Predicate in Java

Question

I am going through the code which uses Predicate in Java  I have never used Predicate  Can someone guide me to any tutorial or conceptual explanation of Predicate and its implementation in Java

User · Answer

You can view the java doc examples or the example of usage of Predicate here

Basically it is used to filter rows in the resultset based on any specific criteria that you may have and return true for those rows that are meeting your criteria:

 // the age column to be between 7 and 10
    AgeFilter filter = new AgeFilter(7, 10, 3);

    // set the filter.
    resultset.beforeFirst();
    resultset.setFilter(filter);

User · Answer

A predicate is a function that returns a true false  i e  boolean  value  as opposed to a proposition which is a true false  i e  boolean  value  In Java  one cannot have standalone functions  and so one creates a predicate by creating an interface for an object that represents a predicate and then one provides  a class that implements that interface  An example of an interface for a predicate might be   public interface Predicate lt ARGTYPE gt        public boolean evaluate ARGTYPE arg       And then you might have an implementation such as   public class Tautology lt E gt  implements Predicate lt E gt         public boolean evaluate E arg            return true             To get a better conceptual understanding  you might want to read about first-order logic   Edit There is a standard Predicate interface  java util function Predicate  defined in the Java API as of Java 8  Prior to Java 8  you may find it convenient to reuse the com google common base Predicate interface from Guava   Also  note that as of Java 8  it is much simpler to write predicates by using lambdas  For example  in Java 8 and higher  one can pass p -  true to a function instead of defining a named Tautology subclass like the above

User · Answer

I m assuming you re talking about com google common base Predicate lt T gt  from Guava   From the API      Determines a true or false value for a given input  For example  a RegexPredicate might implement Predicate lt String gt   and return true for any string that matches its given regular expression     This is essentially an OOP abstraction for a boolean test   For example  you may have a helper method like this   static boolean isEven int num       return  num   2     0     simple     Now  given a List lt Integer gt   you can process only the even numbers like this       List lt Integer gt  numbers   Arrays asList 1 2 3 4 5 6 7 8 9 10       for  int number   numbers            if  isEven number                 process number                     With Predicate  the if test is abstracted out as a type  This allows it to interoperate with the rest of the API  such as Iterables  which have many utility methods that takes Predicate   Thus  you can now write something like this       Predicate lt Integer gt  isEven   new Predicate lt Integer gt               Override public boolean apply Integer number                return  number   2     0                                      Iterable lt Integer gt  evenNumbers   Iterables filter numbers  isEven        for  int number   evenNumbers            process number           Note that now the for-each loop is much simpler without the if test  We ve reached a higher level of abtraction by defining Iterable lt Integer gt  evenNumbers  by filter-ing using a Predicate   API links   Iterables filter   Returns the elements that satisfy a predicate        On higher-order function  Predicate allows Iterables filter to serve as what is called a higher-order function  On its own  this offers many advantages  Take the List lt Integer gt  numbers example above  Suppose we want to test if all numbers are positive  We can write something like this   static boolean isAllPositive Iterable lt Integer gt  numbers        for  Integer number   numbers            if  number  lt  0                return false                      return true           if  isAllPositive numbers         System out println  Yep         With a Predicate  and interoperating with the rest of the libraries  we can instead write this   Predicate lt Integer gt  isPositive   new Predicate lt Integer gt           Override public boolean apply Integer number            return number  gt  0                         if  Iterables all numbers  isPositive         System out println  Yep         Hopefully you can now see the value in higher abstractions for routines like  filter all elements by the given predicate    check if all elements satisfy the given predicate   etc make for better code   Unfortunately Java doesn t have first-class methods  you can t pass methods around to Iterables filter and Iterables all  You can  of course  pass around objects in Java  Thus  the Predicate type is defined  and you pass objects implementing this interface instead   See also   Wikipedia Higher-order function Wikipedia Filter  higher-order function

User · Answer

Adding up to what Micheal has said   You can use Predicate as follows in filtering collections in java   public static  lt T gt  Collection lt T gt  filter final Collection lt T gt  target     final Predicate lt T gt  predicate      final Collection lt T gt  result   new ArrayList lt T gt       for  final T element   target       if  predicate apply element         result add element              return result      one possible predicate can be   final Predicate lt DisplayFieldDto gt  filterCriteria                        new Predicate lt DisplayFieldDto gt         public boolean apply final DisplayFieldDto displayFieldDto        return displayFieldDto isDisplay                Usage    final List lt DisplayFieldDto gt  filteredList    List lt DisplayFieldDto gt  filter displayFieldsList  filterCriteria

[java] Predicate in Java

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