com.google.common.base.Functions Maven / Gradle / Ivy
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/*
* 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 com.google.common.annotations.GwtCompatible;
import static com.google.common.base.Preconditions.checkNotNull;
import java.io.Serializable;
import java.util.Map;
import javax.annotation.Nullable;
/**
* Useful functions.
*
* All methods returns serializable functions as long as they're given
* serializable parameters.
*
* @author Mike Bostock
* @author Vlad Patryshev
* @author Jared Levy
*/
@GwtCompatible
public final class Functions {
private Functions() {}
/**
* Returns a function that calls {@code toString()} on its argument. The
* function does not accept nulls; it will throw a
* {@link NullPointerException} when applied to {@code null}.
*/
public static Function toStringFunction() {
return ToStringFunction.INSTANCE;
}
// enum singleton pattern
private enum ToStringFunction implements Function {
INSTANCE;
public String apply(Object o) {
return o.toString();
}
@Override public String toString() {
return "toString";
}
}
/**
* Returns the identity function.
*/
@SuppressWarnings("unchecked")
public static Function identity() {
return (Function) IdentityFunction.INSTANCE;
}
// enum singleton pattern
private enum IdentityFunction implements Function {
INSTANCE;
public Object apply(Object o) {
return o;
}
@Override public String toString() {
return "identity";
}
}
/**
* Returns a function which performs a map lookup.
*
* 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 all inputs of the correct
* type. The function created by this method returns {@code null} for all
* inputs that do not belong to the map's key set.
*
* @param map source map that determines the function behavior
* @return function that returns {@code map.get(a)} for each {@code a}
*/
public static Function forMap(Map map) {
return new FunctionForMapNoDefault(map);
}
private static class FunctionForMapNoDefault
implements Function, Serializable {
private final Map map;
public FunctionForMapNoDefault(Map map) {
this.map = checkNotNull(map);
}
public V apply(K 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 = 0;
}
/**
* Returns a function which performs a map lookup with a default value. The
* function created by this method returns {@code defaultValue} for all
* inputs that do not belong to the map's key set.
*
* @param map source map that determines the function behavior
* @param defaultValue the value to return for inputs that aren't map keys
* @return function that returns {@code map.get(a)} when {@code a} is a key,
* or {@code defaultValue} otherwise
*/
public static Function forMap(
Map map, @Nullable V defaultValue) {
return new ForMapWithDefault(map, defaultValue);
}
private static class ForMapWithDefault
implements Function, Serializable {
private final Map map;
private final V defaultValue;
public ForMapWithDefault(Map map, V defaultValue) {
this.map = checkNotNull(map);
this.defaultValue = defaultValue;
}
public V apply(K 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)
&& Objects.equal(defaultValue, that.defaultValue);
}
return false;
}
@Override public int hashCode() {
return map.hashCode() + defaultValue.hashCode();
}
@Override public String toString() {
return "forMap(" + map + ", defaultValue=" + defaultValue + ")";
}
private static final long serialVersionUID = 0;
}
/**
* 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(a) == g(f(a))} for each {@code a}.
*
* @see
* function composition
*
* @param g the second function to apply
* @param f the first function to apply
* @return the composition of {@code f} and {@code g}
*/
public static Function compose(
Function g, Function f) {
return new FunctionComposition(g, f);
}
private static class FunctionComposition
implements Function, Serializable {
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() {
return f.hashCode() ^ g.hashCode();
}
@Override public String toString() {
return g.toString() + "(" + f.toString() + ")";
}
private static final long serialVersionUID = 0;
}
/**
* Creates a function that returns the same boolean output as the given
* predicate for all inputs.
*/
public static Function forPredicate(Predicate predicate) {
return new PredicateFunction(predicate);
}
/** @see Functions#forPredicate */
private static class PredicateFunction
implements Function, Serializable {
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 "forPredicate(" + predicate + ")";
}
private static final long serialVersionUID = 0;
}
/**
* Creates a function that returns {@code value} for any input.
*
* @param value the constant value for the function to return
* @return a function that always returns {@code value}
*/
public static Function constant(@Nullable E value) {
return new ConstantFunction(value);
}
private static class ConstantFunction
implements Function, Serializable {
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 static final long serialVersionUID = 0;
}
}