op4j: bending the Java spoon - Functions: FnReduce

1. Overview

Reduce (also known as Fold) functions receive, as a parameter, a function that is applied to two elements on each execution: the function executes on two consecutive elements of the target list and returns another one that will be used as the first target element for the next execution of the function.

FnReduce is a parameterizable function hub class and, before giving you access to the real functions, you will have to specify the type parameter, which will give you access to the functions containing the reduce operations:

FnReduce.onBigDecimal(): returns FnReduceOnBigDecimal
FnReduce.onBigInteger(): returns FnReduceOnBigInteger
FnReduce.onBoolean(): returns FnReduceOnBoolean
FnReduce.onByte(): returns FnReduceOnByte
FnReduce.onDouble(): returns FnReduceOnDouble
FnReduce.onFloat(): returns FnReduceOnFloat
FnReduce.onInteger(): returns FnReduceOnInteger
FnReduce.onLong(): returns FnReduceOnLong
FnReduce.onShort(): returns FnReduceOnShort
FnReduce.onString(): returns FnReduceOnString
FnReduce.onCharacter(): returns FnReduceOn<Character>
FnReduce.onCalendar(): returns FnReduceOn<Calendar>
FnReduce.onDate(): returns FnReduceOn<Date>
FnReduce.onObject(): returns FnReduceOn<Object>
FnReduce.on(final Type<T> type): returns FnReduceOn<T> parameterized with the given type. This is the most generic function in FnReduce

These Reduce operations are executed by means of the reduce method as follows:

Op.on(...)...reduce(FnReduce.onX().Y(...))...;

as in

/* In this case, we insert an initial value: 13 */ 
Op.on(new Integer[] {2, 3, 4}).toList().reduce(FnReduce.onBigDecimal().avg(), BigDecimal.valueOf(13)).get(); 
            
            
Op.on(new Float[] {Float.valueOf(3), Float.valueOf(5), Float.valueOf(8), Float.valueOf(9)}) 
    .toList().reduce(FnReduce.onFloat().div(MathContext.DECIMAL32)).get(); 
            
 
/* In this case, we insert an initial value: 100 */ 
Op.on(new Integer[] {Integer.valueOf(3), Integer.valueOf(5), Integer.valueOf(8), Integer.valueOf(9)}) 
    .toList().reduce(FnReduce.onFloat().div(MathContext.DECIMAL32), Float.valueOf(100)).get();

Note that you can specify an initial value (as in the first and third examples above) which is initially combined with the first target element in the list (as op4j implements left fold).

FnReduceOnBigDecimal

These functions are to be applied on Numbers and all of them return a BigDecimal. Let's see some examples:

/*  The code below would return 3.25 as avg(2, 3, 4) -> avg(avg(2, 3), 4) -> avg(2.5, 4) -> 3.25 */ 
Op.on(new Integer[] {2, 3, 4}).toList() 
    .forEach().exec(FnNumber.toBigDecimal()).endFor()              
    .reduce(FnReduce.onBigDecimal().avg(MathContext.DECIMAL32)).get(); 
                

/*  The code below would return 3 as avg(0, 2, 3, 4) -> avg(avg(avg(0, 2), 3), 4) -> avg(avg(1, 3), 4) -> avg(2, 4) -> 3 */ 
Op.on(new Integer[] {2, 3, 4}).toList()                             
    .reduce(FnReduce.onBigDecimal().avg(MathContext.DECIMAL32), BigDecimal.valueOf(0)).get();

Function name	Type	Params
avg	Function<ValuePair<Number,Number>,BigDecimal>
avg	Function<ValuePair<Number,Number>,BigDecimal>	RoundingMode roundingMode
avg	Function<ValuePair<Number,Number>,BigDecimal>	MathContext mathContext

div	Function<ValuePair<Number,Number>,BigDecimal>
div	Function<ValuePair<Number,Number>,BigDecimal>	RoundingMode roundingMode
div	Function<ValuePair<Number,Number>,BigDecimal>	MathContext mathContext

mult	Function<ValuePair<Number,Number>,BigDecimal>

remainder	Function<ValuePair<Number,Number>,BigDecimal>
remainder	Function<ValuePair<Number,Number>,BigDecimal>	MathContext mathContext

subt	Function<ValuePair<Number,Number>,BigDecimal>

FnReduceOnBigInteger

These functions are to be applied on Numbers and all of them return an Integer. Let's see some examples:

