guava/src/com/google/common/base/Functions.java - platform/external/guava - Git at Google

 /*
  * Copyright (C) 2007 The Guava Authors
  *
  * 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.checkArgument;
 import static com.google.common.base.Preconditions.checkNotNull;

 import com.google.common.annotations.Beta;
 import com.google.common.annotations.GwtCompatible;

 import java.io.Serializable;
 import java.util.Map;

 import javax.annotation.Nullable;

 /**
  * Static utility methods pertaining to {@code Function} instances.
  *
  * <p>All methods return serializable functions as long as they're given serializable parameters.
  *
  * <p>See the Guava User Guide article on <a href=
  * "http://code.google.com/p/guava-libraries/wiki/FunctionalExplained">the use of {@code
  * Function}</a>.
  *
  * @author Mike Bostock
  * @author Jared Levy
  * @since 2.0 (imported from Google Collections Library)
  */
 @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}.
    *
    * <p><b>Warning:</b> The returned function may not be <i>consistent with equals</i> (as
    * documented at {@link Function#apply}). For example, this function yields different results for
    * the two equal instances {@code ImmutableSet.of(1, 2)} and {@code ImmutableSet.of(2, 1)}.
    */
   public static Function<Object, String> toStringFunction() {
     return ToStringFunction.INSTANCE;
   }

   // enum singleton pattern
   private enum ToStringFunction implements Function<Object, String> {
     INSTANCE;

     @Override
     public String apply(Object o) {
       checkNotNull(o);  // eager for GWT.
       return o.toString();
     }

     @Override public String toString() {
       return "toString";
     }
   }

   /**
    * Returns the identity function.
    */
   // implementation is "fully variant"; E has become a "pass-through" type
   @SuppressWarnings("unchecked")
   public static <E> Function<E, E> identity() {
     return (Function<E, E>) IdentityFunction.INSTANCE;
   }

   // enum singleton pattern
   private enum IdentityFunction implements Function<Object, Object> {
     INSTANCE;

     @Override
     @Nullable
     public Object apply(@Nullable Object o) {
       return o;
     }

     @Override public String toString() {
       return "identity";
     }
   }

   /**
    * Returns a function which performs a map lookup. The returned function throws an {@link
    * IllegalArgumentException} if given a key that does not exist in the map. See also {@link
    * #forMap(Map, Object)}, which returns a default value in this case.
    *
    * <p>Note: if {@code map} is a {@link com.google.common.collect.BiMap BiMap} (or can be one), you
    * can use {@link com.google.common.collect.Maps#asConverter Maps.asConverter} instead to get a
    * function that also supports reverse conversion.
    */
   public static <K, V> Function<K, V> forMap(Map<K, V> map) {
     return new FunctionForMapNoDefault<K, V>(map);
   }

   private static class FunctionForMapNoDefault<K, V> implements Function<K, V>, Serializable {
     final Map<K, V> map;

     FunctionForMapNoDefault(Map<K, V> map) {
       this.map = checkNotNull(map);
     }

     @Override
     public V apply(@Nullable K key) {
       V result = map.get(key);
       checkArgument(result != null || map.containsKey(key), "Key '%s' not present in map", key);
       return result;
     }

     @Override public boolean equals(@Nullable 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. See also {@link #forMap(Map)}, which throws an exception in this case.
    *
    * @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 <K, V> Function<K, V> forMap(Map<K, ? extends V> map, @Nullable V defaultValue) {
     return new ForMapWithDefault<K, V>(map, defaultValue);
   }

   private static class ForMapWithDefault<K, V> implements Function<K, V>, Serializable {
     final Map<K, ? extends V> map;
     final V defaultValue;

     ForMapWithDefault(Map<K, ? extends V> map, @Nullable V defaultValue) {
       this.map = checkNotNull(map);
       this.defaultValue = defaultValue;
     }

     @Override
     public V apply(@Nullable K key) {
       V result = map.get(key);
       return (result != null || map.containsKey(key)) ? result : defaultValue;
     }

     @Override public boolean equals(@Nullable 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 Objects.hashCode(map, defaultValue);
     }

     @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}.
    *
    * @param g the second function to apply
    * @param f the first function to apply
    * @return the composition of {@code f} and {@code g}
    * @see <a href="//en.wikipedia.org/wiki/Function_composition">function composition</a>
    */
   public static <A, B, C> Function<A, C> compose(Function<B, C> g, Function<A, ? extends B> f) {
     return new FunctionComposition<A, B, C>(g, f);
   }

   private static class FunctionComposition<A, B, C> implements Function<A, C>, Serializable {
     private final Function<B, C> g;
     private final Function<A, ? extends B> f;

     public FunctionComposition(Function<B, C> g, Function<A, ? extends B> f) {
       this.g = checkNotNull(g);
       this.f = checkNotNull(f);
     }

     @Override
     public C apply(@Nullable A a) {
       return g.apply(f.apply(a));
     }

