Memory Usage in Java

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Memory Usage
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MEMORY USAGE
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As Java programmers we re used to enjoying the freedom of developing programs without considering how much memory we use or when we re using it We torch memory with reckless abandon and let the garbage collector clean up the mess For the most part, we would rather not concern ourselves with memory management issues at all, and there is certainly no requirement to do so when writing TINI applications However, on TINI, we re working with heap sizes as small as a few hundred kilobytes versus a few hundred megabytes on a PC or workstation A little consideration, especially during the design phase, of how your application behaves with respect to memory consumption can go a long way
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Object Creation
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Object creation, initiated by the new operator, is expensive both in terms of memory and CPU consumption All objects that are created are either arrays or class instances When either is created, a malloc (memory allocation operation) is performed by the memory manager on behalf of the VM Further adding to the cost of the malloc operation is the fact that malloc is effectively a calloc that clears all of the allocated memory to 0s This is done so that all array elements, or instance elds, are properly initialized to their speci ed default values The vast majority of the time required to create an array is spent performing the malloc operation When a class instance is created, allocating the memory is just the rst step Next, the object s internal structure is initialized, and then its constructor and its superclasses s constructor (and so on) are all interpreted by the VM The time this takes depends largely on what operations are performed by the individual constructors, but it can easily dwarf the amount of time required to allocate memory for the object In some cases, the amount of object creation can be reduced by reusing previously created objects Depending on the application, this may require care to avoid using stale information from an old object or possibly creating thread-safety problems
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Strings
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Strings have a sneaky way of gobbling up lots of memory (and CPU in the process), but they are extremely useful Every attempt has been made to make string operations ef cient For example, many of the methods in the String class are implemented as native methods However, there isn t much that can be done about the amount of memory consumed by various methods in the String class Methods like toUpperCase, toLowerCase, substring, and so forth all create and return new String objects Imagine a scenario in which an array of strings is being parsed within a loop and in each iteration of the loop the string is compared to the
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lowercase version of a portion of some source string using a statement like the following
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if (s[i]equals(srctoLowerCase()substring(3, 6))) { }
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Two temporary strings are created Each pass through the loop, chewing up both memory and time In this case, the problem can be avoided by creating the string required for comparison outside of the loop and storing a reference to it in a local variable The local reference can then be used for comparison within the loop String concatenation using the + operator is also expensive The java compiler generates code that creates a StringBuffer object and uses its append method to copy the individual strings into the StringBuffer s internal character array The result is then converted back to a string by invoking StringBuffer s toString method The cost of string concatenation can be lowered by creating an appropriately sized StringBuffer directly If the StringBuffer is created using the following constructor
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public StringBuffer(int length)
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with a capacity (the value of length) large enough to contain the nal string, the StringBuffer s internal character array will not have to be resized during concatenation (append) operations This prevents creating new arrays as well as the array copy operations that would be required to copy the contents of the old buffer to the new buffer
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