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A, iteration entrySet () Retrieves the Map of the key and value
When the loop only needs to acquire Map primary key key, iteration keySet () is correct; however, when you need a primary key key and value value, iteration entrySet () is a more efficient approach, which keySet than the first iteration () by then go get the value of better performance.
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Second, the use Collection.isEmpty () detects an empty
Use of a Collection, () is empty is not detected on the logical problems, but using Collection.isEmpty () makes the code more readable, better performance can be obtained; in addition, any Collection.isEmpty () implemented time complexity is O (1), multiple cycles need not traverse, but by a certain time complexity of a Collection, () method implementation may be O (n)
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Third, as far as possible the size specified set of initialization
Try to set the size specified at initialization, can effectively reduce the number of sets of expansion, because each set of expansion when the time complexity is likely to O (n), and time-consuming performance.
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Fourth, the use StringBuilder string concatenation
General string concatenation will be at compile Java optimization, but splice Java compiler optimization of the string can not be executed in a loop, so StringBuilder need to be replaced.
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V. To recall Collection.contains method frequently use the Set
In the Java collection class library, List contains the general method of time complexity of O (n), if the code contains method calls frequently find the first set of data is converted into a list HashSet achieved, the O (n) time complexity will be O (1).
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Sixth, the use of static code block implements static member variable assignment
For static member variables collection types, you should use static code block assignment, instead of using a set of implementation to evaluation.
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Seven, delete unused local variables, method parameters, private methods, fields, and extra parentheses.
IX tools shield constructor
Tools is a collection of a bunch of static fields and functions, which should not be instantiated; however, Java adds an implicit public constructor for each class is not explicitly defined constructor, in order to avoid unnecessary instantiation, should explicit definition of private constructors to shield the implicit public constructor.
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Ten, remove the extra exception caught and ran
After capturing an exception with catch phrase, if nothing is processed, it just makes an exception is thrown again, this result is not like catching exceptions, you can delete or add another piece of code that process.
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Eleven, the string conversion using String.valueOf (value) instead of "" + value
When the type or other objects into a string, using String.valueOf (value) "" + is higher than the efficiency value.
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Twelve, avoid using BigDecimal (double)
BigDecimal (double) the presence of risk of loss of precision, the exact calculation of the comparison values or the scene may cause the business logic exception.
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XIII, returns an empty array rather than null and collections
If the program run returns null, null square mandatory testing needs to be called, otherwise it will throw null pointer exception; returns an empty array or an empty set, effectively avoid the situation because the caller does not detect null null pointer exception is thrown, you can also delete the caller detect null statement makes the code more concise.
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Fourth, priority use constant or call equals method to determine the value
equals method of Object readily shorting pointer exception, should be used with a constant value or determined object to call the equals method.
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Fifth, the enumeration of attribute fields must be private and immutable
Enumeration is generally used as a constant, or if the common attribute field setting field enumeration methods exist, then the enumeration constant properties can easily be modified; ideally, enumeration property fields are private, proprietary and constructor, assignment, there is no corresponding method Setter, preferably together with the final modifier.
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Sixteen, tring.split (String regex) part of a keyword needs translation
When using plit string String method, passing in the delimited string is a regular expression, then part of the keyword (such as [] () |., Etc.) need to be escaped.
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