Joiner.java
/*
* Copyright (C) 2008 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.checkNotNull;
import static java.util.Objects.requireNonNull;
import com.google.common.annotations.Beta;
import com.google.common.annotations.GwtCompatible;
import com.google.errorprone.annotations.CanIgnoreReturnValue;
import java.io.IOException;
import java.util.AbstractList;
import java.util.Arrays;
import java.util.Iterator;
import java.util.Map;
import java.util.Map.Entry;
import javax.annotation.CheckForNull;
import org.checkerframework.checker.nullness.qual.Nullable;
/**
* An object which joins pieces of text (specified as an array, {@link Iterable}, varargs or even a
* {@link Map}) with a separator. It either appends the results to an {@link Appendable} or returns
* them as a {@link String}. Example:
*
* <pre>{@code
* Joiner joiner = Joiner.on("; ").skipNulls();
* . . .
* return joiner.join("Harry", null, "Ron", "Hermione");
* }</pre>
*
* <p>This returns the string {@code "Harry; Ron; Hermione"}. Note that all input elements are
* converted to strings using {@link Object#toString()} before being appended.
*
* <p>If neither {@link #skipNulls()} nor {@link #useForNull(String)} is specified, the joining
* methods will throw {@link NullPointerException} if any given element is null.
*
* <p><b>Warning: joiner instances are always immutable</b>; a configuration method such as {@code
* useForNull} has no effect on the instance it is invoked on! You must store and use the new joiner
* instance returned by the method. This makes joiners thread-safe, and safe to store as {@code
* static final} constants.
*
* <pre>{@code
* // Bad! Do not do this!
* Joiner joiner = Joiner.on(',');
* joiner.skipNulls(); // does nothing!
* return joiner.join("wrong", null, "wrong");
* }</pre>
*
* <p>See the Guava User Guide article on <a
* href="https://github.com/google/guava/wiki/StringsExplained#joiner">{@code Joiner}</a>.
*
* @author Kevin Bourrillion
* @since 2.0
*/
@GwtCompatible
@ElementTypesAreNonnullByDefault
public class Joiner {
/** Returns a joiner which automatically places {@code separator} between consecutive elements. */
public static Joiner on(String separator) {
return new Joiner(separator);
}
/** Returns a joiner which automatically places {@code separator} between consecutive elements. */
public static Joiner on(char separator) {
return new Joiner(String.valueOf(separator));
}
private final String separator;
private Joiner(String separator) {
this.separator = checkNotNull(separator);
}
private Joiner(Joiner prototype) {
this.separator = prototype.separator;
}
/*
* In this file, we use <? extends @Nullable Object> instead of <?> to work around a Kotlin bug
* (see b/189937072 until we file a bug against Kotlin itself). (The two should be equivalent, so
* we normally prefer the shorter one.)
*/
/**
* Appends the string representation of each of {@code parts}, using the previously configured
* separator between each, to {@code appendable}.
*/
@CanIgnoreReturnValue
public <A extends Appendable> A appendTo(A appendable, Iterable<? extends @Nullable Object> parts)
throws IOException {
return appendTo(appendable, parts.iterator());
}
/**
* Appends the string representation of each of {@code parts}, using the previously configured
* separator between each, to {@code appendable}.
*
* @since 11.0
*/
@CanIgnoreReturnValue
public <A extends Appendable> A appendTo(A appendable, Iterator<? extends @Nullable Object> parts)
throws IOException {
checkNotNull(appendable);
if (parts.hasNext()) {
appendable.append(toString(parts.next()));
while (parts.hasNext()) {
appendable.append(separator);
appendable.append(toString(parts.next()));
}
}
return appendable;
}
/**
* Appends the string representation of each of {@code parts}, using the previously configured
* separator between each, to {@code appendable}.
