关于StringJoiner的使用

在项目中,经常有某些集合数据,需要添加逗号或其他符号,拼接成字符串展示,而普通的遍历后追加分隔符,需要单独处理最后一个元素,比较繁琐.Java8中,提供了一个工具类,即StringJoiner可以完美解决此类问题.

1 说明

StringJoiner类, 是Java8中,java.util包下的一个类.基于StringBuilder实现.主要用于对字符串通过分隔符进行拼接的场景.

1 属性和构造方法

public final class StringJoiner {
    // 拼接后字符串前缀
    private final String prefix;
    // 拼接时字符串的分隔符
    private final String delimiter;
    // 拼接后字符串后缀
    private final String suffix;
    // 拼接后字符串的值
    private StringBuilder value;
    // 当value为null时,返回的值  (默认是前缀+后缀)
    private String emptyValue;
    
    // 分隔符构造
    public StringJoiner(CharSequence delimiter) {
        this(delimiter, "", "");
    }
    
    // 前缀,分隔符,后缀构造
    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;
    }
    
}    
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2 普通方法

setEmptyValue : 设置空值

public StringJoiner setEmptyValue(CharSequence emptyValue) {
    this.emptyValue = Objects.requireNonNull(emptyValue,
        "The empty value must not be null").toString();
    return this;
}
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toString : 转成字符串

    @Override
    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;
            }
        }
    }
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add : 添加字符串

public StringJoiner add(CharSequence newElement) {
    prepareBuilder().append(newElement);
    return this;
}
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merge : 合并另一个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;
}
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prepareBuilder : 构建准备,不存在则新增

private StringBuilder prepareBuilder() {
    if (value != null) {
        value.append(delimiter);
    } else {
        value = new StringBuilder().append(prefix);
    }
    return value;
}
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length : 长度,如value为null则返回空值长度

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());
}
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2 使用

1 单个拼接场景

StringBuilder sb = new StringBuilder();
sb.append("(");
sb.append("李白");
sb.append(",");
sb.append("高适");
sb.append(")");
String str = sb.toString();
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改造后

StringJoiner sj = new StringJoiner(",", "(", ")");
sj.add("李白");
sj.add("高适");
String str = sj.toString();
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2 遍历拼接场景

List<String> list = Arrays.asList("李白","高适");
StringBuilder sb = new StringBuilder();
for(String str: list){
    sb.append(str).append(",");
}
// 删除最后一个逗号返回
// 正常需要判断sb不为空即length()大于0  
String str = deleteCharAt(sb.length()-1).toString();
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改造后

List<String> list = Arrays.asList("李白","高适");
StringJoiner sj = new StringJoiner(",");
for(String str: list){
    sj.add(str);
}
String str = sj.toString();
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