Groovy Operators - Coercion Operator

[Last Updated: Jan 23, 2019]

The coercion operator (as) converts an object from one type to another type without them being compatible for assignment.

This is different than type casting. Casting a type which is not assignable to the target type, will produce a ClassCastException at runtime. The coercion operator converts the object to the target type via predefined conversion rules even if it is not castable to the target type . Let's see examples to understand that:

src/Example1AsOpt.groovy

String s = "1.1"
BigDecimal bd = s as BigDecimal;
println bd
println bd.class.name

Output

1.1
java.math.BigDecimal

In above example String is converted to BigDecimal. This conversion is performed in StringGroovyMethods#asType(....).
To have a better idea, following is a quick snippet of StringGroovyMethods:

package org.codehaus.groovy.runtime;
.....
public class StringGroovyMethods extends DefaultGroovyMethodsSupport {
   ....
    public static <T> T asType(String self, Class<T> c) {
        if (c == List.class) {
            return toList((CharSequence)self);
        } else if (c == BigDecimal.class) {
            return toBigDecimal((CharSequence)self);
        } else if (c == BigInteger.class) {
            return toBigInteger((CharSequence)self);
        } else if (c != Long.class && c != Long.TYPE) {
        .......
        }
    }
    ........
}

The super class DefaultGroovyMethodsSupport defines more asType() methods for other type conversions.

Defining asType()

We can also define asType(Class target) method in our own class for as operator to work.

src/Example2AsOpt.groovy

import java.math.MathContext

class Fraction {
    int numerator;
    int denominator;

    Fraction(int numerator, int denominator) {
        this.numerator = numerator
        this.denominator = denominator
    }

    def asType(Class target) {
        switch (target) {
            case BigDecimal.class:
                return new BigDecimal(numerator).divide(
                        new BigDecimal(denominator), MathContext.DECIMAL32)
            case String.class:
                return numerator + "/" + denominator
            case Double.class:
                return Double.valueOf((double) numerator / (double) denominator)
            default:throw new RuntimeException("not supported: "+target);
        }
    }
}


def f = new Fraction(3, 11);

def bd = f as BigDecimal
println bd

def d = f as Double
println d

def s = f as String
println s

Output

0.2727273
0.2727272727272727
3/11

The coercion operator can also be used at other places for example during declaring types.

src/Example3AsOpt.groovy

def list = [1,3,5]
println list.class.name
println list.class.isArray()

def arr = [1,3,5] as int[]
println arr.class.name
println arr.class.isArray()