*/
@CanIgnoreReturnValue
public final <A extends Appendable> A appendTo(A appendable, @Nullable Object[] parts)
throws IOException {
return appendTo(appendable, Arrays.asList(parts));
}
/** Appends to {@code appendable} the string representation of each of the remaining arguments. */
@CanIgnoreReturnValue
public final <A extends Appendable> A appendTo(
A appendable,
@CheckForNull Object first,
@CheckForNull Object second,
@Nullable Object... rest)
throws IOException {
return appendTo(appendable, iterable(first, second, rest));
}
/**
* Appends the string representation of each of {@code parts}, using the previously configured
* separator between each, to {@code builder}. Identical to {@link #appendTo(Appendable,
* Iterable)}, except that it does not throw {@link IOException}.
*/
@CanIgnoreReturnValue
public final StringBuilder appendTo(
StringBuilder builder, Iterable<? extends @Nullable Object> parts) {
return appendTo(builder, parts.iterator());
}
/**
* Appends the string representation of each of {@code parts}, using the previously configured
* separator between each, to {@code builder}. Identical to {@link #appendTo(Appendable,
* Iterable)}, except that it does not throw {@link IOException}.
*
* @since 11.0
*/
@CanIgnoreReturnValue
public final StringBuilder appendTo(
StringBuilder builder, Iterator<? extends @Nullable Object> parts) {
try {
appendTo((Appendable) builder, parts);
} catch (IOException impossible) {
throw new AssertionError(impossible);
}
return builder;
}
/**
* Appends the string representation of each of {@code parts}, using the previously configured
* separator between each, to {@code builder}. Identical to {@link #appendTo(Appendable,
* Iterable)}, except that it does not throw {@link IOException}.
*/
@CanIgnoreReturnValue
public final StringBuilder appendTo(StringBuilder builder, @Nullable Object[] parts) {
return appendTo(builder, Arrays.asList(parts));
}
/**
* Appends to {@code builder} the string representation of each of the remaining arguments.
* Identical to {@link #appendTo(Appendable, Object, Object, Object...)}, except that it does not
* throw {@link IOException}.
*/
@CanIgnoreReturnValue
public final StringBuilder appendTo(
StringBuilder builder,
@CheckForNull Object first,
@CheckForNull Object second,
@Nullable Object... rest) {
return appendTo(builder, iterable(first, second, rest));
}
/**
* Returns a string containing the string representation of each of {@code parts}, using the
* previously configured separator between each.
*/
public final String join(Iterable<? extends @Nullable Object> parts) {
return join(parts.iterator());
}
/**
* Returns a string containing the string representation of each of {@code parts}, using the
* previously configured separator between each.
*
* @since 11.0
*/
public final String join(Iterator<? extends @Nullable Object> parts) {
return appendTo(new StringBuilder(), parts).toString();
}
/**
* Returns a string containing the string representation of each of {@code parts}, using the
* previously configured separator between each.
*/
public final String join(@Nullable Object[] parts) {
return join(Arrays.asList(parts));
}
/**
* Returns a string containing the string representation of each argument, using the previously
* configured separator between each.
*/
public final String join(
@CheckForNull Object first, @CheckForNull Object second, @Nullable Object... rest) {
return join(iterable(first, second, rest));
}
/**
* Returns a joiner with the same behavior as this one, except automatically substituting {@code
* nullText} for any provided null elements.
*/
public Joiner useForNull(final String nullText) {
checkNotNull(nullText);
return new Joiner(this) {
@Override
CharSequence toString(@CheckForNull Object part) {
return (part == null) ? nullText : Joiner.this.toString(part);
}
@Override
public Joiner useForNull(String nullText) {
throw new UnsupportedOperationException("already specified useForNull");
}
@Override
public Joiner skipNulls() {
throw new UnsupportedOperationException("already specified useForNull");
}
};
}
/**
* Returns a joiner with the same behavior as this joiner, except automatically skipping over any
* provided null elements.
