参考文章

StringJoiner

作用

StringJoiner是Java8新出的一个类，用于构造由分隔符分隔的字符序列，并可选择性地从提供的前缀开始和以提供的后缀结尾。省的我们开发人员再次通过StringBuffer或者StingBuilder拼接。

源码

package java.util;/*** {@code StringJoiner} is used to construct a sequence of characters separated* by a delimiter and optionally starting with a supplied prefix* and ending with a supplied suffix.* <p>* Prior to adding something to the {@code StringJoiner}, its* {@code sj.toString()} method will, by default, return {@code prefix + suffix}.* However, if the {@code setEmptyValue} method is called, the {@code emptyValue}* supplied will be returned instead. This can be used, for example, when* creating a string using set notation to indicate an empty set, i.e.* <code>"{}"</code>, where the {@code prefix} is <code>"{"</code>, the* {@code suffix} is <code>"}"</code> and nothing has been added to the* {@code StringJoiner}.** @apiNote* <p>The String {@code "[George:Sally:Fred]"} may be constructed as follows:** <pre> {@code* StringJoiner sj = new StringJoiner(":", "[", "]");* sj.add("George").add("Sally").add("Fred");* String desiredString = sj.toString();* }</pre>* <p>* A {@code StringJoiner} may be employed to create formatted output from a* {@link java.util.stream.Stream} using* {@link java.util.stream.Collectors#joining(CharSequence)}. For example:** <pre> {@code* List<Integer> numbers = Arrays.asList(1, 2, 3, 4);* String commaSeparatedNumbers = numbers.stream()*     .map(i -> i.toString())*     .collect(Collectors.joining(", "));* }</pre>** @see java.util.stream.Collectors#joining(CharSequence)* @see java.util.stream.Collectors#joining(CharSequence, CharSequence, CharSequence)* @since  1.8
*/
public final class StringJoiner {private final String prefix;           // 前缀private final String delimiter;    // 间隔符private final String suffix;      // 后缀/** StringBuilder value -- at any time, the characters constructed from the* prefix, the added element separated by the delimiter, but without the* suffix, so that we can more easily add elements without having to jigger* the suffix each time.*/private StringBuilder value;          // 值/** By default, the string consisting of prefix+suffix, returned by* toString(), or properties of value, when no elements have yet been added,* i.e. when it is empty.  This may be overridden by the user to be some* other value including the empty String.*/private String emptyValue;         // 空值/*** 重载调用* 默认前缀和后缀为 ""*/public StringJoiner(CharSequence delimiter) {this(delimiter, "", "");}/*** 若其中有null 则抛出异常*/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 argumentsthis.prefix = prefix.toString();this.delimiter = delimiter.toString();this.suffix = suffix.toString();this.emptyValue = this.prefix + this.suffix;}/*** 设置空值，检查是否为null*/public StringJoiner setEmptyValue(CharSequence emptyValue) {this.emptyValue = Objects.requireNonNull(emptyValue,"The empty value must not be null").toString();return this;}/*** 重写的toString()*/@Overridepublic String toString() {if (value == null) {return emptyValue;} else {if (suffix.equals("")) {return value.toString();} else {int initialLength = value.length();String result = value.append(suffix).toString();// reset value to pre-append initialLengthvalue.setLength(initialLength);return result;}}}/*** Adds a copy of the given {@code CharSequence} value as the next* element of the {@code StringJoiner} value. If {@code newElement} is* {@code null}, then {@code "null"} is added.** @param  newElement The element to add* @return a reference to this {@code StringJoiner}*/public StringJoiner add(CharSequence newElement) {prepareBuilder().append(newElement);return this;}/*** 合并StringJoiner* 将后面的数据添加进前一个*/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;}/*** 初始化* 先添加前缀，若有数据，则每次先添加间隔符*/private StringBuilder prepareBuilder() {if (value != null) {value.append(delimiter);} else {value = new StringBuilder().append(prefix);}return value;}/*** Returns the length of the {@code String} representation* of this {@code StringJoiner}. Note that if* no add methods have been called, then the length of the {@code String}* representation (either {@code prefix + suffix} or {@code emptyValue})* will be returned. The value should be equivalent to* {@code toString().length()}.** @return the length of the current value of {@code StringJoiner}*/public int length() {// Remember that we never actually append the suffix unless we return// the full (present) value or some sub-string or length of it, so that// we can add on more if we need to.return (value != null ? value.length() + suffix.length() :emptyValue.length());}
}

测试代码

StringJoiner joiner = new StringJoiner(":","{","}");
joiner.add("1");
joiner.add("2");
joiner.add("3");
joiner.add("4");
System.out.println(joiner);

输出 {1:2:3:4}

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StringJoiner

StringJoiner

作用

源码

测试代码

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