     @Override public boolean equals(@Nullable 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 + "(" + f + ")";
     }

     private static final long serialVersionUID = 0;
   }

   /**
    * Creates a function that returns the same boolean output as the given predicate for all inputs.
    *
    * <p>The returned function is <i>consistent with equals</i> (as documented at {@link
    * Function#apply}) if and only if {@code predicate} is itself consistent with equals.
    */
   public static <T> Function<T, Boolean> forPredicate(Predicate<T> predicate) {
     return new PredicateFunction<T>(predicate);
   }

   /** @see Functions#forPredicate */
   private static class PredicateFunction<T> implements Function<T, Boolean>, Serializable {
     private final Predicate<T> predicate;

     private PredicateFunction(Predicate<T> predicate) {
       this.predicate = checkNotNull(predicate);
     }

     @Override
     public Boolean apply(@Nullable T t) {
       return predicate.apply(t);
     }

     @Override public boolean equals(@Nullable 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 <E> Function<Object, E> constant(@Nullable E value) {
     return new ConstantFunction<E>(value);
   }

   private static class ConstantFunction<E> implements Function<Object, E>, Serializable {
     private final E value;

     public ConstantFunction(@Nullable E value) {
       this.value = value;
     }

     @Override
     public E apply(@Nullable Object from) {
       return value;
     }

     @Override public boolean equals(@Nullable 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;
   }

   /**
    * Returns a function that always returns the result of invoking {@link Supplier#get} on {@code
    * supplier}, regardless of its input.
    *
    * @since 10.0
    */
   @Beta
   public static <T> Function<Object, T> forSupplier(Supplier<T> supplier) {
     return new SupplierFunction<T>(supplier);
   }

   /** @see Functions#forSupplier*/
   private static class SupplierFunction<T> implements Function<Object, T>, Serializable {

     private final Supplier<T> supplier;

     private SupplierFunction(Supplier<T> supplier) {
       this.supplier = checkNotNull(supplier);
     }

     @Override public T apply(@Nullable Object input) {
       return supplier.get();
     }

     @Override public boolean equals(@Nullable Object obj) {
       if (obj instanceof SupplierFunction) {
         SupplierFunction<?> that = (SupplierFunction<?>) obj;
         return this.supplier.equals(that.supplier);
       }
       return false;
     }

     @Override public int hashCode() {
       return supplier.hashCode();
     }

     @Override public String toString() {
       return "forSupplier(" + supplier + ")";
     }

     private static final long serialVersionUID = 0;
   }
 }
	/*
	* Copyright (C) 2007 The Guava Authors
	*
	* 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.checkArgument;
	import static com.google.common.base.Preconditions.checkNotNull;

	import com.google.common.annotations.Beta;
	import com.google.common.annotations.GwtCompatible;

	import java.io.Serializable;
	import java.util.Map;

	import javax.annotation.Nullable;

	/**
	* Static utility methods pertaining to {@code Function} instances.
	*
	* <p>All methods return serializable functions as long as they're given serializable parameters.
	*
	* <p>See the Guava User Guide article on <a href=
	* "http://code.google.com/p/guava-libraries/wiki/FunctionalExplained">the use of {@code
	* Function}</a>.
	*
	* @author Mike Bostock
	* @author Jared Levy
	* @since 2.0 (imported from Google Collections Library)
	*/
	@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}.
	*
	* <p><b>Warning:</b> The returned function may not be <i>consistent with equals</i> (as
	* documented at {@link Function#apply}). For example, this function yields different results for
	* the two equal instances {@code ImmutableSet.of(1, 2)} and {@code ImmutableSet.of(2, 1)}.
	*/
	public static Function<Object, String> toStringFunction() {
	return ToStringFunction.INSTANCE;
	}

	// enum singleton pattern
	private enum ToStringFunction implements Function<Object, String> {
	INSTANCE;

	@Override
	public String apply(Object o) {
	checkNotNull(o); // eager for GWT.
	return o.toString();
	}

	@Override public String toString() {
	return "toString";
	}
	}

	/**
	* Returns the identity function.
	*/
	// implementation is "fully variant"; E has become a "pass-through" type
	@SuppressWarnings("unchecked")
	public static <E> Function<E, E> identity() {
	return (Function<E, E>) IdentityFunction.INSTANCE;
	}

	// enum singleton pattern
	private enum IdentityFunction implements Function<Object, Object> {
	INSTANCE;

	@Override
	@Nullable
	public Object apply(@Nullable Object o) {
	return o;
	}

	@Override public String toString() {
	return "identity";
	}
	}

	/**
	* Returns a function which performs a map lookup. The returned function throws an {@link
	* IllegalArgumentException} if given a key that does not exist in the map. See also {@link
	* #forMap(Map, Object)}, which returns a default value in this case.
	*
	* <p>Note: if {@code map} is a {@link com.google.common.collect.BiMap BiMap} (or can be one), you
	* can use {@link com.google.common.collect.Maps#asConverter Maps.asConverter} instead to get a
	* function that also supports reverse conversion.
	*/
	public static <K, V> Function<K, V> forMap(Map<K, V> map) {
	return new FunctionForMapNoDefault<K, V>(map);
	}

	private static class FunctionForMapNoDefault<K, V> implements Function<K, V>, Serializable {
	final Map<K, V> map;

