StringJoiner類拼接字符串的應用

StringJoiner是什麼？

StringJoiner是Java8新出的一個類，用於構造由分隔符分隔的字符序列，並可選擇性地從提供的前綴開始和以提供的後綴結尾。方便開發人員再次通過StringBuffer或者StingBuilder拼接。

應用實例：

StringJoiner sj = new StringJoiner(":", "(", ")");
sj.add("hello").add("world").add("hah");
String sjString = sj.toString();

輸出的代碼爲：

(hello:world:hah)

下面是其源碼加深理解：

package java.util;

public final class StringJoiner {
    private final String prefix;//前綴
    private final String delimiter;//間隔符
    private final String suffix;//後綴
    private StringBuilder value;//值

    private String emptyValue;//空值

    public StringJoiner(CharSequence delimiter) {
        this(delimiter, "", "");//默認前綴和後綴爲"",重載調用
    }

    public StringJoiner(CharSequence delimiter,
                        CharSequence prefix,
                        CharSequence suffix) {
        //間隔符，前綴和後綴判斷是否爲null，null將拋出異常
        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"); 
        // 成員變量賦值
        this.prefix = prefix.toString();
        this.delimiter = delimiter.toString();
        this.suffix = suffix.toString();
        this.emptyValue = this.prefix + this.suffix;//空值被設置爲只有前後綴
    }
	//設置空值，檢查是否爲null
    public StringJoiner setEmptyValue(CharSequence emptyValue) {
        this.emptyValue = Objects.requireNonNull(emptyValue,
            "The empty value must not be null").toString();
        return this;
    }

    @Override
    public String toString() {
        if (value == null) {
            return emptyValue;//沒有值將返回空值或者後續設置的空值
        } else {
            if (suffix.equals("")) {
                return value.toString();//後綴爲""直接返回字符串，不用添加
            } else {
	            //後綴不爲""，添加後綴，然後直接返回字符串，修改長度
                int initialLength = value.length();
                String result = value.append(suffix).toString();
                // reset value to pre-append initialLength
                value.setLength(initialLength);
                return result;
            }
        }
    }
    ////初始化，先添加前綴，有了之後每次先添加間隔符，StringBuilder後續append字符串
    public StringJoiner add(CharSequence newElement) {
        prepareBuilder().append(newElement);
        return this;
    }
	//合併StringJoiner，注意後面StringJoiner 的前綴就不要了，後面的appen進來
    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，先添加前綴，有了之後每次先添加間隔符
    private StringBuilder prepareBuilder() {
        if (value != null) {
            value.append(delimiter);
        } else {
            value = new StringBuilder().append(prefix);
        }
        return value;
    }

    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.
        //不忘添加後綴的長度
        return (value != null ? value.length() + suffix.length() :
                emptyValue.length());
    }
}