StringJoiner与String.join()的使用手册
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1. StringJoiner：
Java 8 有了StringJoiner,发现时觉得真的好用啊，减少了很多代码量，让自己代码更加简洁明了。
StringJoiner的父类就是 Object
成员变量：
prefix：拼接后的字符串前缀
delimiter：拼接时的字符串分隔符
suffix：拼接后的字符串后缀
value：拼接后的值
emptyValue：空值的情况
value为 null 时返回
StringJoiner的字符串拼接也是基于StringBuilder进行字符拼接的：
/**
* Adds the contents of the given {@code StringJoiner} without prefix and
* suffix as the next element if it is non-empty. If the given {@code
* StringJoiner} is empty, the call has no effect.
*
* <p>A {@code StringJoiner} is empty if {@link #add(CharSequence) add()}
* has never been called, and if {@code merge()} has never been called
* with a non-empty {@code StringJoiner} argument.
*
* <p>If the other {@code StringJoiner} is using a different delimiter,
* then elements from the other {@code StringJoiner} are concatenated with
* that delimiter and the result is appended to this {@code StringJoiner}
* as a single element.
*
* @param other The {@code StringJoiner} whose contents should be merged
* into this one
* @throws NullPointerException if the other {@code StringJoiner} is null
* @return This {@code StringJoiner}
*/
public StringJoiner merge(StringJoiner other) {
Objects.requireNonNull(other);
if (other.value != null) {
final int length = other.value.length();
// lock the length so that we can seize the data to be appended
// before initiate copying to avoid interference, especially when
// merge 'this'
StringBuilder builder = prepareBuilder();
builder.append(other.value, other.prefix.length(), length);
}
return this;
}
private StringBuilder prepareBuilder() {
if (value != null) {
value.append(delimiter);
} else {
value = new StringBuilder().append(prefix);
}
return value;
}
StringJoiner的使用方式：
/**
* StringJoiner的使用 简单且实用
*/
void stringJoinerTest() {
StringJoiner stringJoiner = new StringJoiner(",","[","]");
stringJoiner.add("1");
stringJoiner.add("2");
stringJoiner.add("3");
stringJoiner.add("4");
stringJoiner.add("5");
System.out.println(stringJoiner);
}
/**
* StringJoiner的构造方法
*
* 构造一个没有字符的{@code StringJoiner}
* {@code prefix} or {@code suffix}, and a copy of the supplied
* {@code delimiter}.
* 如果没有向{@code StringJoiner}和方法添加字符*访问它的值被调用时，它不会返回a
* {@code前缀}或{@code后缀}(或其属性)，*除非{@code setEmptyValue}已被首次调用。
*
* @param delimiter the sequence of characters to be used between each
* element added to the {@code StringJoiner} value
* @throws NullPointerException if {@code delimiter} is {@code null}
*/
public StringJoiner(CharSequence delimiter) {
this(delimiter, "", "");
}
/**
* 使用副本构造一个没有字符的{@code StringJoiner}*提供的{@code前缀}、{@code分隔符}
* 和{@code后缀}。
* 如果{@code StringJoiner}和方法中没有添加字符*访问它的字符串值时，它将返回
* {@code前缀+后缀}(或其属性)，除非
* {@code setEmptyValue}首次被调用。
*
* @param delimiter the sequence of characters to be used between each
* element added to the {@code StringJoiner}
* @param prefix the sequence of characters to be used at the beginning
* @param suffix the sequence of characters to be used at the end
* @throws NullPointerException if {@code prefix}, {@code delimiter}, or
* {@code suffix} is {@code null}
*/
public StringJoiner(CharSequence delimiter,
CharSequence prefix,
CharSequence suffix) {
Objects.requireNonNull(prefix, "The prefix must not be null");
Objects.requireNonNull(delimiter, "The delimiter must not be null");
Objects.requireNonNull(suffix, "The suffix must not be null");
// make defensive copies of arguments
this.prefix = prefix.toString();
this.delimiter = delimiter.toString();
this.suffix = suffix.toString();
this.emptyValue = this.prefix + this.suffix;
}
2. String.join():
Java 8 同样有了String.join(),是基于StringJoiner进行字符串组合，减少了更多代码量，让自己代码更加简洁明了。
/**
* JAVA 8 出现的新方法 join（）
*
* 属性的副本组成的新字符串
* {@code CharSequence元素}与的副本连接在一起*指定的{@code分隔符}。
*
* <blockquote>For example,
* <pre>{@code
* String message = String.join("-", "Java", "is", "cool");
* // message returned is: "Java-is-cool"
* }</pre></blockquote>
*
* Note that if an element is null, then {@code "null"} is added.
*
* @param delimiter the delimiter that separates each element
* @param elements the elements to join together.
*
* @return a new {@code String} that is composed of the {@code elements}
* separated by the {@code delimiter}
*
* @throws NullPointerException If {@code delimiter} or {@code elements}
* is {@code null}
*
* @see java.util.StringJoiner
* @since 1.8
*/
public static String join(CharSequence delimiter, CharSequence... elements) {
Objects.requireNonNull(delimiter);
Objects.requireNonNull(elements);
// Number of elements not likely worth Arrays.stream overhead.
StringJoiner joiner = new StringJoiner(delimiter);
for (CharSequence cs: elements) {
joiner.add(cs);
}
return joiner.toString();
}
/**
* JAVA 8 出现的新方法 join（）
*
* 属性的副本组成一个新的{@code字符串}
* {@code CharSequence元素}与
* 指定{@code分隔符}。
*
* <blockquote>For example,
* <pre>{@code
* List<String> strings = new LinkedList<>();
* strings.add("Java");strings.add("is");
* strings.add("cool");
* String message = String.join(" ", strings);
* //message returned is: "Java is cool"
*
* Set<String> strings = new LinkedHashSet<>();
* strings.add("Java"); strings.add("is");
* strings.add("very"); strings.add("cool");
* String message = String.join("-", strings);
* //message returned is: "Java-is-very-cool"
* }</pre></blockquote>
*
* Note that if an individual element is {@code null}, then {@code "null"} is added.
*
* @param delimiter a sequence of characters that is used to separate each
* of the {@code elements} in the resulting {@code String}
* @param elements an {@code Iterable} that will have its {@code elements}
* joined together.
*
* @return a new {@code String} that is composed from the {@code elements}
* argument
*
* @throws NullPointerException If {@code delimiter} or {@code elements}
* is {@code null}
*
* @see #join(CharSequence,CharSequence...)
* @see java.util.StringJoiner
* @since 1.8
*/
public static String join(CharSequence delimiter,
Iterable<? extends CharSequence> elements) {
Objects.requireNonNull(delimiter);
Objects.requireNonNull(elements);
StringJoiner joiner = new StringJoiner(delimiter);
for (CharSequence cs: elements) {
joiner.add(cs);
}
return joiner.toString();
}
String.join()使用方法：
/**
* String.join()的使用
*/
@Test
void string_JoinerTest() {
String str = String.join("|", "1", "2", "3");
System.out.println(str);
List<String> list = new ArrayList<String>();
list.add("1");
list.add("2");
list.add("3");
list.add("4");
String join = String.join(",", list);
System.out.println(join);
}
3. StringJoiner 与 String.join() 关系：
String.join() 这是针对 StringJoiner 又封装了一层的 API，同样出自 Java 8，可以传入动态参数或者迭代器。可根据不同的场景使用不同的工具。