StringJoiner类

• 源码
• 使用

StringJoiner是Java8新出的一个类，用于构造由分隔符分隔的字符序列，并可选择性地从提供的前缀开始和以提供的后缀结尾。省的我们开发人员再次通过StringBuffer或者StingBuilder拼接。

package java.util;

public class StringJoiner {

    private final String prefix; //前缀
    private final String delimiter;//间隔符
    private final String suffix;//后缀
    private StringBuilder value;//值
    private String emptyValue;//空值

    
    public StringJoiner(CharSequence delimiter) {
        this(delimiter, "", "");
    }

    
    public StringJoiner(CharSequence delimiter,
                        CharSequence prefix,
                        CharSequence suffix) {
        //间隔符，前缀和后缀判断是否为null，null将抛出异常
        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");
        // make defensive copies of arguments
        this.prefix = prefix.toString();
        this.delimiter = delimiter.toString();
        this.suffix = suffix.toString();
        this.emptyValue = this.prefix + this.suffix;
    }


    
    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;
        } 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;
            }
        }
    }

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

    
    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;
    }

    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());
    }

}

package com.example.demo.test;

import java.util.StringJoiner;



public class StringJoinerTest {
    public static void main(String[] args) {
        StringJoiner stringJoiner = new StringJoiner(",", "[", "]");
        stringJoiner.add("张三").add("李四").add("王五");
        System.out.println(stringJoiner.toString());

        StringJoiner stringJoiner1 = new StringJoiner("..");
        stringJoiner1.add("张三").add("李四").add("王五");
        System.out.println(stringJoiner1.toString());

        StringJoiner merge = stringJoiner1.merge(stringJoiner);
        System.out.println(merge.toString());
        System.out.println(merge.length());

    }
}

输出：
[张三,李四,王五]
张三…李四…王五
张三…李四…王五…张三,李四,王五
20