, mathematical functions If needing special routines, it is always a good idea to look around of what is available before starting implementation

26 Reducing space consumption

27 JVM memory organization The stack –method input- and return parameters, local variables The Heap –for object instances, including strings and arrays Each thread has a separate stack –each growing and shrinking by the depth of method calls The heap is joint for all threads –a garbage collector manages the heap deallocation

28 Three important aspects of memory usage Allocation rate –the rate by which a program creates new objects high rate costs in time and space Retention –the amount of live data on the heap, which is reachable from the method stacks anytime high retention cost space (and garbage collector time) Fragmentation –creating small chunks of unusable memory –Allocation of dynamically growing objects may cause memory fragmentation costs time (to search for useable space) and space (wasted fragments)

29 Instance shared variables Use static declaration for variables shared by all instances of a class –only one field created for all objects Instead of public class Car{ String brand = ”Ferrari” } we can do this public class Car{ final static String brand = ”Ferrari”; }

30 Object allocation When it is not sure the an object will be used, we can apply lazy allocation Lazy allocation means that we postpone the allocation of an object untill it is actually needed, but only once

31 Without Lazy Allocation public class car{ private Button button = new Jbutton(); public Car(){ … initialize button … } public final Jbutton getButton(){ return button; }

32 Lazy Allocation public class car{ private Button button = null; public Car(){…} public final Jbutton getButton(){ if(button == null){ button = new Jbutton(); …initialize button … } return button; }

33 Other resources J. Noble and C. Weir, Small Memory Software, Addison-Wesley, 2001 –A number of design patterns for limited memory systems