String及StringTable(五):java8的新工具类StringJoiner

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在java8中,对于字符串拼接的操作还引入了一个新的类就是StringJoiner,这个类的作用就是提供了一种快捷的字符串拼接的模板方法。

1.使用样例

``` public static void main(String[] args) { StringJoiner stringJoiner = new StringJoiner(",","[","]"); stringJoiner.add("a"); stringJoiner.add("b"); stringJoiner.add("c"); stringJoiner.add("d"); System.out.println(stringJoiner.toString());

StringJoiner stringJoiner1 = new StringJoiner(",");
stringJoiner1.add("1").add("2").add("3");
System.out.println(stringJoiner1.toString());

} 其输出如下: [a,b,c,d] 1,2,3 ``` 可以看到,StringJoiner支持传入分隔符、前缀和后缀,之后就能将这种有规律的字符串进行输出。这非常适合我们平时在对数组的print和序列化的过程中。

2.类结构及成员变量

``` /* * {@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. *

* 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. * "{}", where the {@code prefix} is "{", the * {@code suffix} is "}" and nothing has been added to the * {@code StringJoiner}. * * @apiNote *

The String {@code "[George:Sally:Fred]"} may be constructed as follows: * *

 {@code
 * StringJoiner sj = new StringJoiner(":", "[", "]");
 * sj.add("George").add("Sally").add("Fred");
 * String desiredString = sj.toString();
 * }
*

* 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: * *

 {@code
 * List numbers = Arrays.asList(1, 2, 3, 4);
 * String commaSeparatedNumbers = numbers.stream()
 *     .map(i -> i.toString())
 *     .collect(Collectors.joining(", "));
 * }
* * @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;

} StringJoiner这个类比较单一,就是一个简单的工具类,没有实现任何接口,继承任何父类。其注释大意为: StringJoiner根据分隔符、前缀、后缀来构造分隔的字符串。 在调用add方法添加内容之后,他在默认的情况下是返回前缀+分隔符组合字符串+后缀。但是如果调用setEmptyValue方法之后,如果value为空,将会返回这个设置的字符串。 StringJoiner已经被Collectors引用,在Stream处理中,我们可以类似这样使用: * java.util.stream.Collectors#joining(CharSequence) * java.util.stream.Collectors#joining(CharSequence, CharSequence, CharSequence) ``` 主要的成员变量有5个,分别是String类型的前缀prefix,分隔符delimiter、后缀suffix,实际上内部是一个StringBuilder value,字符串拼接操作采用StringBuilder来完成。还有一个当value为空的时候的默认字符串emptyValue。

2.构造函数

StringJoiner提供的构造函数有两个,分别是只有分隔符的构造函数: public StringJoiner(CharSequence delimiter) { this(delimiter, "", ""); } 和另外一个提供了前后缀的: 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; } 实际上可以看出第一个构造函数只是将前后缀设置为空了,还是使用的第二个构造函数。 对于第二个构造函数,emptyValue默认等于前缀加后缀。

3.其他方法

3.1 setEmptyValue

由于前文提到了emptyValue,那么首先提供了一个setEmptyValue方法: public StringJoiner setEmptyValue(CharSequence emptyValue) { this.emptyValue = Objects.requireNonNull(emptyValue, "The empty value must not be null").toString(); return this; }

3.2 toString

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(); // reset value to pre-append initialLength value.setLength(initialLength); return result; } } } 判断只要value为null则返回emptyValue。之后判断后缀是否为空。否则在value上加上后缀。

3.3 add

public StringJoiner add(CharSequence newElement) { prepareBuilder().append(newElement); return this; } 这是主要的添加元素的方法。

3.4 merge

提供了将两个StringJoiner合并的merge方法: 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; }

3.5 prepareBuilder

添加前缀的方法: private StringBuilder prepareBuilder() { if (value != null) { value.append(delimiter); } else { value = new StringBuilder().append(prefix); } return value; }

3.5 length

得到整个串的长度: 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()); } 如果不为空,由于后缀是在toString的时候才添加的,没有加上后缀,因此长度中需要将后缀的长度加上。