Op.on(new Integer[] {Integer.valueOf(3), Integer.valueOf(5), Integer.valueOf(8), Integer.valueOf(9)}).toList()                      
    .reduce(FnReduce.onBigInteger().div(MathContext.DECIMAL32), BigInteger.valueOf(100)).get();
  
            
Op.on(new BigInteger[] {BigInteger.valueOf(3), BigInteger.valueOf(5), BigInteger.valueOf(8), BigInteger.valueOf(9)}).toList()                          
    .reduce(FnReduce.onBigInteger().div(MathContext.DECIMAL32)).get();

Function name	Type	Params
avg	Function<ValuePair<Number,Number>,BigInteger>
avg	Function<ValuePair<Number,Number>,BigInteger>	RoundingMode roundingMode
avg	Function<ValuePair<Number,Number>,BigInteger>	MathContext mathContext

div	Function<ValuePair<Number,Number>,BigInteger>
div	Function<ValuePair<Number,Number>,BigInteger>	RoundingMode roundingMode
div	Function<ValuePair<Number,Number>,BigInteger>	MathContext mathContext

mult	Function<ValuePair<Number,Number>,BigInteger>

remainder	Function<ValuePair<Number,Number>,BigInteger>
remainder	Function<ValuePair<Number,Number>,BigInteger>	MathContext mathContext

subt	Function<ValuePair<Number,Number>,BigInteger>

FnReduceOnBoolean

These functions are to be applied on Boolean elements and all of them return a Boolean.

Function name	Type	Params	Description
or	Function<ValuePair<Boolean,Boolean>,Boolean>

FnReduceOnByte

These functions are to be applied on Numbers and all of them return a Byte.

Function name	Type	Params
avg	Function<ValuePair<Number,Number>,Byte>
avg	Function<ValuePair<Number,Number>,Byte>	RoundingMode roundingMode
avg	Function<ValuePair<Number,Number>,Byte>	MathContext mathContext

div	Function<ValuePair<Number,Number>,Byte>
div	Function<ValuePair<Number,Number>,Byte>	RoundingMode roundingMode
div	Function<ValuePair<Number,Number>,Byte>	MathContext mathContext

mult	Function<ValuePair<Number,Number>,Byte>

remainder	Function<ValuePair<Number,Number>,Byte>
remainder	Function<ValuePair<Number,Number>,Byte>	MathContext mathContext

subt	Function<ValuePair<Number,Number>,Byte>

FnReduceOnDouble

These functions are to be applied on Numbers and all of them return a Double. Let's see some examples:

/*  The code below would return 3.25 as avg(2, 3, 4) -> avg(avg(2, 3), 4) -> avg(2.5, 4) -> 3.25 */ 
Op.on(new Integer[] {2, 3, 4}).toList() 
    .forEach().exec(FnNumber.toDouble()).endFor()                
    .reduce(FnReduce.onDouble().avg(MathContext.DECIMAL32)).get(); 
    
            
/*  The code below would return 3.525 as avg(4.2, 2, 3, 4) -> avg(avg(avg(4.2, 2), 3), 4) -> avg(avg(3.1, 3), 4) -> avg(3.05, 4) -> 3.525 */ 
Op.on(new Integer[] {2, 3, 4}).toList()                             
    .reduce(FnReduce.onDouble().avg(MathContext.DECIMAL32), Double.valueOf(4.2)).get();

Function name	Type	Params
avg	Function<ValuePair<Number,Number>,Double>
avg	Function<ValuePair<Number,Number>,Double>	RoundingMode roundingMode
avg	Function<ValuePair<Number,Number>,Double>	MathContext mathContext

div	Function<ValuePair<Number,Number>,Double>
div	Function<ValuePair<Number,Number>,Double>	RoundingMode roundingMode
div	Function<ValuePair<Number,Number>,Double>	MathContext mathContext

mult	Function<ValuePair<Number,Number>,Double>

remainder	Function<ValuePair<Number,Number>,Double>
remainder	Function<ValuePair<Number,Number>,Double>	MathContext mathContext

subt	Function<ValuePair<Number,Number>,Double>

FnReduceOnFloat

These functions are to be applied on Numbers and all of them return a Float.

