com.google.common.base.Functions Maven / Gradle / Ivy
/*
* Copyright (C) 2007 Google Inc.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package com.google.common.base;
import static com.google.common.base.Preconditions.checkNotNull;
import java.io.Serializable;
import java.util.Map;
/**
* Useful functions.
*
* @author Mike Bostock
* @author Vlad Patryshev
*/
public final class Functions {
private Functions() { }
/**
* See {@link #toStringFunction()}.
*
* TODO: Consider deprecating this in favor of an accessor method like
* {@link #toStringFunction()}.
*/
public static final Function TO_STRING =
ToStringFunction.INSTANCE;
/**
* A function that returns {@link Object#toString} of its argument.
* Note that this function is not {@literal @Nullable}: it will throw a
* {@link NullPointerException} when applied to {@code null}.
*
* Note also that this is assignable to variables of type
* {@code Function}, but not
* {@code Function}.
*/
public static final Function toStringFunction() {
return ToStringFunction.INSTANCE;
}
private static class ToStringFunction
implements SerializableFunction {
private static final long serialVersionUID = -2979910855853295325L;
public String apply(Object o) {
// avoiding String.valueOf(e) so we get an NPE instead of "null"
return o.toString();
}
private Object readResolve() {
return INSTANCE; /* Preserve singleton property. */
}
@Override public boolean equals(Object obj) {
return obj == INSTANCE;
}
@Override public int hashCode() {
return (int) serialVersionUID;
}
@Override public String toString() {
return "toString";
}
private static final ToStringFunction INSTANCE = new ToStringFunction();
}
/**
* @see #toHashCode()
*/
private static class HashCodeFunction
implements SerializableFunction {
private static final long serialVersionUID = 749417670239189171L;
public Integer apply(@Nullable Object o) {
return (o == null) ? 0 : o.hashCode();
}
private Object readResolve() {
return INSTANCE; /* Preserve singleton property. */
}
@Override public boolean equals(Object obj) {
return obj == INSTANCE;
}
@Override public int hashCode() {
return (int) serialVersionUID;
}
@Override public String toString() {
return "hashCode";
}
static final HashCodeFunction INSTANCE = new HashCodeFunction();
}
/**
* Returns a function that determines the {@link Object#hashCode()} of its
* argument. For null arguments, this returns 0, as used in hash code
* calculations by the Java Collections classes.
*/
public static Function toHashCode() {
return HashCodeFunction.INSTANCE;
}
/*
* For constant Functions a single instance will suffice; we'll cast it to
* the right parameterized type on demand.
*/
/**
* Returns the identity Function.
*/
@SuppressWarnings("unchecked")
public static Function identity() {
return (Function) IdentityFunction.INSTANCE;
}
/**
* @see Functions#identity
*/
private static class IdentityFunction
implements SerializableFunction {
private static final long serialVersionUID = 3129841931134422007L;
public Object apply(Object o) {
return o;
}
private Object readResolve() {
return INSTANCE;
}
@Override public boolean equals(Object obj) {
return obj == INSTANCE;
}
@Override public int hashCode() {
return (int) serialVersionUID;
}
@Override public String toString() {
return "identity";
}
private static final IdentityFunction INSTANCE = new IdentityFunction();
}
/**
* Returns a function which performs key-to-value lookup on {@code map}.
*
* The difference between a map and a function is that a map is defined on
* a set of keys, while a function is defined on a type.
* The function built by this method returns {@code null}
* for all arguments that do not belong to the map's keyset.
*
* @param map Map<A,B> source map
* @return Function<A,B> function that returns map.get(a) for each A a
*/
public static Function forMap(
final Map map) {
return new FunctionForMapNoDefault(map);
}
private static class FunctionForMapNoDefault
implements SerializableFunction {
private final Map map;
public FunctionForMapNoDefault(
Map map) {
this.map = checkNotNull(map);
}
public B apply(A a) {
return map.get(a);
}
@Override public boolean equals(Object o) {
if (o instanceof FunctionForMapNoDefault) {
FunctionForMapNoDefault that = (FunctionForMapNoDefault) o;
return map.equals(that.map);
}
return false;
}
@Override public int hashCode() {
return map.hashCode();
}
@Override public String toString() {
return "forMap(" + map + ")";
}
private static final long serialVersionUID = 3270419028101751025L;
}
/**
* Returns a function which performs key-to-value lookup on {@code map}.
* The function built by this method returns {@code defaultValue}
* for all its arguments that do not belong to the map's keyset.
