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1.为什么使用StringJoiner
您还在为使用代码拼接类似的sql占位符而烦恼吗？要生成如下的代码：
(?,?,?,?,?,?,?,?,?,?)你是使用这种：
StringBuffer sb= new StringBuffer();
sb.append("(");for (int i = 0; i < 10; i++) {if (i == 9) {sb.append("?");} else {sb.append("?").append(",");}}
sb.append(")");还是使用这种：
StringBuffer sb = new StringBuffer();
sb.append("(");
for (int i = 0; i < 10; i++) {if (i == 0) {sb.append("?");} else {sb.append(",").append("?");}
}
sb.append(")");而使用StringJoiner，你只需要这样就能生成：
// public StringJoiner(CharSequence delimiter,
// CharSequence prefix,
// CharSequence suffix)
StringJoiner joiner = new StringJoiner(",", "(", ")");
for (int i = 0; i < 10; i++) {joiner.add("?");
}StringJoiner还能合并合并其他的StringJoiner，实现内容的合并。
StringJoiner joiner = new StringJoiner(",", "(", ")");
for (int i = 0; i < 10; i++) {joiner.add("?");
}
StringJoiner joiner1 = new StringJoiner(",", "(", ")");
for (int i = 0; i < 10; i++) {joiner1.add("&");
}
joiner.merge(joiner1);
System.out.println(joiner);
//输出：(?,?,?,?,?,?,?,?,?,?,&,&,&,&,&,&,&,&,&,&)2.StringJoiner的主要功能
StringJoiner 是 Java 8 引入的一个实用工具类，位于 java.util 包中。它用于构建由分隔符分隔的字符串序列，可以选择性地开始于一个前缀并以一个后缀结束。StringJoiner 是设计来简化多个字符串组合操作的，特别是当你需要创建一个由多个部分组成的字符串时，比如在生成列表、表格或任何由多个元素按照特定格式排列的字符串时。StringJoiner的主要功能如下：
-
添加分隔符：
StringJoiner的主要功能之一是在添加到StringJoiner的每个元素之间自动添加指定的分隔符。这使得构建如逗号分隔值（CSV）或其他由特定字符分隔的字符串变得非常简单和直接。 -
可选的前缀和后缀：
StringJoiner允许你指定一个可选的前缀和后缀，这些前缀和后缀将被添加到最终构建的字符串的开始和结束位置。这在生成如 JSON 数组或括号包围的列表时非常有用。 -
处理空值：默认情况下，如果没有元素添加到
StringJoiner，则它不会添加前缀和后缀，只有在添加了至少一个元素后，前缀和后缀才会出现在结果字符串中。这种行为可以通过使用setEmptyValue方法来改变，允许你为一个空的StringJoiner实例定义一个自定义的字符串。 -
合并：
StringJoiner提供了merge方法，允许将另一个StringJoiner的内容合并到当前的StringJoiner中，而不会添加额外的前缀、后缀或分隔符。这在需要组合来自多个源的字符串时非常有用。 -
易于使用：与直接使用字符串连接或
StringBuilder相比，StringJoiner提供了一种更高级、更易于理解和使用的方式来构建复杂的字符串，尤其是在涉及到多个字符串元素和分隔符时。
3.StringJoiner源码解析
1.主要的属性
//前缀 "("
private final String prefix;
//分隔符 ","
private final String delimiter;
//后缀缀 ")"
private final String suffix;
/** StringBuilder 用于存储当前所有已添加元素的字符串表示，* 如果 StringJoiner 为空，则此字段可能为 null。*/
private StringBuilder value;prefix、delimiter和suffix是在构造时设置的，分别代表前缀、分隔符和后缀。value是一个StringBuilder实例，用于构建和存储最终的字符串。初始时可能为null，直到添加了第一个元素。
StringJoiner 提供了几个构造函数，允许指定分隔符、前缀和后缀：
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;}这个构造函数接受三个参数：delimiter、prefix 和 suffix。所有参数都不能为 null（使用 Objects.requireNonNull 进行检查）。
add方法：
public StringJoiner add(CharSequence newElement) {prepareBuilder().append(newElement);return this;
}private StringBuilder prepareBuilder() {if (value != null) {value.append(delimiter);} else {value = new StringBuilder().append(prefix);}return value;}- 如果
value（StringBuilder实例）不为null，则意味着之前已经添加过元素，因此在添加新元素之前会先追加分隔符。 - 如果
value为null，则表示这是第一个被添加的元素，因此会创建一个新的StringBuilder实例，并先追加前缀。
merge方法如下：
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;}最终的字符串是通过 toString 方法生成的：
public 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();// Now reset the StringBuilder to its original statevalue.setLength(initialLength);return result;}}
}
- 如果
value为null，则返回一个预定义的空值字符串。 - 否则，会在
value（StringBuilder实例）上追加后缀，生成最终的字符串，并在操作完成后重置StringBuilder的长度，以便移除后缀（这样做是为了保持value的状态，以便可以继续添加元素）。
StringJoiner 主要特性：
StringJoiner是不可变的，每次添加元素或合并其他StringJoiner实例时，都会在内部更新StringBuilder实例。StringJoiner的设计非常高效，特别适合于构建复杂的字符串，尤其是当涉及到多个部分需要通过特定分隔符连接时。
4.StringJoiner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;private final String delimiter;private final String suffix;/** 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;/** 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;/*** Constructs a {@code StringJoiner} with no characters in it, with no* {@code prefix} or {@code suffix}, and a copy of the supplied* {@code delimiter}.* If no characters are added to the {@code StringJoiner} and methods* accessing the value of it are invoked, it will not return a* {@code prefix} or {@code suffix} (or properties thereof) in the result,* unless {@code setEmptyValue} has first been called.** @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, "", "");}/*** Constructs a {@code StringJoiner} with no characters in it using copies* of the supplied {@code prefix}, {@code delimiter} and {@code suffix}.* If no characters are added to the {@code StringJoiner} and methods* accessing the string value of it are invoked, it will return the* {@code prefix + suffix} (or properties thereof) in the result, unless* {@code setEmptyValue} has first been called.** @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 argumentsthis.prefix = prefix.toString();this.delimiter = delimiter.toString();this.suffix = suffix.toString();this.emptyValue = this.prefix + this.suffix;}/*** Sets the sequence of characters to be used when determining the string* representation of this {@code StringJoiner} and no elements have been* added yet, that is, when it is empty. A copy of the {@code emptyValue}* parameter is made for this purpose. Note that once an add method has been* called, the {@code StringJoiner} is no longer considered empty, even if* the element(s) added correspond to the empty {@code String}.** @param emptyValue the characters to return as the value of an empty* {@code StringJoiner}* @return this {@code StringJoiner} itself so the calls may be chained* @throws NullPointerException when the {@code emptyValue} parameter is* {@code null}*/public StringJoiner setEmptyValue(CharSequence emptyValue) {this.emptyValue = Objects.requireNonNull(emptyValue,"The empty value must not be null").toString();return this;}/*** Returns the current value, consisting of the {@code prefix}, the values* added so far separated by the {@code delimiter}, and the {@code suffix},* unless no elements have been added in which case, the* {@code prefix + suffix} or the {@code emptyValue} characters are returned** @return the string representation of this {@code StringJoiner}*/@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;}/*** 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;}/*** 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());}
}
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