Function name	Type	Params
avg	Function<ValuePair<Number,Number>,Float>
avg	Function<ValuePair<Number,Number>,Float>	RoundingMode roundingMode
avg	Function<ValuePair<Number,Number>,Float>	MathContext mathContext

div	Function<ValuePair<Number,Number>,Float>
div	Function<ValuePair<Number,Number>,Float>	RoundingMode roundingMode
div	Function<ValuePair<Number,Number>,Float>	MathContext mathContext

mult	Function<ValuePair<Number,Number>,Float>

remainder	Function<ValuePair<Number,Number>,Float>
remainder	Function<ValuePair<Number,Number>,Float>	MathContext mathContext

subt	Function<ValuePair<Number,Number>,Float>

FnReduceOnInteger

These functions are to be applied on Numbers and all of them return an Integer.

Function name	Type	Params
avg	Function<ValuePair<Number,Number>,Integer>
avg	Function<ValuePair<Number,Number>,Integer>	RoundingMode roundingMode
avg	Function<ValuePair<Number,Number>,Integer>	MathContext mathContext

div	Function<ValuePair<Number,Number>,Integer>
div	Function<ValuePair<Number,Number>,Integer>	RoundingMode roundingMode
div	Function<ValuePair<Number,Number>,Integer>	MathContext mathContext

mult	Function<ValuePair<Number,Number>,Integer>

remainder	Function<ValuePair<Number,Number>,Integer>
remainder	Function<ValuePair<Number,Number>,Integer>	MathContext mathContext

subt	Function<ValuePair<Number,Number>,Integer>

FnReduceOnLong

These functions are to be applied on Numbers and all of them return a Long.

Function name	Type	Params
avg	Function<ValuePair<Number,Number>,Long>
avg	Function<ValuePair<Number,Number>,Long>	RoundingMode roundingMode
avg	Function<ValuePair<Number,Number>,Long>	MathContext mathContext

div	Function<ValuePair<Number,Number>,Long>
div	Function<ValuePair<Number,Number>,Long>	RoundingMode roundingMode
div	Function<ValuePair<Number,Number>,Long>	MathContext mathContext

mult	Function<ValuePair<Number,Number>,Long>

remainder	Function<ValuePair<Number,Number>,Long>
remainder	Function<ValuePair<Number,Number>,Long>	MathContext mathContext

subt	Function<ValuePair<Number,Number>,Long>

FnReduceOnShort

These functions are to be applied on Numbers and all of them return a Short.

Function name	Type	Params
avg	Function<ValuePair<Number,Number>,Short>
avg	Function<ValuePair<Number,Number>,Short>	RoundingMode roundingMode
avg	Function<ValuePair<Number,Number>,Short>	MathContext mathContext

div	Function<ValuePair<Number,Number>,Short>
div	Function<ValuePair<Number,Number>,Short>	RoundingMode roundingMode
div	Function<ValuePair<Number,Number>,Short>	MathContext mathContext

mult	Function<ValuePair<Number,Number>,Short>

remainder	Function<ValuePair<Number,Number>,Short>
remainder	Function<ValuePair<Number,Number>,Short>	MathContext mathContext

subt	Function<ValuePair<Number,Number>,Short>

FnReduceOnString

These functions are to be applied on Strings and all of them return a String as well.

Function name	Type	Params	Description
join	Function<ValuePair<String,String>,String>	String separator

FnReduceOn

The functions in FnReduceOn are applied on objects of any type. FnReduceOn is a generic class and, by calling FnReduce.onCharacter(), FnReduce.onCalendar(), FnReduce.onDate(), FnReduce.onObject() or FnReduce.on(final Type<T> type), the type parameter can be replaced with the type you need at any time so that chaining functions becomes easier and less error-prone.

Op.on(new Calendar[] {Calendar.getInstance(), Calendar.getInstance()}).toList()                         
    .reduce(FnReduce.onCalendar().min()).get();
    
            
Op.on(aListOfComprables))                      
    .reduce(FnReduce.on(Types.forClass(MyClassOfComparables.class)).min()).get());

Function name	Type	Params
max	Function<ValuePair<T,T>,T>

maxBy	<X extends Comparable<? super X>> Function<ValuePair<T,T>,T>	IFunction<? super T,X> function

min	Function<ValuePair<T,T>,T>

minBy	<X extends Comparable<? super X>> Function<ValuePair<T,T>,T>	IFunction<? super T,X> function

The op4j Project

Extensions

Functions: FnReduce