*/
public Joiner skipNulls() {
return new Joiner(this) {
@Override
public <A extends Appendable> A appendTo(
A appendable, Iterator<? extends @Nullable Object> parts) throws IOException {
checkNotNull(appendable, "appendable");
checkNotNull(parts, "parts");
while (parts.hasNext()) {
Object part = parts.next();
if (part != null) {
appendable.append(Joiner.this.toString(part));
break;
}
}
while (parts.hasNext()) {
Object part = parts.next();
if (part != null) {
appendable.append(separator);
appendable.append(Joiner.this.toString(part));
}
}
return appendable;
}
@Override
public Joiner useForNull(String nullText) {
throw new UnsupportedOperationException("already specified skipNulls");
}
@Override
public MapJoiner withKeyValueSeparator(String kvs) {
throw new UnsupportedOperationException("can't use .skipNulls() with maps");
}
};
}
/**
* Returns a {@code MapJoiner} using the given key-value separator, and the same configuration as
* this {@code Joiner} otherwise.
*
* @since 20.0
*/
public MapJoiner withKeyValueSeparator(char keyValueSeparator) {
return withKeyValueSeparator(String.valueOf(keyValueSeparator));
}
/**
* Returns a {@code MapJoiner} using the given key-value separator, and the same configuration as
* this {@code Joiner} otherwise.
*/
public MapJoiner withKeyValueSeparator(String keyValueSeparator) {
return new MapJoiner(this, keyValueSeparator);
}
/**
* An object that joins map entries in the same manner as {@code Joiner} joins iterables and
* arrays. Like {@code Joiner}, it is thread-safe and immutable.
*
* <p>In addition to operating on {@code Map} instances, {@code MapJoiner} can operate on {@code
* Multimap} entries in two distinct modes:
*
* <ul>
* <li>To output a separate entry for each key-value pair, pass {@code multimap.entries()} to a
* {@code MapJoiner} method that accepts entries as input, and receive output of the form
* {@code key1=A&key1=B&key2=C}.
* <li>To output a single entry for each key, pass {@code multimap.asMap()} to a {@code
* MapJoiner} method that accepts a map as input, and receive output of the form {@code
* key1=[A, B]&key2=C}.
* </ul>
*
* @since 2.0
*/
public static final class MapJoiner {
private final Joiner joiner;
private final String keyValueSeparator;
private MapJoiner(Joiner joiner, String keyValueSeparator) {
this.joiner = joiner; // only "this" is ever passed, so don't checkNotNull
this.keyValueSeparator = checkNotNull(keyValueSeparator);
}
/**
* Appends the string representation of each entry of {@code map}, using the previously
* configured separator and key-value separator, to {@code appendable}.
*/
@CanIgnoreReturnValue
public <A extends Appendable> A appendTo(A appendable, Map<?, ?> map) throws IOException {
return appendTo(appendable, map.entrySet());
}
/**
* Appends the string representation of each entry of {@code map}, using the previously
* configured separator and key-value separator, to {@code builder}. Identical to {@link
* #appendTo(Appendable, Map)}, except that it does not throw {@link IOException}.
*/
@CanIgnoreReturnValue
public StringBuilder appendTo(StringBuilder builder, Map<?, ?> map) {
return appendTo(builder, map.entrySet());
}
/**
* Appends the string representation of each entry in {@code entries}, using the previously
* configured separator and key-value separator, to {@code appendable}.
*
* @since 10.0
*/
@Beta
@CanIgnoreReturnValue
public <A extends Appendable> A appendTo(A appendable, Iterable<? extends Entry<?, ?>> entries)
throws IOException {
return appendTo(appendable, entries.iterator());
}
/**
* Appends the string representation of each entry in {@code entries}, using the previously
* configured separator and key-value separator, to {@code appendable}.
