关于StringJoiner的使用

本文主要是介绍关于StringJoiner的使用，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！

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 argumentsthis.prefix = prefix.toString();this.delimiter = delimiter.toString();this.suffix = suffix.toString();this.emptyValue = this.prefix + this.suffix;}}    

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;
}

toString : 转成字符串

    @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;}}}

add : 添加字符串

public StringJoiner add(CharSequence newElement) {prepareBuilder().append(newElement);return this;
}

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;
}

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;
}

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());
}

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();

改造后

StringJoiner sj = new StringJoiner(",", "(", ")");
sj.add("李白");
sj.add("高适");
String str = sj.toString();

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();

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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