*
* @param map Map<A,B>
* @param defaultValue B
* @return Function {@code f} such that {@code f(a)=map.get(a)}
* if {@code map.containsKey(x)}, and {@code defaultValue} otherwise.
*
* @see #forMap(Map)
*/
public static Function forMap(
final Map map,
@Nullable final B defaultValue) {
if (defaultValue == null) {
return forMap(map);
}
return new ForMapWithDefault(map, defaultValue);
}
private static class ForMapWithDefault
implements SerializableFunction {
private static final long serialVersionUID = 1652422010500531299L;
private final Map map;
private final B defaultValue;
public ForMapWithDefault(Map map, B defaultValue) {
this.map = checkNotNull(map);
this.defaultValue = checkNotNull(defaultValue);
}
public B apply(A a) {
return map.containsKey(a) ? map.get(a) : defaultValue;
}
@Override public boolean equals(Object o) {
if (o instanceof ForMapWithDefault) {
ForMapWithDefault that = (ForMapWithDefault) o;
return map.equals(that.map) && defaultValue.equals(that.defaultValue);
}
return false;
}
@Override public int hashCode() {
return map.hashCode() + defaultValue.hashCode();
}
@Override public String toString() {
return "forMap(" + map + ", defaultValue=" + defaultValue + ")";
}
}
/**
* Returns the composition of two functions. For {@code f: A->B} and
* {@code g: B->C}, composition is defined as the function h such that
* {@code h(x) == g(f(x))} for each {@code x}.
*
* @see
* function composition
*
* @param g Function<B,C>
* @param f Function<A,B>
* @return Function<A,C> composition of f and g
*/
public static Function compose(
final Function g,
final Function f) {
return new FunctionComposition(g, f);
}
private static class FunctionComposition
implements SerializableFunction {
private final Function g;
private final Function f;
public FunctionComposition(Function g,
Function f) {
this.g = checkNotNull(g);
this.f = checkNotNull(f);
}
public C apply(A a) {
return g.apply(f.apply(a));
}
@Override public boolean equals(Object obj) {
if (obj instanceof FunctionComposition) {
FunctionComposition that = (FunctionComposition) obj;
return f.equals(that.f) && g.equals(that.g);
}
return false;
}
@Override public int hashCode() {
/*
* TODO: To leave the door open for future enhancement, this
* calculation should be coordinated with the hashCode() method of the
* corresponding composition method in Predicates. To construct the
* composition:
* predicate(function2(function1(x)))
*
* There are two different ways of composing it:
* compose(predicate, compose(function2, function1))
* compose(compose(predicate, function2), function1)
*
* It would be nice if these could be equal.
*/
return f.hashCode() ^ g.hashCode();
}
@Override public String toString() {
return g.toString() + "(" + f.toString() + ")";
}
private static final long serialVersionUID = 2530922454216511764L;
}
/**
* Returns a boolean-valued function that evaluates to the same result as the
* given predicate.
*/
public static Function forPredicate(
Predicate predicate) {
checkNotNull(predicate);
return new PredicateFunction(predicate);
}
/** @see Functions#forPredicate */
private static class PredicateFunction
implements SerializableFunction {
private final Predicate predicate;
private PredicateFunction(Predicate predicate) {
this.predicate = checkNotNull(predicate);
}
public Boolean apply(T t) {
return predicate.apply(t);
}
@Override public boolean equals(Object obj) {
if (obj instanceof PredicateFunction) {
PredicateFunction that = (PredicateFunction) obj;
return predicate.equals(that.predicate);
}
return false;
}
@Override public int hashCode() {
return predicate.hashCode();
}
@Override public String toString() {
return "asFunction(" + predicate + ")";
}
private static final long serialVersionUID = 7159925838099303368L;
}
/**
* Returns a {@link Function} that returns {@code value} for any input.
*
* @param value the constant value for the {@code Function} to return
* @return a {@code Function} that always returns {@code value}.
*/
public static Function constant(@Nullable final E value) {
return new ConstantFunction(value);
}
private static class ConstantFunction
implements SerializableFunction {
private static final long serialVersionUID = 2347134351918525179L;
private final E value;
public ConstantFunction(@Nullable E value) {
this.value = value;
}
public E apply(Object from) {
return value;
}
@Override public boolean equals(Object obj) {
if (obj instanceof ConstantFunction) {
ConstantFunction that = (ConstantFunction) obj;
return Objects.equal(value, that.value);
}
return false;
}
@Override public int hashCode() {
return (value == null) ? 0 : value.hashCode();
}
@Override public String toString() {
return "constant(" + value + ")";
}
}
private interface SerializableFunction
extends Function, Serializable {}
}