*
* @since 11.0
*/
@Beta
@CanIgnoreReturnValue
public <A extends Appendable> A appendTo(A appendable, Iterator<? extends Entry<?, ?>> parts)
throws IOException {
checkNotNull(appendable);
if (parts.hasNext()) {
Entry<?, ?> entry = parts.next();
appendable.append(joiner.toString(entry.getKey()));
appendable.append(keyValueSeparator);
appendable.append(joiner.toString(entry.getValue()));
while (parts.hasNext()) {
appendable.append(joiner.separator);
Entry<?, ?> e = parts.next();
appendable.append(joiner.toString(e.getKey()));
appendable.append(keyValueSeparator);
appendable.append(joiner.toString(e.getValue()));
}
}
return appendable;
}
/**
* Appends the string representation of each entry in {@code entries}, using the previously
* configured separator and key-value separator, to {@code builder}. Identical to {@link
* #appendTo(Appendable, Iterable)}, except that it does not throw {@link IOException}.
*
* @since 10.0
*/
@Beta
@CanIgnoreReturnValue
public StringBuilder appendTo(StringBuilder builder, Iterable<? extends Entry<?, ?>> entries) {
return appendTo(builder, entries.iterator());
}
/**
* Appends the string representation of each entry in {@code entries}, using the previously
* configured separator and key-value separator, to {@code builder}. Identical to {@link
* #appendTo(Appendable, Iterable)}, except that it does not throw {@link IOException}.
*
* @since 11.0
*/
@Beta
@CanIgnoreReturnValue
public StringBuilder appendTo(StringBuilder builder, Iterator<? extends Entry<?, ?>> entries) {
try {
appendTo((Appendable) builder, entries);
} catch (IOException impossible) {
throw new AssertionError(impossible);
}
return builder;
}
/**
* Returns a string containing the string representation of each entry of {@code map}, using the
* previously configured separator and key-value separator.
*/
public String join(Map<?, ?> map) {
return join(map.entrySet());
}
/**
* Returns a string containing the string representation of each entry in {@code entries}, using
* the previously configured separator and key-value separator.
*
* @since 10.0
*/
@Beta
public String join(Iterable<? extends Entry<?, ?>> entries) {
return join(entries.iterator());
}
/**
* Returns a string containing the string representation of each entry in {@code entries}, using
* the previously configured separator and key-value separator.
*
* @since 11.0
*/
@Beta
public String join(Iterator<? extends Entry<?, ?>> entries) {
return appendTo(new StringBuilder(), entries).toString();
}
/**
* Returns a map joiner with the same behavior as this one, except automatically substituting
* {@code nullText} for any provided null keys or values.
*/
public MapJoiner useForNull(String nullText) {
return new MapJoiner(joiner.useForNull(nullText), keyValueSeparator);
}
}
CharSequence toString(@CheckForNull Object part) {
/*
* requireNonNull is not safe: Joiner.on(...).join(somethingThatContainsNull) will indeed throw.
* However, Joiner.on(...).useForNull(...).join(somethingThatContainsNull) *is* safe -- because
* it returns a subclass of Joiner that overrides this method to tolerate null inputs.
*
* Unfortunately, we don't distinguish between these two cases in our public API: Joiner.on(...)
* and Joiner.on(...).useForNull(...) both declare the same return type: plain Joiner. To ensure
* that users *can* pass null arguments to Joiner, we annotate it as if it always tolerates null
* inputs, rather than as if it never tolerates them.
*
* We rely on checkers to implement special cases to catch dangerous calls to join(), etc. based
* on what they know about the particular Joiner instances the calls are performed on.
*
* (In addition to useForNull, we also offer skipNulls. It, too, tolerates null inputs, but its
* tolerance is implemented differently: Its implementation avoids calling this toString(Object)
* method in the first place.)
*/
requireNonNull(part);
return (part instanceof CharSequence) ? (CharSequence) part : part.toString();
}
private static Iterable<@Nullable Object> iterable(
@CheckForNull final Object first,
@CheckForNull final Object second,
final @Nullable Object[] rest) {
checkNotNull(rest);
return new AbstractList<@Nullable Object>() {
@Override
public int size() {
return rest.length + 2;
}
@Override
@CheckForNull
public Object get(int index) {
switch (index) {
case 0:
return first;
case 1:
return second;
default:
return rest[index - 2];
}
}
};
}
}