由Jdk1.8提供,在android 7.0及其以上提供支持。内部基于StringBuilder实现，提供更加便捷的方式来添加分隔符、前缀以及后缀。
用法
由前缀prefix、需要拼接的字符串、分隔符delimiter、以及后缀suffix组成。
//Kotlin
class Demo {
@RequiresApi(Build.VERSION_CODES.N)
fun test() {
var stringJoiner = StringJoiner("", "--->", "<---")
//result:---><---
println(stringJoiner.toString())
stringJoiner = StringJoiner("|", "--->", "<---")
//result:---><---
println(stringJoiner.toString())
stringJoiner.add("1")
//result:--->1<---
println(stringJoiner.toString())
stringJoiner.add("2")
//result:--->1|2<---
println(stringJoiner.toString())
var stringJoiner1 = StringJoiner(",", "前", "后")
stringJoiner1.add("a")
stringJoiner1.add("b")
//result:前a,b后
println(stringJoiner1.toString())
stringJoiner.merge(stringJoiner1)
//结果--->1|2|a,b<---
println(stringJoiner.toString())
stringJoiner = StringJoiner("|", "--->", "<---")
stringJoiner.merge(stringJoiner1)
//result:--->a,b<---
println(stringJoiner.toString())
}
}
源码分析
public final class StringJoiner {
//前缀
private final String prefix;
//分割符，只存在两个拼接的字符串之间
private final String delimiter;
//后缀
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.
*/
//在任何时候只包含前缀+带有分隔符的字符串组成的集合
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.
*/
//由前缀+后缀组成的字符串，默认结果为"";如果没有设置前缀或后缀，将未设置的替换“”，不允许设置null
//当value==null,toString方法返回的是它
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 arguments
this.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}
*/
@Override
public String toString() {
//检测是否已添加需要拼接的字符串
if (value == null) {
return emptyValue;
} else {
//检查后缀是否为“”，value中包含前缀
if (suffix.equals("")) {
return value.toString();
} else {
int initialLength = value.length();
//将value添加后缀并作为结果返回
String result = value.append(suffix).toString();
// reset value to pre-append initialLength
//重新设置value的长度，将value还原到还未添加后缀的状态,也就是说去掉刚添加的后缀，保证下一次add正确性
value.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) {
//首先调用prepareBuilder()对value进行操作，如果value为null，则创建value并添加前缀，否则添加分隔符
//接着添加字符串，就变成前缀+需要拼接的字符串或者前缀+需要拼接的字符串+分割符+需要拼接的字符串
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());
}
}