	FunctionForMapNoDefault(Map<K, V> map) {
	this.map = checkNotNull(map);
	}

	@Override
	public V apply(@Nullable K key) {
	V result = map.get(key);
	checkArgument(result != null \|\| map.containsKey(key), "Key '%s' not present in map", key);
	return result;
	}

	@Override public boolean equals(@Nullable 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. See also {@link #forMap(Map)}, which throws an exception in this case.
	*
	* @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 <K, V> Function<K, V> forMap(Map<K, ? extends V> map, @Nullable V defaultValue) {
	return new ForMapWithDefault<K, V>(map, defaultValue);
	}

	private static class ForMapWithDefault<K, V> implements Function<K, V>, Serializable {
	final Map<K, ? extends V> map;
	final V defaultValue;

	ForMapWithDefault(Map<K, ? extends V> map, @Nullable V defaultValue) {
	this.map = checkNotNull(map);
	this.defaultValue = defaultValue;
	}

	@Override
	public V apply(@Nullable K key) {
	V result = map.get(key);
	return (result != null \|\| map.containsKey(key)) ? result : defaultValue;
	}

	@Override public boolean equals(@Nullable 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 Objects.hashCode(map, defaultValue);
	}

	@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}.
	*
	* @param g the second function to apply
	* @param f the first function to apply
	* @return the composition of {@code f} and {@code g}
	* @see <a href="//en.wikipedia.org/wiki/Function_composition">function composition</a>
	*/
	public static <A, B, C> Function<A, C> compose(Function<B, C> g, Function<A, ? extends B> f) {
	return new FunctionComposition<A, B, C>(g, f);
	}

	private static class FunctionComposition<A, B, C> implements Function<A, C>, Serializable {
	private final Function<B, C> g;
	private final Function<A, ? extends B> f;

	public FunctionComposition(Function<B, C> g, Function<A, ? extends B> f) {
	this.g = checkNotNull(g);
	this.f = checkNotNull(f);
	}

	@Override
	public C apply(@Nullable A a) {
	return g.apply(f.apply(a));
	}

	@Override public boolean equals(@Nullable 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 + "(" + f + ")";
	}

	private static final long serialVersionUID = 0;
	}

	/**
	* Creates a function that returns the same boolean output as the given predicate for all inputs.
	*
	* <p>The returned function is <i>consistent with equals</i> (as documented at {@link
	* Function#apply}) if and only if {@code predicate} is itself consistent with equals.
	*/
	public static <T> Function<T, Boolean> forPredicate(Predicate<T> predicate) {
	return new PredicateFunction<T>(predicate);
	}

	/** @see Functions#forPredicate */
	private static class PredicateFunction<T> implements Function<T, Boolean>, Serializable {
	private final Predicate<T> predicate;

	private PredicateFunction(Predicate<T> predicate) {
	this.predicate = checkNotNull(predicate);
	}

	@Override
	public Boolean apply(@Nullable T t) {
	return predicate.apply(t);
	}

	@Override public boolean equals(@Nullable 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 <E> Function<Object, E> constant(@Nullable E value) {
	return new ConstantFunction<E>(value);
	}

	private static class ConstantFunction<E> implements Function<Object, E>, Serializable {
	private final E value;

	public ConstantFunction(@Nullable E value) {
	this.value = value;
	}

	@Override
	public E apply(@Nullable Object from) {
	return value;
	}

	@Override public boolean equals(@Nullable 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;
	}

	/**
	* Returns a function that always returns the result of invoking {@link Supplier#get} on {@code
	* supplier}, regardless of its input.
	*
	* @since 10.0
	*/
	@Beta
	public static <T> Function<Object, T> forSupplier(Supplier<T> supplier) {
	return new SupplierFunction<T>(supplier);
	}

	/** @see Functions#forSupplier*/
	private static class SupplierFunction<T> implements Function<Object, T>, Serializable {

	private final Supplier<T> supplier;

	private SupplierFunction(Supplier<T> supplier) {
	this.supplier = checkNotNull(supplier);
	}

	@Override public T apply(@Nullable Object input) {
	return supplier.get();
	}

	@Override public boolean equals(@Nullable Object obj) {
	if (obj instanceof SupplierFunction) {
	SupplierFunction<?> that = (SupplierFunction<?>) obj;
	return this.supplier.equals(that.supplier);
	}
	return false;
	}

	@Override public int hashCode() {
	return supplier.hashCode();
	}

	@Override public String toString() {
	return "forSupplier(" + supplier + ")";
	}

	private static final long serialVersionUID = 0;
	}
	}